U.S. patent application number 13/725280 was filed with the patent office on 2013-05-02 for digital-to-analog converter, analog-to-digital converter, and semiconductor device.
This patent application is currently assigned to SONY CORPORATION. The applicant listed for this patent is SONY CORPORATION. Invention is credited to Kiyotaka AMANO, Go ASAYAMA, Noriyuki FUKUSHIMA, Yoshinori MURAMATSU, Yoshikazu NITTA.
Application Number | 20130106636 13/725280 |
Document ID | / |
Family ID | 37910630 |
Filed Date | 2013-05-02 |
United States Patent
Application |
20130106636 |
Kind Code |
A1 |
ASAYAMA; Go ; et
al. |
May 2, 2013 |
DIGITAL-TO-ANALOG CONVERTER, ANALOG-TO-DIGITAL CONVERTER, AND
SEMICONDUCTOR DEVICE
Abstract
A DA conversion device includes the following elements. A
higher-bit current source cell portion includes uniformly weighted
higher-bit current source cells to generate an identical constant
current. A lower-bit current source cell portion includes a
lower-bit current source cells that are weighted to generate
1/two-to-the-power-of-certain-numbers constant currents. A constant
current source selection controller includes a lower-bit controller
having a scaler that uses clocks scaled down to
1/two-to-the-power-of-certain-numbers to select the lower-bit
current source cells, and a higher-bit controller having shift
registers and using a signal indicating a carry bit or a borrow bit
used in the lower-bit controller to sequentially activate shift
outputs of the shift registers, and uses the shift outputs to
select the higher-bit current source cells. Constant current
outputs of the selected current source cells are added and output
so that an output current corresponding to the digital input signal
is obtained.
Inventors: |
ASAYAMA; Go; (Kanagawa,
JP) ; FUKUSHIMA; Noriyuki; (Kanagawa, JP) ;
NITTA; Yoshikazu; (Kanagawa, JP) ; MURAMATSU;
Yoshinori; (Kanagawa, JP) ; AMANO; Kiyotaka;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SONY CORPORATION; |
Tokyo |
|
JP |
|
|
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
37910630 |
Appl. No.: |
13/725280 |
Filed: |
December 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13098206 |
Apr 29, 2011 |
8339300 |
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13725280 |
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12426731 |
Apr 20, 2009 |
7936294 |
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13098206 |
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|
11974266 |
Oct 12, 2007 |
7522082 |
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12426731 |
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11526512 |
Sep 25, 2006 |
7324033 |
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11974266 |
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Current U.S.
Class: |
341/144 ;
341/164 |
Current CPC
Class: |
H03M 1/747 20130101;
H03M 1/745 20130101; H03M 1/123 20130101; H03M 1/56 20130101; H03M
1/0863 20130101; H03M 1/664 20130101; H03M 1/66 20130101; H03M
1/687 20130101; H03M 1/34 20130101 |
Class at
Publication: |
341/144 ;
341/164 |
International
Class: |
H03M 1/66 20060101
H03M001/66; H03M 1/34 20060101 H03M001/34 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2005 |
JP |
2005-277190 |
Claims
1. A digital-to-analog conversion device comprising: a higher-bit
current source cell portion including a plurality of higher-bit
current source cells that are uniformly weighted to generate an
identical constant current; a lower-bit current source cell portion
including a plurality of lower-bit current source cells that are
weighted to generate constant currents which are
1/two-to-the-power-of-certain-numbers of the constant current
generated by the higher-bit current source cells; and a constant
current source selection controller operable to select the
higher-bit current source cells of the higher-bit current source
cell portion and the lower-bit current source cells of the
lower-bit current source cell portion in accordance with a data
value of a digital input signal, wherein the constant current
source selection controller includes a lower-bit controller and a
higher-bit controller, and the lower-bit controller includes a
scaler that performs a scaling operation based on an input count
clock and uses clocks scaled down to
1/two-to-the-power-of-certain-numbers obtained by the scaling
operation as a selection control signal to select the lower-bit
current source cells weighted with current values corresponding to
the clocks scaled down to 1/two-to-the-power-of-certain-numbers,
and the higher-bit controller includes the same number of shift
registers as the number of the higher-bit current source cells and
uses, as a shift clock, a signal indicating a carry bit or a borrow
bit used in the scaling operation controlled by the lower-bit
controller to sequentially activate shift outputs of the shift
registers, and uses the shift outputs as a selection control signal
to select the higher-bit current source cells, and wherein constant
current outputs of the selected lower-bit current source cells and
the higher-bit current source cells are added and output so that an
output current corresponding to the digital input signal is
obtained.
2. The digital-to-analog conversion device according to claim 1,
wherein the higher-bit current source cell portion includes the
number of the higher-bit current source cells corresponding to the
data value of the higher bits of the digital input signal, and the
lower-bit current source cell portion includes the number of the
lower-bit current source cells corresponding to the bit number of
the lower bits of the digital input signal.
3. The digital-to-analog conversion device according to claim 2,
wherein the higher-bit controller stops a shift operation performed
by the shift registers when the shift output of the shift register
reaches the data value of the higher bits of the digital input
signal.
4. The digital-to-analog conversion device according to claim 3,
wherein the lower-bit controller stops the scaling operation
performed by the scaler when the clock scaled down to
1/two-to-the-power-of-certain-number reaches the bit number of the
lower bits of the digital input signal.
5. The digital-to-analog conversion device according to claim 1,
wherein each of the lower-bit current source cells and the
higher-bit current source cells includes a constant current source
and a differential switch that switches an output current of the
constant current source based on a complementary signal, and at
least one of the higher-bit controller and the lower-bit controller
includes delay means for inverting the logical level of an input
signal to generate an inverted signal and for delaying the inverted
signal by a predetermined period, and means for supplying the input
signal that is not delayed by the delay means and the inverted
signal that is delayed by the delay means to the differential
switch as the complementary signal.
6. The digital-to-analog conversion device according to claim 1,
wherein the shift register at a first stage is disposed near a
position at which the shift clock is output from the lower-bit
controller, and the remaining shift registers are disposed in a
two-dimensional matrix such that they turn around from one line to
another line.
7. The digital-to-analog conversion device according to claim 6,
wherein the length of a wiring pattern between each of the shift
registers and the other shift registers is the same according to an
isometric wiring pattern.
8. An analog-to-digital conversion device comprising: a reference
signal generator operable to generate a reference signal for
converting an analog signal into a digital signal; a comparator
operable to compare the analog signal with the reference signal
generated by the reference signal generator; and a counter operable
to perform a counting operation with a predetermined count clock,
simultaneously with a comparison operation performed by the
comparator, and to store a count value at a time when the
comparison operation is finished, wherein the reference signal
generator includes a higher-bit current source cell portion
including a plurality of higher-bit current source cells that are
uniformly weighted to generate an identical constant current, a
lower-bit current source cell portion including a plurality of
lower-bit current source cells that are weighted to generate
constant currents which are 1/two-to-the-power-of-certain-numbers
of the constant current generated by the higher-bit current source
cells, and a constant current source selection controller operable
to select the higher-bit current source cells of the higher-bit
current source cell portion and the lower-bit current source cells
of the lower-bit current source cell portion in accordance with a
data value of a digital input signal, wherein the constant current
source selection controller includes a lower-bit controller and a
higher-bit controller, and the lower-bit controller includes a
scaler that performs a scaling operation based on an input count
clock and uses clocks scaled down to
1/two-to-the-power-of-certain-numbers obtained by the scaling
operation as a selection control signal to select the lower-bit
current source cells weighted with current values corresponding to
the clocks scaled down to 1/two-to-the-power-of-certain-numbers,
and the higher-bit controller includes the same number of shift
registers as the number of the higher-bit current source cells and
uses, as a shift clock, a signal indicating a carry bit or a borrow
bit used in the scaling operation controlled by the lower-bit
controller to sequentially activate shift outputs of the shift
registers, and uses the shift outputs as a selection control signal
to select the higher-bit current source cells, and wherein constant
current outputs of the selected lower-bit current source cells and
the higher-bit current source cells are added and output so that an
output current corresponding to the digital input signal is
obtained.
9. A physical-quantity-distribution detecting semiconductor device
comprising: an effective area including, in a unit element, a
charge generator that generates charge in response to an incident
electromagnetic wave and a unit signal generator that generates an
analog unit signal corresponding to the charge generated by the
charge generator; a reference signal generator operable to generate
a reference signal for converting the analog unit signal into a
digital signal; a comparator operable to compare the analog signal
with the reference signal generated by the reference signal
generator; and a counter operable to perform a counting operation
with a predetermined count clock, simultaneously with a comparison
operation performed by the comparator, and to store a count value
at a time when the comparison operation is finished, the reference
signal generator, the comparator, and the counter operating as
function elements for converting the analog unit signal into the
digital signal, wherein the reference signal generator includes a
higher-bit current source cell portion including a plurality of
higher-bit current source cells that are uniformly weighted to
generate an identical constant current, a lower-bit current source
cell portion including a plurality of lower-bit current source
cells that are weighted to generate constant currents which are
1/two-to-the-power-of-certain-numbers of the constant current
generated by the higher-bit current source cells, and a constant
current source selection controller operable to select the
higher-bit current source cells of the higher-bit current source
cell portion and the lower-bit current source cells of the
lower-bit current source cell portion in accordance with a data
value of a digital input signal, wherein the constant current
source selection controller includes a lower-bit controller and a
higher-bit controller, and the lower-bit controller includes a
scaler that performs a scaling operation based on an input count
clock and uses clocks scaled down to
1/two-to-the-power-of-certain-numbers obtained by the scaling
operation as a selection control signal to select the lower-bit
current source cells weighted with current values corresponding to
the clocks scaled down to 1/two-to-the-power-of-certain-numbers,
and the higher-bit controller includes the same number of shift
registers as the number of the higher-bit current source cells and
uses, as a shift clock, a signal indicating a carry bit or a borrow
bit used in the scaling operation controlled by the lower-bit
controller to sequentially activate shift outputs of the shift
registers, and uses the shift outputs as a selection control signal
to select the higher-bit current source cells, and wherein constant
current outputs of the selected lower-bit current source cells and
the higher-bit current source cells are added and output so that an
output current corresponding to the digital input signal is
obtained.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The subject matter of application Ser. No. 13/098,206 is
incorporated herein by reference. The present application is a
continuation of U.S. application Ser. No. 13/098,206, filed Apr.
29, 2011, now U.S. Pat. No. 8,339,300, issued Dec. 25, 2012, which
is a Continuation of U.S. application Ser. No. 12/426,731, filed
Apr. 20, 2009, now U.S. Pat. No. 7,936,294, issued May 3, 2011,
which is a continuation of U.S. application Ser. No. 11/974,266,
filed Oct. 12, 2007, now U.S. Pat. No. 7,522,082, issued Apr. 21,
2009, which is a continuation of Ser. No. 11/526,512, filed Sep.
25, 2006, now U.S. Pat. No. 7,324,033, issued Jan. 29, 2008, which
claims priority to Japanese Patent Application JP 2005-277190 filed
in the Japanese Patent Office on Sep. 26, 2005, the entire contents
of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a digital-to-analog (DA)
converter for converting digital signals into analog signals, an
analog-to-digital (AD) converter including this DA converter, and a
semiconductor device, such as an imaging device. More particularly,
the invention relates to a DA conversion mechanism using a current
source cell matrix including an array-structured current source
cells.
[0004] 2. Description of the Related Art
[0005] Various DA converters for converting digital signals into
analog signals are used in electronic apparatuses.
[0006] Physical-quantity-distribution detecting semiconductor
devices including a plurality of unit elements (for example,
pixels) which are disposed in a line or a matrix and which are
responsive to electromagnetic waves, such as light or radiation,
input from an external source are used in various fields.
[0007] For example, in the field of video equipment, charge-coupled
device (CCD), metal oxide semiconductor (MOS), or complementary
metal-oxide semiconductor (CMOS)-type solid-state imaging devices
for detecting light, which is one example of physical quantities,
are used. Such imaging devices read a physical quantity
distribution obtained by converting light into an electric signal
by using the unit elements (pixels in the solid-state imaging
apparatuses). In the solid-state imaging devices, "solid-state"
means that the imaging devices are formed of semiconductor.
[0008] One type of solid-state imaging device is an amplifying
solid-state imaging device including active pixel sensors (APS),
which are also referred to as "gain cells", each active pixel
sensor using an amplifying transistor in an image signal generator
for generating pixel signals in accordance with signal charge
generated by a charge generator. Many CMOS-type solid-state imaging
devices have this type of structure.
[0009] In such an amplifying solid-state imaging device, to read
out pixel signals to the exterior from the imaging device, address
control is performed on a pixel portion including a plurality of
unit pixels so that pixel signals can be selected from the
corresponding unit pixels. That is, the amplifying solid-state
imaging device is one example of address-control solid-state
imaging device.
[0010] For example, in an amplifying solid-state imaging device,
which is one type of X-Y address solid-state imaging device
including unit pixels disposed in a matrix, MOS active elements
(MOS transistors) are used for forming the pixels so that the
pixels have an amplifying function by themselves. That is, signal
charge (photoelectrons) stored in photodiodes, which are
photoelectric transducers, is amplified by the active elements and
is read as image information.
[0011] In this type of X-Y address solid-state imaging device, many
pixel transistors are disposed in a two-dimensional matrix to form
a pixel portion. Signal charge corresponding to incident light is
stored line by line or pixel by pixel, and current or voltage
signals in accordance with the stored signal charge are
sequentially read out from the individual pixels by addressing. In
most of the MOS (including CMOS)-type solid-state imaging devices,
unit pixels are accessed line by line and pixel signals in one line
are read out from the pixel portion to the exterior. In some of the
MOS (or CMOS)-type solid-state imaging devices, analog pixel
signals read out from the pixel portion are converted into digital
signals by an AD converter before being output to the exterior
(see, for example, Japanese Unexamined Patent Application
Publication Nos. 2000-152082 and 2002-232291).
[0012] As disclosed in the above-described publications, there are
various methods for performing AD conversion in terms of the
circuit scale, processing speed, and resolution. One of the AD
conversion methods is so-called "single-slope-integrating or
ramp-signal-comparison AD conversion method". In this method, an
analog unit signal is compared with a reference signal which varies
monotonously and used for digital signal conversion, and
simultaneously with this comparison operation, counting is started.
Then, based on the count value when the comparison operation is
finished, a digital signal is obtained. In this type of AD
conversion method, a DA converter is sometimes used for generating
the reference signal.
[0013] There are also various methods for performing DA conversion
in terms of the circuit scale, processing speed, and resolution. In
one of the DA conversion methods, many current source cells
weighted with predetermined current values are used, and among
those current source cells, predetermined current source cells are
selected based on a multi-bit digital input signal, and then,
constant current outputs of the selected current source cells are
added to each other. As a result, an analog current output
corresponding to a digital input signal can be obtained.
[0014] In this type of DA converter, various modes can be employed
in selecting current source cells corresponding to a digital input
signal. One mode is a decoding mode (see Japanese Unexamined Patent
Application Publication No. 5-191290). In the decoding mode, many
current source cells uniformly weighted with a predetermined
current value are used, and a multi-bit digital input signal is
decoded into a decimal number. Then, current source cells are
selected in accordance with the decimal number. Another mode is a
binary mode in which a plurality of current source cells for
outputting currents weighted with two to the power of certain
numbers (or 1/two-to-the-power-of-certain-numbers) are used, and
current source cells in accordance with the bit value of a
multi-bit digital input signal are selected. Another mode is a
composite mode (see Japanese Unexamined Patent Application
Publication No. 11-17545). In the composite mode, a multi-bit
digital input signal is divided into higher bits and lower bits,
and the decoding mode is applied to the higher bits and the binary
mode is applied to the lower bit.
[0015] In the composite mode, in the case of the higher bits, many
current source cells uniformly weighted with a predetermined
current value are prepared, and then, the higher bits of the
multi-bit digital input signal are decoded into a decimal number,
and the decimal number is latched. Then, current source cells are
selected in accordance with the decimal number. In the case of the
lower bits, many current source cells uniformly weighted with
1/two-to-the-power-of-certain-numbers of the current value weighted
for the current source cells corresponding to the higher bits are
prepared, and then, the lower bits of the multi-bit digital input
signal are latched. Then, current source cells are selected in
accordance with the lower bits. Subsequently, by adding the output
currents of the current source cells selected for the higher bits
and the lower bits on the basis of the latched values, an analog
current output corresponding to the digital input signal value can
be obtained. For the decoding of the higher bits, full decoding is
performed.
SUMMARY OF THE INVENTION
[0016] In the known composite mode, however, higher bits subjected
to the full decoding mode and lower bits subjected to the binary
mode are operated independently. Even if the latching timing is the
same between the higher bits and the lower bits, if a digital
signal is input at high speed, it becomes difficult to perform
decoding and latching reliably and speedily to select current
source cells even though the full decoding mode and the binary mode
are operated at the same time. The reason for this is that, in
particular, the full decoding mode is not suitable for high speed,
and as a result, glitch or erroneous coding occurs and it is
difficult to obtain stable analog signals.
[0017] It is thus desirable to provide a DA conversion mechanism
that can solve the problem of the occurrence of glitch or erroneous
coding, for example, a DA conversion mechanism suitable for
generating a reference signal which monotonously varies and which
is used for single-slope integrating AD conversion method.
[0018] In an embodiment of the present invention, as in the
invention disclosed in Japanese Unexamined Patent Application
Publication No. 11-17545, a digital input is first divided into
higher bits and lower bits, and then, the selection of current
source cells is controlled for each of the higher bits and the
lower bits. Accordingly, in a higher-bit current source cell
portion controlled by a higher-bit controller, a plurality of
higher-bit current source cells that are uniformly weighted to
generate an identical constant current are disposed. In a lower-bit
current source cell portion controlled by a lower-bit controller, a
plurality of lower-bit current source cells that generate
bit-weighted constant currents are disposed.
[0019] The lower-bit controller includes a scaler that performs a
scaling operation, i.e., a binary counter operation, based on an
input count clock and that uses clocks scaled down to
1/two-to-the-power-of-certain-numbers obtained by the scaling
operation as a selection control signal to select the lower-bit
current source cells weighted with the corresponding current
values. The higher-bit controller includes the same number of shift
registers as the number of higher-bit current source cells and
uses, as a shift clock, a signal indicating a carry bit or a borrow
bit used in the scaling operation controlled by the lower-bit
controller to sequentially activate shift outputs of the shift
registers, and uses the shift outputs as a selection control signal
to select the higher-bit current source cells.
[0020] The lower-bit controller performs the scaling operation
based on the count clock, and the higher-bit controller performs
the shift operation by using, as a shift clock, a signal indicating
a carry bit or a borrow bit generated while the scaling operation
is performed. Accordingly, the lower-bit controller and the
higher-bit controller perform the operations, not independently,
but cooperatively (synchronously). Then, the higher-bit controller
can reliably select the current source cells corresponding to the
subsequent bit data.
[0021] With this configuration, the occurrence of glitch or
erroneous coding, which would be caused by the independent
operation of higher bits subjected to the decoding mode and lower
bits subjected to the binary mode, can be prevented. As a result,
it is possible to provide a DA conversion device suitable for
single-slope-integrating AD conversion that requires a reference
signal presenting monotonous changing characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic diagram illustrating a CMOS-type
solid-state imaging device according to an embodiment of the
present invention;
[0023] FIG. 2 illustrates functions of a DA conversion circuit of a
reference signal generator used in a solid-state imaging
device;
[0024] FIG. 3 is a timing chart illustrating signal difference
processing, which is a basic operation, in a column AD circuit of
the solid-state imaging device shown in FIG. 1;
[0025] FIG. 4 is a block diagram illustrating a specific
configuration of a reference signal generator;
[0026] FIG. 5 is a timing chart illustrating the overall operation
of a DA converter, mainly a lower-bit controller and a higher-bit
controller;
[0027] FIG. 6 illustrates a basic configuration of each current
source cell disposed in a current source cell portion;
[0028] FIG. 7 illustrates a detailed configuration of a current
source cell portion;
[0029] FIG. 8 illustrates a detailed configuration of a scaling
processor;
[0030] FIG. 9 illustrates a detailed configuration of a shift
register unit;
[0031] FIGS. 10A and 10B respectively illustrate a detailed
configuration and an operation of a glitch suppression
processor;
[0032] FIGS. 11A and 11B illustrate a first example of the layout
of a DA converter;
[0033] FIGS. 12A and 12B illustrate a second example of the layout
of the DA converter;
[0034] FIG. 13 illustrates the principle that the slope of a
reference signal is changed;
[0035] FIG. 14 illustrates an increase in the dynamic range;
and
[0036] FIG. 15 illustrates gamma correction by changing the slope
of a reference signal to increase the dynamic range.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] An embodiment of the present invention is described in
detail below with reference to the accompanying drawings in the
context of a CMOS-type imaging device, which is an example of an
X-Y address solid-state imaging device. A description is given
below, assuming that all pixels used in the CMOS-type imaging
device are NMOS or PMOS pixels.
[0038] The CMOS-type imaging device is an example only, and the
type of device used in the following embodiment is not restricted
to a MOS imaging device, and the following embodiment is applicable
to all physical-quantity-distribution detecting semiconductor
devices including a plurality of unit elements which are disposed
in a line or matrix and which are responsive to electromagnetic
waves, such as light or radiation, input from an external
source.
Configuration of Solid-State Imaging Device
[0039] FIG. 1 is a schematic diagram illustrating a CMOS-type
solid-state imaging device (CMOS-type image sensor) 1, which serves
as a semiconductor device according to an embodiment of the present
invention.
[0040] The solid-state imaging device 1 includes a
two-dimensional-matrix pixel portion 10. In the pixel portion 10, a
plurality of pixels having photodetectors (which serve as a charge
generator) that output signals in accordance with the quantity of
incident light are disposed in rows and columns, i.e., in a matrix
form. A signal output from each pixel serves as a voltage signal.
The solid-state imaging device 1 also includes correlated double
sampling (CDS) processing functions and AD converters, which are
disposed in parallel in the column direction.
[0041] More specifically, a plurality of CDS processing functions
and AD converters are provided substantially parallel with vertical
signal lines 19. When viewed from the top of the solid-state
imaging device 1, the plurality of CDS processing functions and AD
converters may be disposed only at one edge (output side at the
bottom in FIG. 1) of the pixel portion 10, or may be disposed both
at one edge (output side at the bottom in FIG. 1) and at the other
edge (output side at the top in FIG. 1) of the pixel portion 10. If
the CDS processing functions and AD converters are disposed at both
the edges of the pixel portion 10, it is preferable that horizontal
scanners for reading in the row direction (horizontal scanning) are
also disposed at both the edges and are operated independently.
[0042] A typical form of arranging CDS processing functions and AD
converters is a column type in which a plurality of CDS processing
functions and AD converters are provided in a column region
disposed at the output side of the imaging portion 10, one CDS
processing function and one AD converter being provided for each
vertical column, and signals are sequentially read out to the
output side. Alternatively, a CDS processing function and an AD
converter may be assigned to a plurality of adjacent (for example,
two) vertical signal lines (vertical columns) 19, or a CDS
processing function and an AD converter may be assigned to N
vertical signal lines 19 in every N signal lines 19 (N is a
positive integer).
[0043] In any form other than the column type, a plurality of
vertical signal lines 19 (vertical columns) use the common CDS
processing function and the common AD converter. Accordingly, a
changeover circuit (switch) is provided for switching pixel signals
for a plurality of columns output from the pixel portion 10 to be
supplied to a common CDS processing function and a common AD
converter. It may be necessary to provide a memory for storing
output signals depending on the processing of a subsequent
stage.
[0044] In any form other than the column type, by applying one CDS
processing function and one AD converter to a plurality of vertical
signal lines (vertical columns) 19, signal processing can be
performed on pixel signals column by column after the pixel signals
are read from the pixel columns. Accordingly, compared with
performing similar signal processing in each unit pixel, the
configuration of a unit pixel can be simplified, and thus, a
multi-pixel, smaller, and less expensive image sensor can be
implemented.
[0045] Additionally, pixel signals for one row can be
simultaneously processed in a plurality of
column-parallel-structured signal processors (CDS processing
functions and AD converters) disposed. Accordingly, compared with a
case where pixel signals are processed by a single CDS processing
function or a single AD converter in an output circuit or outside
the imaging device, the signal processors can be operated at lower
speed, which is advantageous in terms of the power consumption,
band performance, noise, etc. In other words, if the power
consumption and band performance are the same, the entire imaging
sensor can be operated at higher speed.
[0046] The column type, which can operate at a lower speed, is
advantageous in terms of the power consumption, band performance,
noise, etc. Additionally, the provision of a change-over circuit
(switch) is not necessary. In the following description, it should
be assumed that the column type is used unless otherwise
stated.
[0047] The solid-state imaging device 1 includes, as shown in FIG.
1, the pixel portion (imaging portion) 10 in which a plurality of
unit pixels 3 formed generally in the shape of a square are
disposed in rows and columns (i.e., in a matrix), a drive
controller 7, a column processor 26, a reference signal generator
27 that supplies an AD conversion reference signal to the column
processor 26, and an output circuit 28. The drive controller 7, the
column processor 26, the reference signal generator 27, and the
output circuit 28 are disposed outside the pixel portion 10.
[0048] If necessary, at the stage before or after the column
processor 26, an auto gain control (AGC) circuit having a signal
amplifying function may be provided in the same semiconductor
region as the column circuit 26. If AGC is performed before the
column processor 26, analog amplification is employed, and if AGC
is performed after the column processor 26, digital amplification
is employed. If an n-bit digital signal is directly amplified, the
grayscale may be impaired. Accordingly, it is preferable that
analog amplification is conducted first, and then, the amplified
signal is converted into a digital signal.
[0049] The drive controller 7 includes control circuit functions of
sequentially reading signals from the pixel portion 10. More
specifically, the drive controller 7 includes a horizontal scanning
circuit (column scanning circuit) 12 for controlling column
addressing or column scanning, a vertical scanning circuit (row
scanning circuit) 14 for controlling row addressing or row
scanning, and a communication/timing controller 20 having a
function of generating an internal clock.
[0050] As indicated by the broken lines in the vicinity of the
communication/timing controller 20 in FIG. 1, a clock converter 23,
which is an example of a high-speed clock generator, for generating
pulses having a clock frequency higher than an input clock
frequency may be disposed. The communication/timing controller 20
generates an internal clock based on an input clock (master clock)
CLK0 input through a terminal 5a or a high-speed clock generated in
the clock converter 23.
[0051] The use of a signal based on a high-speed clock generated in
the clock converter 23 achieves fast AD conversion processing. The
use of a high-speed clock also makes it possible to perform motion
extraction or compression that requires fast computation. Parallel
data output from the column processor 26 can be converted into
serial data and is output to the exterior of the imaging device 1
as video data D1. With this configuration, a fast operation can be
implemented with a smaller number of terminals than the number of
bits of a digital signal converted from an analog signal.
[0052] The clock converter 23 has a built-in multiplier circuit for
generating pulses having a clock frequency higher than the input
clock frequency. The clock converter 23 receives a low-speed clock
CLK2 from the communication/timing controller 20, and generates a
clock having a frequency at least twice as high as the low-speed
clock CLK2. If k1 is a factor of the low-speed clock CLK2, a k1
multiplier circuit is provided for the clock converter 23, and
known various circuits can be used as the multiplier circuit.
[0053] In FIG. 1, for simple representation, not all of the rows
and columns are shown. In reality, several tens of to several
thousands unit pixels 3 are disposed in each row or each column to
form the pixel portion 10. Typically, each unit pixel 3 includes a
photodiode, which serves as a photodetector (charge generator), and
an inter-pixel amplifier having an amplifying semiconductor element
(for example, a transistor).
[0054] As the inter-pixel amplifier, a floating diffusion
amplifier, may be used. For example, a floating diffusion amplifier
including four transistors, which are generally used in a CMOS-type
sensor, i.e., a floating diffusion amplifier including a read-out
selection transistor, which is an example of a charge reader
(transfer gate/read-out gate), a reset transistor, which is an
example of a reset gate, a vertical selection transistor, and a
source-follower amplifying transistor, which is an example of a
detector for detecting a potential change of a floating diffusion,
can be used.
[0055] Alternatively, an inter-pixel amplifier, such as that
disclosed in Japanese Patent No. 2708455, including three
transistors, i.e., an amplifying transistor connected to a drain
line (DRN) to amplify a signal voltage corresponding to signal
charge generated by a charge generator, a reset transistor for
resetting the charge generator, and a read-out selection transistor
(transfer gate) which is scanned by a vertical shift register via a
transfer wiring (TRF), may be used.
[0056] The pixel portion 10 includes, not only an effective image
area (effective portion) 10a, but also a reference pixel area
disposed around the effective image area to optically provide a
black color. For example, reference pixels for several lines (for
example, one to ten lines) that optically provide a black color are
disposed above and below the image area 10a, and several to several
tens of reference pixels (for example, 3 to 40 reference pixels)
that optically provide a black color are disposed at the left and
right sides of the effective image area 10a.
[0057] The light-receiving surfaces of the reference pixels are
shielded so that light does not enter the charge generators, such
as photodiodes. Pixel signals from the reference pixels are used as
a reference for a black color of a video signal.
[0058] In the solid-state imaging device 1, the pixel portion 10 is
configured so that color imaging can be implemented. That is, on
the light-receiving surface of each charge generator on which
electromagnetic waves (light in this example) are incident, one of
a plurality of color filters, which are used for capturing color
images, forming a color separation filter is disposed.
[0059] In the example shown in FIG. 1, a basic color filter having
a Bayer pattern is used. That is, in the pixel portion 10, the same
pixel pattern repeats in units of 2.times.2 pixels so that the unit
pixels 3 disposed in a square lattice correspond to three color
filters, i.e., red (R), green (G), and blue (B) color filters.
[0060] For example, first color pixels for sensing a first color
(R) are disposed at the odd rows and odd columns; second color
pixels for sensing a second color (G) are disposed for the odd row
and even columns and even rows and odd columns; and third color
pixels for sensing a third color (B) are disposed at the even rows
and even columns. In this manner, two different color pixels, i.e.,
R/G or G/B pixels, are disposed every other line according to the
checkerboard pattern.
[0061] In such a basic color filter having a Bayer pattern, two
patterns of combinations of two colors, i.e., R/G and G/B,
alternately repeat in the row direction and in the column
direction.
[0062] The unit pixels 3 are connected to the vertical scanning
circuit 14 through row control lines 15 for selecting rows and are
also connected to the column processor 26 through the corresponding
vertical signal lines 19. In the column processor 26, a column AD
circuit 25 is disposed for each vertical column. The row control
lines 15 represent the overall wiring from the vertical scanning
circuit 14 to the unit pixels 3.
[0063] The horizontal scanning circuit 12 serves as a read-out
scanner for reading count values from the column processor 26. The
elements forming the drive controller 7, such as the horizontal
scanning circuit 12 and the vertical scanning circuit 14, are
integrally formed, together with the pixel portion 10, in a
semiconductor region, such as single crystal silicon, by using a
technique similar to a semiconductor integrated circuit
manufacturing technique, and are formed as solid-state imaging
elements, which are an example of a semiconductor system.
[0064] The horizontal scanning circuit 12 and the vertical scanning
circuit 14 include a horizontal decoder 12a and a vertical decoder
14a, respectively, which are discussed below, and start a shifting
(scanning) operation in response to control signals CN2 and CN1,
respectively, supplied from the communication/timing controller 20.
Accordingly, various pulse signals (for example, reset pulse RST,
transfer pulse TRF, DRN control pulse DRN, etc.) for driving the
unit pixels 3 are transmitted through the row control lines 15.
[0065] The communication/timing controller 20 includes a functional
block of a timing generator TG, which is an example of a read-out
address controller, and a functional block of a communication
interface, though they are not shown. The timing generator TG
supplies clocks and pulse signals having predetermined timing
required for the operation of the elements forming the drive
controller 7. The communication interface receives the master clock
CLK0 through the terminal 5a or receives data DATA indicating the
operation mode through a terminal 5b, and outputs data including
information concerning the solid-state imaging device 1.
[0066] For example, the communication/timing controller 20 outputs
a horizontal address signal to the horizontal decoder 12a or a
vertical address signal to the vertical decoder 14a, and the
horizontal decoder 12a or the vertical decoder 14a receives the
corresponding address signal and selects the corresponding column
or row.
[0067] In this case, since the unit pixels 3 are disposed in a
matrix, the reading of pixel signals and pixel data is facilitated
by the following scanning operation. Vertical scanning is first
performed such that analog pixel signals generated by a pixel
signal generator and output via the vertical signal lines 19 are
accessed and read row by row, and then, horizontal scanning is
performed such that pixel signals (digital pixel data in this
example) are accessed and read in the column direction to the
output side. Instead of the above-described scanning operation,
random access may be performed by directly addressing the desired
unit pixel 3 and by reading information only concerning the
required unit pixel 3.
[0068] The communication/timing controller 20 supplies a clock CLK1
having the same frequency as the master clock CLK0 input through
the terminal 5a and also supplies a low-speed clock which is scaled
down to one half the clock CLK1 (i.e., half the frequency the clock
CLK1) or lower speed clocks, to, for example, the horizontal
scanning circuit 12, the vertical scanning circuit 14, and the
column processor 26. Clocks scaled down to one half the master
clock CLK0 or lower are collectively referred to as the "low-speed
clock CLK2".
[0069] The vertical scanning circuit 14 selects rows of the pixel
portion 10 and supplies required pulses to the selected rows. The
vertical scanning circuit 14 includes the vertical decoder 14a for
defining rows of the pixel portion 10 and a vertical drive circuit
14b for supplying pulses to the row control lines 15 corresponding
to the rows defined by the vertical decoder 14a to drive the unit
pixels 3 of the selected rows. The vertical decoder 14a selects,
not only rows from which signals are read, but also, a row used for
an electronic shutter.
[0070] The horizontal scanning circuit 12 sequentially selects the
column AD circuits 25 of the column processor 26 in synchronization
with the low-speed clock CLK2, and supplies the signals of the
selected column AD circuits 25 to a horizontal signal line
(horizontal output lines) 18. The horizontal scanning circuit 12
includes the horizontal decoder 12a for defining columns of the
pixel portion 10 (selecting the corresponding column AD circuits 25
of the column processor 26) and a horizontal drive circuit 12b for
supplying the signals in the column processor 26 to the horizontal
signal line 18 according to the readout addresses defined by the
horizontal decoder 12a. The same number of lines forming the
horizontal signal line 18 as the number n (n is an integer) of bits
handled by the column AD circuits 25 are provided. For example, if
the number of bits is 10, the horizontal signal line 18 includes 10
lines.
[0071] In the solid-state imaging device 1 configured as described
above, pixel signals output from the unit pixels 3 in each vertical
column are supplied to the column AD circuit 25 of the
corresponding column processor 26 via the vertical signal line
19.
[0072] Each column AD circuit 25 of the column processor 26
receives pixel signals for one column and processes them. For
example, each column AD circuit 25 has an analog digital converter
(ADC) circuit for converting an analog signal into, for example,
10-bit digital data, based on the low-speed clock CLK2.
[0073] To perform AD conversion in the column processor 26, analog
signals stored in the individual rows are converted into digital
signals in parallel by using the corresponding column AD circuits
25 disposed in the column direction. In this case, the
single-slope-integrating (or ramp-signal-comparison) AD conversion
technique disclosed in, for example, Japanese Patent No. 2532374 or
literature "Column-kan FPN no nai column-gata AD-henkanki wo
tousaishita CMOS image sensor (CMOS-type Image Sensor with
Column-type ADCs without Inter-column FPN)", The Institute of Image
Information and Television Engineers, IPU2000-57, pp. 79-84. Since
this technique can implement an AD converter with a simple
structure, the circuit scale is not increased even if AD converters
are disposed in parallel.
[0074] The configuration of the ADC circuit is briefly discussed,
though details thereof are described later. A subject analog signal
is converted into a digital signal based on a time from the start
of AD conversion until the voltage of the analog signal coincides
with a reference voltage RAMP. The mechanism for implementing this
is, in principle, as follows. A ramp reference signal RAMP is
supplied to a comparator (voltage comparator), and at the same
time, counting is started with clock signals. Counting is continued
until a pulse signal is obtained as a result of comparing an analog
pixel signal input via the vertical signal line 19 with the
reference voltage RAMP.
[0075] In this case, by modifying the circuit configuration, for a
voltage-mode pixel signal input via the vertical signal line 19,
the difference between the signal level (noise level) immediately
after resetting the pixel and the true signal level Vsig (based on
the quantity of light received) can be calculated in addition to AD
conversion. With this arrangement, noise signal components, such as
fixed pattern noise (FPN) or reset noise, can be eliminated.
[0076] Pixel data digitized in each column AD circuit 25 is
transferred to the horizontal signal line 18 via a horizontal
selection switch (not shown) driven by a horizontal selection
signal supplied from the horizontal scanning circuit 12, and is
then input into the output circuit 28. In this case, the number of
bits of a digital signal is 10, but this is an example only. The
number of bits may be less than 10 (for example, 8) or more than 10
(for example, 14 bits).
[0077] With this configuration, from the pixel portion 10 including
the photodetectors, which serve as charge generators, disposed in a
matrix, pixel signals of the associated vertical columns are
sequentially output row by row. Then, an image corresponding to the
pixel portion 10, i.e., a frame image, can be represented by a set
of pixel signals of the entire pixel portion 10.
Details of Column AD Circuit and Reference Signal Generator
[0078] The reference signal generator 27 includes a DA conversion
circuit (DA converter (DAC)), which has a function of generating a
reference signal for AD conversion, according to the types or
pattern of the colors of color filters forming the color separation
filter of the pixel portion 10. In this embodiment, the reference
signal generator 27 uses a current-output-type DA conversion
circuit, and details thereof are given below.
[0079] Once the pixel portion 10 to be used is determined, the
types or pattern of colors of the color filters forming the color
separation filter are determined, and the types of colors of the
color filters located at certain positions of the two-dimensional
lattice are also uniformly determined. The repeating cycle (same
pattern) of the pixels of the color filters in the row direction
and in the column direction is also uniformly determined by the
pattern of the color filters. In one row to be processed by each
column AD circuit 25, not pixel signals having all colors used in
the color separation filter, but pixel signals having a combination
of a smaller number of predetermined colors determined by the
repeating cycle are contained.
[0080] In this embodiment, by focusing on this characteristic of
the color filters, in an AD conversion circuit including a
comparator circuit and a counter, DA conversion circuits, which are
functional elements for generating an AD conversion reference
signal to be supplied to the comparator circuit and which are an
example of color-dependent reference signal output units, are
formed in the following manner. Instead of providing DA conversion
circuits having the same number of all colors used in the color
separation filter, the same number of DA conversion circuits as the
number of predetermined colors existing in the repeating cycle of a
combination of color filters in one row, which is the unit for
reading pixel signals, are provided. With this arrangement, the
number of DA conversion circuits becomes smaller than the number of
all colors of the color filters existing in the repeating cycles of
the color filters in the two-dimensional matrix. When processing a
row, if x colors exist (x is a positive integer of two or greater)
in that row, it is sufficient if reference signals associated with
the x colors are supplied to the comparator circuits, and it is
therefore sufficient if x DA conversion circuits are provided.
[0081] From the viewpoint of supplying individual reference signals
indicating variation characteristics and initial values of the
individual colors to the comparator circuits, it is necessary to
switch the row to be processed. It is thus preferable that a
switching mechanism for supplying a reference signal corresponding
to a color to be processed in the column direction, which is
orthogonal to the row direction, be provided for each of the x DA
conversion circuits.
[0082] That is, in the direction different from the row direction,
which is the unit for reading pixel signals, i.e., in the vertical
column direction, the same number of color-dependent reference
signal generators as the predetermined number of colors of a
combination of color filters existing in the repeating cycle in the
vertical column direction are provided for the corresponding DA
conversion circuits (reference signal output units). A reference
signal generated by the color-dependent reference signal generator
has a variation characteristic (more specifically, the slope)
corresponding to the color characteristic of the color pixels and
an initial value defined based on black references or circuit
offset components, which are irrelevant to the color
characteristics. A selector is provided for selecting one of the
reference signals generated in the reference signal generators and
for supplying the selected reference signal to the comparator
circuit.
[0083] In this case, if the same color filter exits in the
repeating cycle of the two-dimensional color filters, such as in a
Bayer pattern, the DA conversion circuits (reference signal output
units) may use the single color-dependent reference signal
generator for that same color filter.
[0084] Regardless of the configuration of the DA conversion
circuits, every time a row to be processed is switched, a
combination of predetermined colors in that row is also switched.
In response to this, each DA conversion circuit switches the
variation characteristic (more specifically, the slope) of the
reference signal (analog reference voltage) generated by the DA
conversion circuit in accordance with the characteristics of the
switched color filters, i.e., analog pixel signals, and outputs the
switched variation characteristics. The initial values are set
based on, for example, black references and offset components,
which are irrelevant to the color characteristics.
[0085] With this arrangement, the number of reference voltage
generators (corresponding to the DA conversion circuits in this
embodiment) or the number of wiring patterns extending from the
reference voltage generators can be reduced to be smaller than the
number of color filters forming the color separation filter.
Additionally, selectors (multiplexers) disposed for the individual
vertical columns to selectively output analog reference voltages
(corresponding to reference signals in this embodiment) supplied
from the reference voltage generators, which are necessary in a
case where a reference voltage generator is provided for each color
filter (see Japanese Unexamined Patent Application Publication No.
2000-152082), becomes unnecessary. As a result, the circuit scale
can be reduced. In addition, the number of signal lines for
transmitting the reference signals based on the color pixels to the
input sides of the comparator circuits can be reduced to be smaller
than the number of color components of the color filters.
[0086] Alternatively, when a row to be processed is switched, the
communication/timing controller 20 may set variation
characteristics (slope) corresponding to the color characteristics
of the color pixels and the initial value, such as black references
and circuit offset components, in accordance with a change in a
combination of colors forming the repeating cycle of the color
filters. With this modification, the provision of a color-dependent
reference signal generator for each DA conversion circuit
(reference signal output unit) or the provision of a selector for
selecting a color-dependent reference signal generator becomes
unnecessary.
[0087] That is, in this modification, every time a row to be
processed is switched, variation characteristics (slope) and an
initial value are set in the corresponding DA conversion circuit in
accordance with a change in a combination of colors forming the
repeating cycle of the color filters. Then, it is not necessary to
provide a color-dependent reference signal generator for each DA
conversion circuit or to provide a selector for selecting a
color-dependent reference signal generator in accordance with a row
to be switched. Then, the scale of the overall reference signal
generator 27 can further be reduced. In this case, however, the
processing performed by a control system for the reference signal
generator 27 may become complicated.
[0088] In this embodiment, in the solid-state imaging device 1, a
Bayer basic pattern is used. As stated above, the same pattern
repeats in units of 2.times.2 pixels. Pixel signals are read row by
row and are input into the corresponding column-parallel-structured
column AD circuits 25 via the corresponding vertical signal lines
19. Accordingly, in one row to be processed, pixel signals
including only two colors, i.e., R/G or G/B, are contained. Thus,
in this embodiment, two DA conversion circuits, i.e., a DA
conversion circuit 27a corresponding to the odd columns and a DA
conversion circuit 27b corresponding to the even columns, are
provided.
[0089] Reference signals RAMPa and RAMPb output from the DA
conversion circuits 27a and 27b, respectively, are transmitted to a
voltage comparator unit 252 via common reference signal lines 251a
and 251b (may be collectively referred to as the "common reference
signal line 251"), respectively, which are independent of each
other. The voltage comparator unit 252 includes a plurality of
voltage comparators 252a (for the odd columns) and a plurality of
voltage comparators 252b (for the even columns) connected to the
common reference signal lines 251a and 251b, respectively.
[0090] In this case, the reference signals RAMPa and RAMPb are
substantially directly transmitted to the plurality of voltage
comparators 252a and 252b, respectively, via the independent common
reference signal lines 251a and 251b, respectively. Each of a set
of voltage comparators 252a and 252b corresponds to color filters
having the same color characteristics. Substantially directly
transmitting via the common reference signal lines 251a and 251b
means that selectors, such as multiplexers, are not disposed
between the common reference signal lines 251a and 251b and the
corresponding voltage comparators 252a and 252b. This configuration
is very different from that disclosed in Japanese Unexamined Patent
Application Publication No. 2000-152082. In this configuration, a
reference signal output from each analog reference voltage
generator is transmitted to the input side of a comparator provided
for each vertical column, and a selector (multiplexer) is provided
immediately before the input side of each comparator to selectively
output one of the reference signals from the corresponding analog
reference voltage generator.
[0091] The DA conversion circuits 27a and 27b generate stepped-ramp
saw tooth waves (ramp voltages) from initial values indicated by
control data CN4 (CN4a and CN4b) supplied from the
communication/timing controller 20 in synchronization with count
clocks CKdaca and CKdacb (may be the count clock CK0),
respectively, output from the communication/timing controller 20,
and supplies the generated stepped-ramp saw tooth waves to the
corresponding column AD circuits 25 of the column processor 26 as
the AD conversion reference signals (ADC reference signals) RAMPa
and RAMPb, respectively. It is preferable that a noise suppression
filter be provided, though it is not shown.
[0092] In this embodiment, when performing AD conversion on a
signal component Vsig of a pixel signal Vx located at a
predetermined position by using the voltage comparators 252 and
counters 254, the DA conversion circuit 27a or 27b sets the initial
voltage of the reference signal RAMPa or RAMPb generated by the DA
conversion circuit 27a or 27b, respectively, to a value different
from that when AD conversion is performed on a reset component
.DELTA.V by reflecting the characteristics of the pixel and
variations in the circuit. The DA conversion circuit 27a or 27b
also sets the slope .beta.a or .beta.b, respectively, by
considering the pattern of the color filters so that the variation
characteristics match the pixel characteristics.
[0093] More specifically, the initial values Vas and Vbs of the
reference signals RAMPa and RAMPb for the signal component Vsig are
calculated based on a signal obtained from a plurality of
predetermined black references. The pixels for generating black
references are pixels covered with a light-shielding layer, such as
photodiodes, which serve as photoelectric transducers, forming
charge generators disposed around the color pixels. The positions
and the number of pixels for generating black references and the
type of light-shielding layer are not particularly restricted, and
a known technique may be employed.
[0094] The initial voltage includes variations unique to the DA
conversion circuit 27a or 27b. Generally, the initial voltage Vas
or Vbs is set to be lower than the initial voltage Var or Vbr of
the reference signal RAMPa or RAMPb for the reset component
.DELTA.V by an offset OFFa or OFFb, respectively.
[0095] Even if the initial voltages Var and Vbr of the reference
signal RAMPa or RAMPb for the reset component .DELTA.V are set to
be equal to each other, the initial voltages Vas and Vbs of the
reference signals RAMPa and RAMPb for the signal component Vsig
become different from each other since the offsets OFFa and OFFb
are different.
[0096] The initial voltages Vas and Vbs of the reference signals
RAMPa and RAMPb for the signal component Vsig may contain offsets
other than those contained in pixels for generating black
references.
[0097] The DA conversion circuits 27a and 27b of the reference
signal generator 27 may control the offsets OFFa and OFFb in the
following manner. The function of calculating initial values based
on a signal obtained from reference pixels for generating a
plurality of black references is provided for the
communication/timing controller 20, and the communication/timing
controller 20 outputs control data CN4 representing initial values.
Then, the DA conversion circuits 27a and 27b may control the
offsets OFFa and OFFb based on the initial values. Alternatively,
the DA conversion circuits 27a and 27b may have the function of
calculating the initial values.
[0098] Alternatively, instead of providing the function of
calculating the initial values of reference voltages for the
communication/timing controller 20 or the DA conversion circuits
27a and 27b within the chip forming the imaging device 1, an
external system outside the chip may calculate the initial values
based on a signal obtained from reference pixels for generating
black references, and sends information concerning the calculated
initial values to the communication/timing controller 20 through
the terminal 5b. Then, the communication/timing controller 20 sends
control data CN4 indicating the initial values to the reference
signal generator 27.
[0099] Stepped reference signals output from the reference signal
generator 27, and more specifically, the reference signal RAMPa and
the reference signal RAMPb output from the DA conversion circuits
27a and 27b, respectively, can be generated based on a high-speed
clock generated by the clock converter 23, for example, a
multiplied clock generated by a multiplier circuit. Then, the
stepped reference signals can be changed at higher speed than when
they are generated based on the master clock CLK0 input via the
terminal 5a.
[0100] The control data CN4a and CN4b supplied from the
communication/timing controller 20 to the DA conversion circuits
27a and 27b, respectively, contain information indicating the slope
(the degree by which the voltage is changed, i.e., the amount by
which the voltage is changed with respect to the time) of a ramp
voltage to be used every time a comparison operation is
performed.
[0101] Each column AD circuit 25 includes the voltage comparator
252 and the counter 254 and has an n-bit AD conversion function.
The voltage comparator 252 compares an analog pixel signal obtained
from the unit pixels 3 corresponding to each of the row control
lines 15 (V1, V2, . . . , Vv) through the corresponding vertical
signal line 19 with the reference signal RAMP generated by the DA
conversion circuit 27a or 27b of the reference signal generator 27.
The counter 254 counts time until the voltage comparator 252
finishes the comparison operation, and stores the counting result
therein.
[0102] The communication/timing controller 20 has a function of
controlling the counting mode to be used in the counter 254
according to whether the reset component .DELTA.V or the signal
component Vsig of the pixel signal is used for a comparison
operation in the voltage comparator 252. A mode control signal CN5
for designating whether the counter 254 is operated in a
down-counting mode or an up-counting mode is input from the
communication/timing generator 20 into the counter 254 of each
column AD circuit 25.
[0103] The stepped-ramp reference signal RAMP generated in the
reference signal generator 27 is input into one input terminal RAMP
of each of the voltage comparators 252, and the other input
terminals of the voltage comparators 252 are connected to the
corresponding vertical signal lines 19 and receive the pixel signal
voltages from the pixel portion 10. The output signals from the
voltage comparators 252 are supplied to the corresponding counters
254.
[0104] The count clock CK0 is input from the communication/timing
controller 20 to the clock terminals CK of the counters 254.
[0105] Although the configuration of the counter 254 is not shown,
the counter 254 can be implemented by changing the wiring
arrangement of a data storage unit formed of latches into a
synchronous counter, and the internal counting is performed by
receiving a single count clock CK0. As in the stepped voltage
waveform, the count clock CK0 can also be generated based on a
high-speed clock (for example, a multiplied clock) from the clock
converter 23 so that it is operated faster than the master clock
CLK0 input via the terminal 5a.
[0106] The n-bit counter 254 can be implemented by a combination of
n latches, and the circuit scale can be reduced to one half the
circuit scale of a data storage unit formed of two blocks, each
block having n latches. Additionally, the provision of a counter
for each column is not necessary, and the overall configuration is
much smaller.
[0107] Although details of the configuration of the counters 254
are given later, the main feature of the counters 254 is as
follows. By the use of the common up/down counter (U/D CNT)
regardless of the counting mode, counting can be performed by
switching alternately between the down-counting operation and the
up-counting operation. As the counter 254, a synchronous counter
that outputs counter output values in synchronization with the
count clock CK0 is used.
[0108] In the case of the synchronous counter, the operations of
all flip-flops (counter basic elements) are restricted by the count
clock CK0. Thus, if a higher frequency operation is required, the
use of an asynchronous counter suitable for the fast operation is
preferable since the operation limit frequency is determined only
by the limit frequency of the first flip-flop (counter basic
element).
[0109] A control pulse is input from the horizontal scanning
circuit 12 into the counter 254 via a control line 12c. The counter
254 has a latch function of latching a counting result, and latches
the counter output value until the control pulse is received via
the control line 12c.
[0110] The column AD circuit 25 configured as described above is
disposed for each of the vertical signal lines 19 (H1, H2, . . . ,
and Hh) to form the column processor 26, which is a column-parallel
structured ADC block.
[0111] The output sides of the column AD circuits 25 are connected
to the horizontal signal line 18. As described above, the
horizontal signal line 18 includes the same n number of signal
lines as the n-bit column AD circuit 25, and the signal lines are
connected to the output circuit 28 via n sensing circuits (not
shown) corresponding to output lines.
[0112] With this configuration, during the pixel signal read-out
period corresponding to a horizontal blanking period, the column AD
circuit 25 performs a counting operation and outputs the counting
result at a predetermined time. More specifically, the voltage
comparator 252 first compares a ramp waveform voltage output from
the reference signal generator 27 with a pixel signal voltage input
via the vertical signal line 19, and when the two voltages become
equal to each other, the comparator output of the voltage
comparator 252 is inverted (in this embodiment, from the H level to
the L level).
[0113] The counter 254 has already started the counting operation
in the down-counting mode or the up-counting mode in
synchronization with the ramp waveform voltage input from the
reference signal generator 27, and when receiving information
indicating that the comparator output is inverted, the counter 254
stops the counting operation and latches (stores) the count value
as pixel data. Then, AD conversion is completed.
[0114] Thereafter, the counter 254 sequentially outputs the stored
pixel data to the outside the column processor 26 or the outside
the chip having the pixel portion 10 via an output terminal 5c
based on a shift operation of horizontal selection signals CH(i)
input from the horizontal scanning circuit 12 via the control line
12c at a predetermined time.
[0115] Other various signal processing circuits may be contained in
the elements forming the solid-state imaging device 1, though they
are not shown since they are not directly related to this
embodiment.
Functions of Reference Signal Generator
[0116] FIG. 2 illustrates the functions of the DA conversion
circuits (DACs) 27a and 27b of the reference signal generator 27
used in the solid-state imaging device 1.
[0117] The DA conversion circuit 27a or 27b receives a count clock
CKdaca or CKdacb, respectively, for DA conversion from the
communication/timing controller 20, generates a linearly-decreasing
stepped-ramp saw tooth wave (ramp waveform) in synchronization with
the count clock CKdaca or CKdacb, respectively, and supplies the
generated saw tooth wave as an AD conversion reference voltage (ADC
reference signal) to the voltage comparators 252 of the individual
column AD circuits 25.
[0118] The DA conversion circuit 27a or 27b first sets an initial
voltage based on information indicating the initial value of a ramp
voltage, which is used every time a comparison operation is
performed, contained in the control data CN4a or CN4b,
respectively. The DA conversion circuit 27a or 27b also sets a
voltage change .DELTA.RAMP per clock pulse based on information
indicating the slope (degree of change) of the ramp voltage, and
changes each count value per unit time (count clock CKdac). In
actuality, it is sufficient if the maximum voltage of the maximum
count number (for example, 1024 corresponding to 10 bits) of the
count clock CKdac is set. Any desired circuit configuration to set
initial voltages can be formed.
[0119] With this configuration, the DA conversion circuit 27a or
27b reduces the voltage (for example, 3 V) indicating the initial
value contained in the control data CN4 by .DELTA.RAMP per clock
pulse of the count clock CKdaca or CKdacb.
[0120] When setting the factor for a pixel signal (more
specifically, a true signal component) output from the unit pixel
3, the communication/timing controller 20 supplies a count clock
CKdacm scaled down to 1/m the reference cycle of the count clock
CKdac1 having a factor 1 to the DA conversion circuit 27a. The DA
conversion circuit 27a reduces the voltage by .DELTA.RAMP per clock
pulse of each count clock CKdacm from a voltage (for example, 3 V)
indicating the initial value contained in the control data CN4.
[0121] Accordingly, the slope of the reference signals RAMPa and
RAMPb supplied to the voltage comparators 252 is increased by the
factor of 1/m compared with when the reference signals RAMPa and
RAMPb are generated with the count clock CKdac1 (=CK0). In the
counter 254, the count value is increased by the factor of m for
the same pixel voltage. That is, the factor m can be set.
[0122] That is, by adjusting the cycle of the count clock CKdaca or
CKdacb, the slope of the reference signal RAMPa or RAMPb can be
changed. For example, if a clock scaled down to 1/m the reference
clock is used, the slope results in 1/m. If the count clock CK0 in
the counter 254 is the same, the count value is increased by the
factor of m for the same pixel voltage. That is, changing the slope
of the reference signal RAMPa or RAMPb can adjust the factor for
difference processing, which is described below.
[0123] FIG. 2 shows that, as the slope of the reference signal
RAMPa or RAMPb becomes larger, the factor applied to the amount of
information stored in the unit pixels 3 becomes smaller, and
conversely, as the slope becomes smaller, the factor becomes
larger. For example, by supplying the count clock CKdac2 scaled
down to 1/2 the reference cycle of the count clock CKdac1, the
factor can be set to be 2, and by supplying the count clock CKdac4
scaled down to 1/4, the factor can be set to be 4. By supplying the
count clock CKdacnm scaled down to n/m, the factor can be set to be
m/n.
[0124] By adjusting the slope, the analog gain when AD conversion
is performed can be controlled. That is, adjusting the factor can
change the slope of a reference voltage, and changing the slope of
a reference voltage can adjust the amplitude of the reference
signal per unit time. Accordingly, the gain for a pixel signal to
be compared can be adjusted. By dynamically changing the slope
during DA conversion (setting the frequency of a clock higher than
CK0 by progressively increasing the slope thereof), AD conversion
by reflecting gamma (.gamma.) correction on an analog signal can be
conducted.
[0125] In this manner, the factor can be set simply and precisely
by adjusting the cycle of the count clock CKdacnm supplied to the
reference signal generator 27 while changing (reducing in this
embodiment) a voltage by .DELTA.RAMP per clock pulse of each count
clock CKdacm. The sign (+/-) of the factor can be specified by
adjusting the counting operation mode for signal components Vsig of
a pixel signal.
[0126] The above-described technique for setting the factor by
using the slope of the reference signal RAMPa or RAMPb is an
example only, and another technique can be used for setting the
factor. For example, if the cycles of the count clocks CKdaca and
CKdacb supplied to the reference signal generator 27 are fixed, and
if the counter output value is x and the slope of the ramp voltage
contained in the control data CN4 is 1, the potential calculated by
the equation y=.alpha. (initial value)-.beta.*x is output. In this
manner, according to information indicating the slope (rate of
change) of a ramp voltage contained in the control data CN4, a
change in the voltage .DELTA.RAMP per count pulse of each count
clock CKdac can be adjusted. As stated above, the slope .beta. of
the ramp voltage can be adjusted by changing the number of pulses
of a clock, or may be adjusted by changing the current of a unit
current source.
[0127] The technique for setting .alpha. (initial value) providing
an offset or .beta. (factor) providing the slope can be determined
in accordance with a circuit configuration for generating a ramp
waveform that is changed per clock pulse of each of the count
clocks CKdaca and CKdacb. For example, if the circuit for
generating a ramp waveform is formed of a set of constant current
sources and a selection circuit for selecting one or more constant
current sources, .alpha. and .beta. can be set by adjusting the
current flowing in the selected constant current source.
[0128] No matter which technique is used for generating a reference
signal, the generated reference signal contains a slope based on
the color characteristics of color pixels and also contains an
initial value based on non-color characteristics, such as black
references or circuit offset components. Then, by using such a
reference signal having suitable color characteristics and
non-color characteristics, AD conversion can be performed.
Operation of Solid-State Imaging Device
[0129] FIG. 3 is a timing chart illustrating signal difference
processing, which is a basic operation, in the column AD circuits
25 of the solid-state imaging device 1 shown in FIG. 1.
[0130] The mechanism for converting an analog pixel signal sensed
in each pixel unit 3 of the pixel portion 10 into a digital signal
is, for example, as follows. The time when the ramp-waveform
reference signal RAMP which is progressively reduced with a
predetermined slope becomes equal to the voltage of the reference
components or the signal components of a pixel signal of the unit
pixel 3 is searched. Then, the time from when the reference signal
RAMP used for a comparison operation is generated until when an
electric signal corresponding to the reference components or the
signal components of the pixel signal coincides with the reference
signal is counted with count clocks. Then, the count value
associated with the level of the reference components or the signal
components can be obtained.
[0131] For the pixel signal output from the vertical signal line
19, time-sequentially, the signal components Vsig appear after the
reset components .DELTA.V, i.e., the reference components,
containing noise of the pixel signal. If the first operation is
performed on the reference components (reset components .DELTA.V),
the second operation is performed on the signal obtained by adding
the signal components Vsig to the reference components (reset
components .DELTA.V). This operation is specifically discussed
below.
[0132] For the first read-out operation, the communication/timing
controller 20 sets the mode control signal CN5 to the low level so
that the counter 254 is set to be in the down-counting mode, and
also sets the reset control signal CN6 to be active (in this
embodiment, the high level) for a predetermined period so that the
count value of the counter 254 is reset to the initial value 0
(t9). After the first read-out operation for reading the pixel
signals from a selected row V.alpha. to the corresponding vertical
signal line 19 (H0, H1, . . . , Hh) is stabilized, the
communication/timing controller 20 supplies the control data CN4a
or CN4b for generating the reference signal RAMPa or RAMPb,
respectively, to the reference signal generator 27.
[0133] Upon receiving the control data CN4a, the DA conversion
circuit 27a of the reference signal generator 27 generates and
outputs the reference signal RAMPa, as a comparison voltage, having
a stepped waveform (RAMP waveform) which has a slope .beta.a based
on the color pixel characteristic of one of two sets of colors
(i.e., R or G in the odd-numbered columns) in the row V.alpha. and
which is changed in the shape of a saw tooth wave (RAMP) over time
to one input terminal RAMP of the voltage comparator 252 of the
column AD circuit 25 corresponding to an odd column.
[0134] Similarly, upon receiving the control data CN4b, the DA
conversion circuit 27b of the reference signal generator 27
generates and outputs the reference signal RAMPb, as a comparison
voltage, having a stepped waveform (RAMP waveform) which has a
slope .beta.b based on the color pixel characteristic of the other
set of colors (i.e., G or B in the even-numbered columns) in the
row V.alpha. and which is changed in the shape of a saw tooth wave
(RAMP) over time to one input terminal RAMP of the voltage
comparator 252 of the column AD circuit corresponding to an even
column.
[0135] The voltage comparator 252 compares the pixel signal voltage
of the selected vertical signal line 19 (V.alpha.) supplied from
the pixel portion 10 with the RAMP waveform comparison voltage.
[0136] Simultaneously with the input of the reference signal RAMPa
or RAMPb into the input terminal RAMP of the voltage comparator
252, to measure the comparison time in the voltage comparator 252
by the counter 254, the count clock CK0 is input from the
communication/timing controller 20 into the clock terminal of the
counter 254 in synchronization with the ramp waveform voltage
output from the reference signal generator 27 (t10), and
down-counting is started from the initial value 0 as the first
counting operation. That is, the counting operation is started in
the negative direction.
[0137] The voltage comparator 252 compares the pixel signal voltage
Vx input via the vertical signal line 19 with the ramp reference
signal RAMP supplied from the reference signal generator 27, and
when the two voltages become equal to each other, the voltage
comparator 252 inverts the comparator output from the H level to
the L level (t12). That is, upon comparing the voltage signal
corresponding to the reset components Vrst with the reference
signal RAMP, the voltage comparator 252 generates an active-low (L)
pulse signal after the lapse of the time corresponding to the level
of the reset components Vrst, and supplies the generated pulse
signal to the counter 254.
[0138] Upon receiving this result, the counter 254 stops the
counting operation almost at the same time as the inversion of the
comparator output, and latches (retains/stores) the corresponding
count value as the pixel data. Then, AD conversion is completed
(t12). That is, the counter 254 starts down-counting simultaneously
with the generation of the ramp reference signal RAMP to be
supplied to the voltage comparator 252, and continues counting with
the clock CK0 until the active-low (L) pulse signal is received as
a result of comparison processing. Then, the count value
corresponding to the level of the reset components Vrst is
obtained.
[0139] After the lapse of a predetermined down-counting period
(t14), the communication/timing controller 20 stops supplying the
control data CN4 to the voltage comparator 252 and supplying the
count clock CK0 to the counter 254. Then, the voltage comparator
252 stops generating the ramp reference signal RAMP.
[0140] In this first read-out operation, since the counting
operation is performed by detecting the reset level Vrst of the
pixel signal voltage Vx by the voltage comparator 252, the reset
components .DELTA.V of the unit pixels 3 are read.
[0141] The reset components .DELTA.V contain offset noise which
varies depending on the unit pixel 3. Generally, however, a
variation in the reset components .DELTA.V is small, and the reset
level Vrst is generally common for all the pixels. Accordingly, the
output value of the reset components .DELTA.V of the pixel signal
voltage Vx of the selected vertical signal line 19 is generally
known.
[0142] Accordingly, in the first read-out operation for the reset
components .DELTA.V, by adjusting the RAMP voltage, the
down-counting period (comparison period from t10 to t14) can be
reduced. In this embodiment, a comparison operation is performed
for the reset components .DELTA.V by setting the maximum period for
the comparison operation to be a 7-bit counting period (128 clock
pulses).
[0143] Subsequently, in the second read-out operation, in addition
to the reset components .DELTA.V, the electric signal components
Vsig for each unit pixel 3 in accordance with the quantity of
incident light are read, and an operation similar to the first
read-out operation is performed. More specifically, the
communication/timing controller 20 first changes the mode control
signal CN5 to the high level so that the counter 254 is set to be
in the up-counting mode (t18). Then, after the second read-out
operation for reading the signal components Vsig from the unit
pixels 3 of the selected row V.alpha. to the vertical signal line
19 (H1, H2, . . . , Hh) is stabilized, the communication/timing
controller 20 supplies the control data CN4a (containing offset
OFFa and slope .beta.a) for generating the reference signal RAMPa
to the DA conversion circuit 27a and the control data CN4b
(containing offset OFFb and slope .beta.b) for generating the
reference signal RAMPb to the DA conversion circuit 27b to perform
AD conversion for the signal components Vsig.
[0144] Upon receiving the control data CN4a, the DA conversion
circuit 27a of the reference signal generator 27 generates and
outputs a stepped waveform (RAMP waveform), as a comparison
voltage, which has the slope .beta.a based on the color pixel
characteristic of one of the two sets of colors (R or G in the
odd-numbered columns) in the row V.alpha. and changes in the shape
of a saw tooth (RAMP shape) over time and which is lower than the
initial value Var for the reset components .DELTA.V by the offset
OFFa to one input terminal RAMP of the voltage comparator 252
corresponding to an odd-numbered column.
[0145] Similarly, upon receiving the control data CN4b, the DA
conversion circuit 27b of the reference signal generator 27
generates and outputs a stepped waveform (RAMP waveform), as the
comparison voltage, which has the slope .beta.b based on the color
pixel characteristic of the other set of colors (G or B in the
even-numbered columns) in the row V.alpha. and changes in the shape
of a saw tooth (RAMP shape) over time and which is lower than the
initial value Vbr for the reset components .DELTA.V by the offset
OFFb to one input terminal RAMP of the voltage comparator 252
corresponding to an even-numbered column.
[0146] The voltage comparator 252 compares the pixel signal voltage
of the selected vertical signal line 19 (Vx) supplied from the
pixel portion 10 with the RAMP-waveform comparison voltage.
[0147] As stated above, the initial values of the reference
voltages are calculated based on a signal obtained from pixels for
generating a plurality of black references. The initial values of
the reference signals RAMPa and RAMPb generated by the DA
conversion circuits 27a and 27b, respectively, are difference since
they contain unique variation components. The initial voltages of
the reference signals RAMPa and RAMPb for the signal components
Vsig may contain offsets other than those contained in pixels for
generating black references.
[0148] Simultaneously with the input of the reference signal RAMPa
or RAMPb into the input terminal RAMP of the voltage comparator
252, to measure the comparison time in the voltage comparator 252
by the counter 254, the count clock CK0 is input from the
communication/timing controller 20 to the clock terminal of the
counter 254 in synchronization with the ramp waveform supplied from
the reference signal generator 27 (t20), and, in contrast to the
first counting operation, in the second counting operation, the
counter 254 starts up-counting from the count value corresponding
to the reset components .DELTA.V of the unit pixels 3 obtained in
the first read-out operation. That is, the counting operation is
started in the positive direction.
[0149] The voltage comparator 252 compares the pixel signal voltage
Vx input via the vertical signal line 19 with the ramp reference
signal RAMP supplied from the reference signal generator 27, and
when the two voltages become equal to each other, the voltage
comparator 252 inverts the comparator output from the H level to
the L level (t22). That is, upon comparing the voltage signal
corresponding to the signal components Vsig with the reference
signal RAMP, the voltage comparator 252 generates the active-low
(L) pulse signal after the lapse of the time corresponding to the
level of the signal components Vsig, and supplies the generated
pulse signal to the counter 254.
[0150] Upon receiving this result, the counter 254 stops the
counting operation almost at the same time as the inversion of the
comparator output, and latches (retains/stores) the corresponding
count value as the pixel data. Then, AD conversion is completed
(t22). That is, simultaneously with the generation of the ramp
reference signal RAMP to be supplied to the voltage comparator 252,
the counter 254 starts up-counting, and continues counting with the
clock CK0 until the active-low (L) pulse signal is received as a
result of comparison processing. Then, the count value
corresponding to the level of the signal components Vsig can be
obtained.
[0151] After the lapse of a predetermined up-counting period (t24),
the communication/timing controller 20 stops supplying the control
data CN4 to the voltage comparator 252 and supplying the count
clock CK0 to the counter 254. Then, the voltage comparator 252
stops generating the ramp reference signal RAMP.
[0152] In the second read-out operation, since the counting
operation is performed by detecting the signal components Vsig of
the pixel signal voltage Vx by the voltage comparator 252, the
signal components Vsig of the unit pixels 3 are read.
[0153] In this embodiment, as the counting operation by the counter
254, down-counting is performed during the first read-out
operation, and then, up-counting is performed during the second
read-out operation. Accordingly, the subtraction expressed by
equation (1) is executed automatically in the counter 254, and the
count value as a result of the subtraction is retained in the
counter 254.
(Count value in the second comparison period)-(count value in the
first comparison period). (1)
[0154] In this case, equation (1) can be modified into equation
(2), and as a result, the count value retained in the counter 254
corresponds to the signal components Vsig.
( Count value in the second comparison period ) - ( count value in
the first comparison period ) = ( signal components Vsig 1 + reset
components .DELTA. V + offset components in the column AD circuit
25 - ( black reference components ) ) - ( reset components .DELTA.
V + offset components in the column AD circuit 25 ) = ( signal
components Vsig ) - ( black reference components ) ( 2 )
##EQU00001##
[0155] That is, as stated above, according to the difference
processing in the counter 254 by the two read-out operations and
two counting operations, such as down-counting during the first
read-out operation and up-counting during the second read-out
operation, the reset components .DELTA.V containing noise which
varies depending on the unit pixel 3 and the offset components
depending on the column AD circuit 25 can be eliminated, and only a
digital signal obtained by adding black reference components to the
signal components Vsig in accordance with the quantity of incident
light in each pixel unit 3 can be extracted with a simple
configuration. In this case, circuit variations and reset noise can
also be eliminated.
[0156] Accordingly, the column AD circuit 25 in this embodiment
serves as, not only a digital converter for converting an analog
pixel signal into digital pixel data, but also as a CDS processing
function.
[0157] Since the pixel data represented by the count value obtained
in equation (2) is a positive signal voltage, it is not necessary
to perform complementary computation. Thus, the compatibility with
known systems is high.
[0158] During the second read-out operation, the electric signal
components Vsig based on the quantity of incident light are read.
Accordingly, since the level of the quantity of light is determined
in a wide range, it is necessary to set the up-counting period
(comparison period from t20 to t24) to be long so that the ramp
voltage to be supplied to the voltage comparator 252 can be sharply
changed.
[0159] Accordingly, in this embodiment, the comparison operation is
performed for the signal components Vsig by setting the maximum
period for the comparison operation to be a 10-bit counting period
(1024 clock pulses). That is, the maximum period for the comparison
operation for the reset components .DELTA.V (reference components)
is set to be shorter than the maximum period for the comparison
operation for the signal components Vsig. Instead of setting the
maximum period (i.e., the maximum value of the AD conversion
period) for the comparison operation for the reset components
.DELTA.V (reference components) to be the same as that for the
signal components Vsig, the maximum period for the comparison
operation for the reset components .DELTA.V (reference components)
is set to be shorter than that for the signal components Vsig,
thereby decreasing the total AD conversion period of the two
operations.
[0160] In this case, the number of comparison bits is different for
the first operation and the second operation. Then, the control
data CN4a and CN4b are supplied from the communication/timing
controller 20 to the reference signal generator 27, and based on
the control data CN4a and CN4b, ramp voltages are generated in the
DA conversion circuits 27a and 27b so that the slopes of the ramp
voltages, i.e., the rates of changes, in the reference signals
RAMPa and RMAPb, become the same between the first operation and
the second operation. It is easy to set the slopes of the ramp
voltages to be the same for the first operation and the second
operation since the ramp voltages are generated by digital control.
Accordingly, the precision of the AD conversion for the first
operation can be equal to that for the second operation, thereby
making it possible to obtain a correct subtraction result expressed
by equation (1) by the up/down counter.
[0161] At a predetermined time (t28) after the completion of the
second counting operation, the communication/timing controller 20
instructs the horizontal scanning circuit 12 to read out the pixel
data. In response to this instruction, the horizontal scanning
circuit 12 sequentially shifts the horizontal selection signals
CH(i) to be supplied to the counters 254 via the control lines
12c.
[0162] With this operation, the count values expressed by equation
(2) stored/retained in the counters 254, i.e., the pixel data
represented by n-bit digital data, are sequentially output to the
outside the column processor 26 or the outside the chip having the
pixel portion 10 from the output terminal 5c via the n lines
forming the horizontal signal line 18. Subsequently, the operation
is similarly repeated row by row, thereby obtaining the video data
D1 representing a two-dimensional image.
[0163] As described above, according to the solid-state imaging
device 1, the counting operation is performed twice by using the
up/down counter and by switching the counting operation mode
thereof. Additionally, in association with the unit pixels 3
disposed in a matrix, the column AD circuits 25 are disposed in a
column-parallel structure.
[0164] Instead of providing DA conversion circuits, which are
functional elements for generating AD conversion reference signals
to be supplied to the comparators 252, having the same number of
all colors used in color filters forming the color separation
filter, the same number of DA conversion circuits as the number of
predetermined colors existing in the repeating cycle of a
combination of color filters, which is determined by the types or
pattern of colors, are provided. Additionally, when a row to be
processed is switched, the combination of predetermined colors is
also changed. In response to this change, the characteristic (more
specifically, the slope) or the initial value of a reference signal
(analog reference voltage) generated by the DA conversion circuit
is switched in accordance with the color filters, i.e., the
characteristics of the analog pixel signal.
[0165] With this arrangement, the number of DA conversion circuits,
which serve as reference voltage generators, or the number of
wiring patterns extending from the DA conversion circuits can be
reduced to be smaller than the number of color filters forming the
color separation filter. Additionally, multiplexers for selectively
outputting analog reference voltages (reference signals), which are
necessary in a case where a reference voltage generator is provided
for each color filter, becomes unnecessary. As a result, the
circuit scale can be significantly reduced.
[0166] In response to a change in the combination of predetermined
colors existing in a row to be processed, the characteristic (more
specifically, the slope) of a reference signal generated by the DA
conversion circuit is also changed. Accordingly, different
reference voltages can be generated in accordance with the
characteristics of the color pixels forming the pixel portion 10,
and then, comparison processing can be performed by using the
generated reference voltages. Thus, when converting an analog pixel
signal output from unit pixels into a digital signal, the slope of
the reference signal can be adjusted in accordance with each color
so that the characteristic of each color can be precisely
controlled.
[0167] Additionally, the initial value of a reference signal
generated by the DA conversion circuit is changed in accordance
with variation components generated in the DA conversion circuit
and black reference components. Thus, circuit variations can be
corrected, and also, analog signals adjusted by black reference
components can be converted into digital signals with a simple
structure.
[0168] The result of the subtraction of the reference components
(reset components) from the signal components can be directly
obtained for every vertical column as the result of the second
counting operation. A memory device for storing the counting result
of each of the reference components and the signal components can
be implemented by a latch function provided for each of the
counters 254. This eliminates the need to provide dedicated memory
devices for storing AD converted data separately from the counters
254.
[0169] Additionally, a special subtractor for calculating the
difference between the reference components and the signal
components is not necessary. Accordingly, the circuit scale or
circuit area can be reduced compared with that of the related art,
and noise can be eliminated and the current consumption or power
consumption can be suppressed.
[0170] Since each column AD circuit (AD converter) is formed by a
comparator and a counter, a counting operation can be controlled by
a single count clock for operating the counter regardless of the
number of bits and by a control line for switching the counting
mode. This eliminates the need to provide signal lines for sending
count values of the counters to the memory devices, which are
necessary in the configuration of the related art. As a result,
noise and power consumption can be suppressed.
[0171] That is, in the solid-state imaging device 1 having AD
converters mounted on the same chip, a pair of the voltage
comparator 252 and the counter 254 forms the column AD circuit 25
as an AD converter, and also, the counter 254 is operated as a
combination of a down-counting operation and an up-counting
operation, and the difference between reference components (reset
component in this embodiment) and signal components of a signal to
be processed is calculated as a digital signal. Thus, the circuit
scale or circuit area, the power consumption, or the number of
interface wiring patterns with other functions can be reduced, and
noise or current consumption caused by the provision of interface
wiring patterns can be suppressed.
[0172] An n-bit data storage unit for storing counting results
retained in each counter 254 may be disposed after the counter 254,
though it is not shown. A control pulse is input into the data
storage unit from the horizontal scanning circuit 12 via the
control line 12c. The data storage unit stores the count values
read from the counter 254 until an instruction represented by a
control pulse is given via the control line 12c. The horizontal
scanning circuit 12 serves as a reader scanner for reading a count
value stored in each data storage unit while the voltage comparator
252 and the counter 254 are executing corresponding processing.
With this configuration, pipeline processing can be
implemented.
[0173] Before the operation of the counter 254 (t6), the counting
result obtained by the processing on the previous row Hx-1 is
transferred to the data storage unit based on a memory transfer
instruction pulse CN8 from the communication/timing controller
20.
[0174] According to the operation shown in FIG. 3, it is difficult
to output pixel data to the outside the column processor 26 before
completing the second read-out operation, i.e., AD conversion, on a
pixel signal to be processed. Accordingly, the read-out operation
is restricted. In contrast, if a data storage unit is disposed
after the counter 254, the count value indicating the previous
subtraction processing can be transferred to the data storage unit
before performing the first read-out operation (AD conversion
processing) on a pixel signal to be processed. Accordingly, the
read-out operation is not restricted.
[0175] Thus, with this configuration, a counting operation, i.e.,
AD conversion, by the counter 254 and a reading operation for
reading out signals to the horizontal signal line 18 can be
independently controlled, and thus, pipeline processing for
independently and simultaneously performing AD conversion and
reading signals to the outside the imaging device 1 via the
horizontal signal line 18 can be implemented.
Specific Example of Configuration of Reference Signal Generator
[0176] FIG. 4 is a block diagram illustrating the specific
configuration of the reference signal generator 27. As discussed
above, a current-output-type DA conversion circuit is used as the
basic configuration of the reference signal generator 27. The
current-output-type DA conversion circuit is a current source cell
matrix including cell arrays disposed in a matrix.
[0177] As this type of DA conversion circuit, the following DA
conversion circuit is known. By using a plurality of current source
cells weighted with a predetermined current value to generate the
same constant current, current source cells in accordance with the
data value of a multi-bit digital input signal are selected from
the plurality of current source cells, and then, the constant
current outputs of the selected current source cells are added. As
a result, an analog current output in accordance with the digital
input signal can be obtained.
[0178] To select current source cells, various modes, typical
examples of which are the decoding mode, binary mode, a composite
mode having a combination of the decoding mode and the binary mode,
can be employed. Among others, the following mode is widely known
in which a multi-bit digital input signal is divided into higher
bits and lower bits and the higher bits and the lower bits are then
converted into an analog signal (see Japanese Unexamined Patent
Application Publication No. 11-17545).
[0179] In the composite mode, the higher bits are decoded into a
decimal number, and the lower bits are divided into binary bits.
The decimal number decoded from the higher bits is latched, and
then, current source cells are selected in accordance with the
decimal number. Accordingly, if a digital signal is input at high
speed, it becomes difficult to simultaneously operate a device used
in the decoding mode and a device used in the binary mode and to
perform decoding and latching reliably and speedily to select
current source cells. As a result, glitch or erroneous coding
occurs and it is difficult to obtain stable analog signals.
[0180] Accordingly, in this embodiment, a mechanism for solving
this problem is employed.
[0181] A DA converter 300 forming the reference signal generator 27
(DA conversion circuits 27a and 27b) includes, as shown in FIG. 4,
a DAC controller 310 that controls the overall DA converter 300 and
serves as a constant current source selection controller, a current
source cell unit 350 including a plurality of constant current, and
a voltage amplitude controller 360 for controlling the amplitude of
a reference signal, which serves as a current setting unit for
setting the operation currents of the current source cells in the
current source cell unit 350.
[0182] The DAC controller 310 includes a block controller 320 for
controlling the operation of the elements forming the DAC
controller 310, a lower-bit controller 330 disposed at an input
stage of an input digital signal Din to control the processing of
lower bits, and a higher-bit controller 340 disposed at an input
stage of the input digital signal Din to control the processing of
higher bits.
[0183] The current source cell unit 350 includes a lower-bit
current source cell portion 352 having a plurality of lower-bit
current source cells 353 for outputting predetermined weighted
currents and a higher-bit current source cell portion 354 having
higher current source cells 355 disposed in a matrix to output an
identical predetermined weighted current.
[0184] In the lower-bit current source cell portion 352, the j
lower-bit current source cells 353 assigned to the j lower bits of
the digital input signal are disposed in parallel. The output
terminals of the lower-bit current source cells 353 are connected
to a selection output line 396 for combining the output currents of
the lower-bit current source cells 353. The selection output line
396 is connected to a DAC output terminal DACout.
[0185] The weights applied to the currents of the lower-bit current
source cells 353 of the lower-bit current source cell portion 352
are 1/2, 1/4, . . . , 1/2.sup.j of the current of the higher-bit
current source cells 355 of the higher-bit current source cell
portion 354. The lower-bit current source cells 353 are
individually selected by the lower-bit controller 330 in accordance
with the logical levels (1 or 0) of the j lower bits of the input
digital signal Din. The output currents of the lower-bit current
source cells 353 selected by the lower-bit controller 330 on the
basis of the j lower bits of the input digital signal Din are added
by a reference resistor 398 connected at one end to a reference
voltage Vref via the DAC output terminal DACout.
[0186] In the higher-bit current source cell portion 354, at least
2.sup.i (more preferably, 2.sup.i+y (y is a certain value))
higher-bit current source cells 355 assigned to the i higher bits
are disposed in a two-dimensional matrix. Matrix selection lines
358 for selecting the higher-bit current source cells 355 are
disposed around the higher-bit current source cells 355 in
accordance with the arrangement of the higher-bit current source
cells 355.
[0187] In the higher-bit current source cell portion 354, the
higher-bit current source cells 355 are selected by the higher-bit
controller 340 in accordance with the higher bit data of the
digital input signal. As in the lower-bit current source cell
portion 352, the output currents of the higher-bit current source
cells 355 are added in the DAC output terminal DACout. The output
voltage of the DAC output terminal DACout is defined by the product
of the added current Idac and the resistance Rref of the reference
resistor 398.
[0188] Within the voltage amplitude controller 360, current source
cells for adjusting currents are disposed. The current-adjusting
current source cells form a current mirror between the current
source cells 353 of the lower-bit current source cell portion 352
and the current source cells 355 of the higher-bit current source
cell portion 354. Because of the formation of a current mirror, the
operation currents of the current source cells 353 and 355 within
the current source cell unit 350 can be set, and a DA conversion
circuit that is operable with a certain gain can be implemented. By
adjusting the operation current, the gain can be varied in a range
from -3 dB to 20 dB. Since the current source cell unit 350 is
controlled by digital signals, the DA converter 300 is operable as
a DA conversion circuit that can convert an input digital signal
Din into an analog signal with a desired gain. This is described in
detail below.
[0189] The block controller 320 controls the higher-bit controller
340 and the lower-bit controller 330 based on the multi-bit digital
signal Din supplied from the communication/timing controller 20.
For example, the block controller 320 decodes the i higher bits
into a decimal number, and controls shift registers forming a shift
register unit 342 based on the decimal number.
[0190] The block controller 320 receives various control signals
J320 for controlling the DA conversion mode, DA conversion start,
DA conversion resolution, etc., and based on the input digital
signal Din (for example, decoded value) and control signals J320,
the block controller 320 supplies control signals J330 and control
signals J340 to the lower-bit controller 330 and the higher-bit
controller 340, respectively.
[0191] The control signals J330 include a reset signal for
resetting an output of a scaling processor 332 to a constant
logical level (clear value in this embodiment). The control signals
J340 include a reset signal and a set signal for setting an output
of the shift register unit 342 to a constant logical level (clear
value and full value in this embodiment).
[0192] The DA conversion count clock CKdac is input into the block
controller 320 from the communication/timing controller 20 as an
external clock. The block controller 320 then shapes the count
clock CKdac into an internal count clock CKcnt and supplies it to
the lower-bit controller 330.
[0193] The block controller 320 also supplies control signals J342
to the higher-bit controller 340 to restrict the number of
higher-bit current source cells 355 to define the DA digital
resolution (for example, "i" of i bits). The control signals J342
include, for example, enable signals for controlling the activation
of the shift registers. When the enable signals to be supplied to
the output enable (OE) terminals of the shift registers are active,
the outputs of the shift registers become enabled, and when the
enable signals are inactive, the output of the shift registers
become disabled (for example, the non-inverted output terminal Q is
maintained at L level and the inverted output terminal xQ is
maintained at H level).
[0194] For example, it is now assumed that the maximum resolution
of the higher-bit current source cells 355 is i bits (2.sup.i). If
the higher-bit current source cell portion 354 is used with the
resolution of x (x.ltoreq.i), the activation of the shift registers
can be controlled so that only the 2.sup.x higher-bit current
source cells 355 are activated. In this case, it is sufficient if i
control signals J342 (control signals J342_1 through J342_j) are
supplied.
[0195] The control signal J342_1 activates the first higher-bit
current source cell 355 and the control signal J342_2 activates
second through 2.sup.2-th higher-bit current source cells 355.
Similarly, the control signals J342_1 through J342_x activate the
1+2.sup.(x-1)-th through 2.sup.x-th higher-bit current source cells
355. That is, the total number of 2.sup.i (may be 2.sup.i+y)
higher-bit current source cells 355 have been divided into x
blocks, and only the higher-bit current source cells 355 having a
desired digital resolution have been selected.
[0196] The lower-bit controller 330 includes the scaling processor
332 and a glitch suppression processor 336. The scaling processor
332 has a counter (i.e., scaler for scaling the clock) for counting
the internal count clock CKcnt supplied from the block controller
320. The glitch suppression processor 336 has a plurality of glitch
suppression circuits for suppressing glitch occurring in an output
of the DA converter 300. The same number of glitch suppression
circuits as the number of current source cells 353 disposed in the
lower-bit current source cell portion 352 are provided.
[0197] The scaling processor 332 can execute DA conversion
corresponding to one least significant bit (LSB) during one clock
period. More specifically, the scaling processor 332 includes (j-1)
scalers for generating currents weighted with
1/two-to-the-power-of-certain-numbers, and supplies a .times.1
clock, which is the count clock CKcnt itself, and (j-1)-bit lower
binary outputs, i.e., clocks scaled down to 1/2, 1/4, . . . ,
1/2.sup.(j-1) by the scalers, to the corresponding glitch
suppression circuits of the glitch suppression processor 336 as
selection control signals (details of the scaling operation are
described below). That is, clocks scaled down to 1/2.sup.k (k is
from 0 to j-1) corresponding to the j lower-bit binary data are
supplied to the glitch suppression processor 336_j-k. The scaling
processor 332 serves as a selector for the j lower-bit current
source cells 353 disposed in the lower-bit current source cell
portion 352.
[0198] The glitch suppression processor 336 performs glitch
suppression processing by using the corresponding glitch
suppression circuits and supplies the clocks scaled down to
1/2.sup.k (k is from 0 to (j-1)) to the current source cells 353
having current values 1/2.sup.(j-k). For example, the .times.1
clock is supplied to the lower-bit current source cell 353.sub.--j
having a current value 1/2.sup.j, the 1/2-scaled-down clock is
supplied to the lower-bit current source cell 353_j-1 having a
current value 2.sup.(j-1), and the 1/4-scaled-down clock is
supplied to the lower-bit current source cell 353_j-2 having a
current value 2.sup.(j-2). Thereafter, similarly, the
1/2.sup.(j-2)-scaled-down clock is supplied to the lower-bit
current source cell 353_2 having a current value 1/4, and the
1/2.sup.(j-1)-scaled-down clock is supplied to the lower-bit
current source cell 353_1 having a current value 1/2.
[0199] The lower-bit controller 330 includes the same number of
selection lines 338 as the number of lower-bit current source cells
353 to select the lower-bit current source cells 353 of the
lower-bit current source cell portion 352. The lower-bit controller
330 controls the selection lines 338 to select the lower-bit
current source cells 353 corresponding to the lower-bit data values
of a digital input signal. In this example, the glitch suppression
processor 336 is interposed on the selection lines 338 between the
scaling processor 332 and the lower-bit current source cell portion
352.
[0200] The glitch suppression processor 336 almost simultaneously
outputs positive logic output Q and negative logic output xQ
(hereinafter also referred to as the "complementary output") from
the output terminals based on the scaled-down clocks output from
the scaling processor 332. In accordance with the complementary
output of the glitch suppression processor 336, two selection lines
forming the selection lines 338 are connected to each of the
lower-bit current source cells 353 (more specifically, the input
terminal of the differential switch of each cell 353). Details of
the glitch suppression processor 336 are given below.
[0201] The lower-bit controller 330 includes a shift controller 333
for supplying a signal indicating a carry bit or a borrow bit to
the shift register unit 342 of the higher-bit controller 340 as a
shift clock CKsr. For example, the shift controller 333 includes an
inverter 334 having a buffer function of inverting the logical bit
of the clock scaled down to 1/2.sup.(j-1) of the lower-bit binary
outputs of the scaling processor 332 to generate the shift clock
CKsr indicating a carry bit. The scaling processor 332 inverts the
phase of the clock scaled down to 1/2.sup.(j-1) via the inverter
334, and supplies one of the changed edges to the higher-bit
controller 340 as the shift clock CKsr.
[0202] The above-described configuration of the shift controller
333 is an example only. Alternatively, a carry pulse that can be
generated during the up-counting operation may be used as the shift
clock CKsr indicating a carry bit, and a borrow pulse that can be
generated during the down-counting operation can be used as the
shift clock CKsr indicting a borrow bit.
[0203] The higher-bit controller 340 includes the shift register
unit 342 having the same number of shift registers as the i higher
bits (2.sup.i) and a glitch suppression processor 346 having a
plurality of glitch suppression circuits for suppressing the
occurrence of glitch in an output of the DA converter 300. The same
number (2.sup.i) of glitch suppression circuits as the number of
current source cells 355 disposed in the higher-bit current source
cell portion 354 are provided. The shift clock CKsr is supplied to
the shift register unit 342 from the lower-bit controller 330.
[0204] The shift register unit 342 can execute DA conversion
corresponding to the i higher bit data values. More specifically,
the shift register unit 324 includes cascade-connected shift
registers to sequentially handle the higher i-bit digital signals.
The shift output terminals of the shift registers are sequentially
activated in a predetermined direction based on the shift clock
CKsr supplied from the lower-bit controller 330 so that the i
higher bits of the input digital signal can be decoded into a
decimal number and the decimal number can be output to the shift
output terminals.
[0205] The shift outputs from the shift registers are supplied as
selection control signals to the corresponding glitch suppression
circuits of the glitch suppression processor 346 (details thereof
are given below). The glitch suppression processor 346 performs
glitch suppression processing on the selection control signals by
using the corresponding glitch suppression circuits, and supplies
the selection control signals to the corresponding higher-bit
current source cells 355 of the higher-bit current source cell
portion 354.
[0206] The higher-bit controller 340 includes the same number of
matrix selection lines 348 (corresponding to the matrix selection
lines 358) as the number of higher-bit current source cells 355 to
select the higher-bit current source cells 355 of the higher-bit
current source cell portion 354. The higher-bit controller 340
controls the matrix selection lines 348 to select the corresponding
higher-bit current source cells 355 associated with the higher bit
data values of the digital input signal. In this embodiment, the
glitch suppression processor 346 is interposed on the matrix
selection lines 348 between the shift register unit 342 and the
higher-bit current source cell portion 354.
[0207] The glitch suppression processor 346 performs complementary
output based on the shift outputs from the shift register unit 342.
Two selection lines forming the matrix selection lines 358 disposed
around the higher-bit current source cell portion 354 in accordance
with the outputs Q and xQ of the glitch suppression processor 346
are connected to each of the higher-bit current source cells 355
(more specifically, the input terminal of the differential switch
of the cell 355).
[0208] In the shift register unit 342, the control signals J342_1
through J342.sub.--i for controlling the digital resolution are
supplied to the output enable terminals of the shift registers from
the block controller 320. If the shift registers are used with the
resolution of x (x.ltoreq.i) bits, the enable signals are output so
that only the 2.sup.x shift registers are activated.
[0209] The control signal J342_1 activates the first shift register
and the control signal J342_2 activates second through 2.sup.2-th
shift registers. Similarly, the control signals J342_1 through
J342_x activate the 1+2.sup.(x-1)-th through 2.sup.x-th shift
registers. That is, the total number of 2.sup.i (may be 2.sup.i+y)
shift registers have been divided into x blocks and only the shift
registers corresponding to a desired digital resolution have been
selected. Then, only the higher-bit current source cells 355 having
a desired digital resolution can be used.
[0210] The digital resolution may be controlled in units of bits,
as described above. Alternatively, it may be controlled by a
certain value z. For example, it is now assumed that z is 34 when i
higher bits are 7 bits and a certain value y is 8 and when each
block has four shift registers. In this case, a control signal
associated with a desired digital resolution is input into every z
shift registers.
[0211] The higher-bit controller 340 selects the number of
higher-bit current source cells 355 associated with the higher-bit
data value of a digital input signal to perform DA conversion on
the higher-bit data. In this case, control is performed so that
only the number of shift registers or higher-bit current source
cells 355 associated with the digital resolution can be selected.
Then, DA conversion is not conducted after the target digital
resolution is reached. That is, the higher-bit controller 340
starts DA conversion on higher-bit data based on the shift clock
CKsr supplied from the lower-bit controller 330, and also,
automatically stops DA conversion when the target digital
resolution is reached.
[0212] The shift register unit 342 sequentially activates the shift
output terminals of the cascade-connected shift registers in a
predetermined direction based on the shift clock CKsr supplied from
the lower-bit controller 330. In this case, if the lower-bit
controller 330 performs a shift-up operation, the shift clock CKsr
indicates a carry bit, and then, the shift register unit 342
performs a shift operation so that one higher-bit current source
cell 355 is further activated. On the other hand, if the lower-bit
controller 330 performs a shift-down operation, the shift clock
CKsr indicates a borrow bit, and then, the shift register unit 342
performs a shift operation so that the final-stage higher-bit
current source cell 355 which is being inactivated is further
inactivated.
[0213] The operation of the DA converter 300 is briefly discussed
below. To count bits speedily and reliably, the required number of
bits are divided into the i higher bits and j lower bits by using
the scaling processor 332, which operates as a clock counter, and
the higher-bit controller 340. For the j lower bits, clocks scaled
down to 1/two-to-the-power-of-certain-numbers are generated, and
the scaling processor 332 selects the j lower-bit current source
cells 353 disposed in the lower-bit current source cell portion
352. For the i higher bits, based on the shift clock CKsr supplied
from the scaling processor 332, the shift registers of the
higher-bit controller 340 are activated or inactivated in a
predetermined direction for each clock, and then, the higher-bit
current source cells 355 of the higher-bit current source cell
portion 354 are selected and driven.
[0214] With this configuration, the lower-bit current source cell
portion 352 controlled in the binary counter mode and the
higher-bit current source cell portion 354 controlled in the
decoding mode can be operated in cooperation with each other. Even
if a digital signal is input at high speed, two portions divided
from the current source cell unit 350 based on the binary mode and
the decoding mode can be operated almost simultaneously, and thus,
the current source cells 353 and 355 corresponding to the input
digital signal can be speedily and reliably selected. As a result,
the occurrence of glitch or erroneous coding can be prevented, and
the stable DA conversion operation can be implemented.
[0215] That is, in the DA converter 300 of this embodiment, an
n-bit digital signal is divided into i higher bits and j lower bits
(n=i+j). In the case of the i higher bits, the matrix-type
higher-bit current source cell portion 354 having the higher-bit
current cells 355 which are uniformly weighted to generate the same
current value for the higher digital signal is formed, and the
higher-bit current source cell portion 354 is controlled by the
higher-bit controller 340 in the decoding mode. In the case of the
j lower bits, the lower-bit current source cell portion 352 having
the parallel-disposed weighted current source cells 353 to generate
1/two-to-the-power-of-certain-numbers currents is formed, and the
lower-bit current source cell portion 352 is controlled by the
lower-bit controller 330 in the binary counter mode.
[0216] By shifting the shift registers built in the higher-bit
controller 340 in the direction associated with a carry bit or a
borrow bit in the lower-bit controller 330 in association with the
shift-up or shift-down operation of the lower-bit controller 330,
the selection of the lower-bit current source cells 353 associated
with the lower bits of the input digital value and the selection of
the higher-bit current source cells 355 associated with the higher
bits of the input digital value can be almost simultaneously
performed. Details of the functions and operations thereof are
specifically described below.
Overview of Overall Operation
[0217] FIG. 5 is a timing chart illustrating an overview of the
overall operation of the DA converter 300, mainly the lower-bit
controller 330 and the higher-bit controller 340. It is now assumed
that the j lower bits are 5 and the 2.sup.i higher-bit current
source cells 355 are disposed in the higher-bit current source cell
portion 354.
[0218] In FIG. 5, as well as the scaled-down clocks output from the
scaling processor 332 of the lower-bit controller 330 and the shift
outputs from the shift register unit 342 of the higher-bit
controller 340, digital codes corresponding to the counter values
are also shown. Although the digital codes are indicated by
hexadecimal (Hex) in FIG. 5, they may be indicated in decimal or
binary notation.
[0219] It is now assumed that, when the outputs from the scaling
processor 332 and the shift register unit 342 are at low level, the
lower-bit current source cells 353 and the higher-bit current
source cells 355 are turned ON. Accordingly, the initial output
values of the scaling processor 332 and the shift register unit 342
are at high level, and all the current source cells 353 and 355 are
turned OFF.
[0220] When the reset signal is HIGH and the enable signal is HIGH,
an input clock can be transmitted to the scaling processor 332.
Accordingly, when the reset signal is LOW or the enable signal is
LOW, the outputs from all the scaled-down clocks are at high level,
and the current source cells 353 and 355 are turned OFF. The
operation is started from the initial state (reset state) by the
enable signal, and the set signal is used for turning ON some of
the shift registers from the reset state.
[0221] In the scaling processor 332, when the .times.1 clock (the
same as CK0) is shifted to high level, the 1/2 scaler performs the
switching operation, and when the .times.1 clock is shifted to high
level 16 times, the 1/16 scaler performs switching.
[0222] In the shift register unit 342, the shift operation is
performed by using the shift clock CKsr generated based on the
1/16-scaled-down clock output from the 1/16 scaler of the scaling
processor 332. Accordingly, the outputs from the shift registers
are sequentially activated every 16 clocks.
[0223] In the example shown in FIG. 5, when the 1/16-scaled-down
clock is shifted to high level the second time, the non-inverted
output of the shift register is shifted to the high level and the
inverted output is shifted to the low level, and then, the first
higher-bit current source cell 355_1 is turned ON. In this manner,
every time the 1/16-scaled-down clock is shifted to the high level,
the non-inverted outputs of the shift registers are sequentially
shifted to the high level and the inverted outputs are sequentially
shifted to the low level, and then, the corresponding k-th
lower-bit current source cell 353.sub.--k is turned ON. This
operation is continued until the final 2.sup.i-th higher-bit
current source cell 355_2.sup.i is turned ON unless the control
signals J330 or J340 output a reset signal.
Basic Configuration of Current Source Cell
[0224] FIG. 6 illustrates the basic configuration of each current
source cell 353 or 355 disposed in the current source cell unit
350. A basic current source cell 500 is operated upon receiving
complementary signals in accordance with the
complementary-output-type lower-bit controller 330 or higher-bit
controller 340.
[0225] The basic current source cell 500 includes, as shown in FIG.
6, a unit current source 510 and a changeover switch 520 for
switching an output current of the unit current source 510.
[0226] The unit current source 510 includes an NMOS transistor 512,
which serves as a reference current source. The changeover switch
520 includes two NMOS transistors 524 and 526 differentially
connected to each other. The NMOS transistor 524 serves as a
differential switch 1 and the NMOS transistor 526 serves as a
differential switch 2.
[0227] The NMOS transistor 512 is connected at a source terminal to
an analog ground line 590 and at a drain terminal to the source
terminals of the NMOS transistors 524 and 526. A bias voltage Vb,
which is applied to all the cells, is applied to the gate terminal
of the NMOS transistor 512 from the voltage amplitude controller
360 via a current control line 592 based on the concept of a
current mirror, and the current flowing in the NMOS transistor 512
is changed depending on the potential difference Vgs.
[0228] In the two NMOS transistors 524 and 526 forming the
changeover switch 520, complementary control signals Qin and xQin
(x indicates a logical inverted signal) are input into the gate
terminals of the NMOS transistors 524 and 526, respectively, and
lead lines 594 and 596 are connected to the drain terminals of the
NOMS transistors 524 and 526, respectively. For example, the
active-high (H) control signal (non-inverted input) Qin is input
into the gate terminal of the NMOS transistor 524, and the drain
terminal thereof is connected to the lead line 594, and the lead
line 594 serves as the current-output selection output line 396.
The control signal (inverted input) xQin is input into the gate
terminal of the NMOS transistor 526, and the drain terminal thereof
is connected to the lead line 596, and the lead line 596 is
connected to a power supply Vdd. In the basic current source cell
500, the active-H control signal (non-inverted input) Qin is input
into the selection output line 396 to turn ON the NMOS transistor
524, and then, the corresponding current source cell is turned
ON.
Details of Current Source Cell Unit
[0229] FIG. 7 illustrates the detailed configuration of the current
source cell unit 350. As the individual current source cells, the
basic current source cells 500 shown in FIG. 6 are used, and the
operation currents to be supplied to the individual current source
cells and the number of combinations of the basic current source
cells 500 can be adjusted, and then, the current source cells 353
and 355 for generating bit-dependent weighted current values are
formed. Basically, the potential applied to the gate of the NOMS
transistor 512 is adjusted so that a basic current source having a
current value weighted with a predetermined output current is
prepared. Then, current source cells that can output a current in
proportional to two to the power of certain numbers or
1/two-to-the-power-of-certain-numbers of the potential input into
the gate of the NMOS transistor 512 are disposed in accordance with
the basic current source.
[0230] More specifically, the lower-bit current source cell 353_1
having a 1/2 current value corresponding to the 1/2.sup.(j-1)
scaled-down clock is formed by the basic current source cell 500
shown in FIG. 6. The level of the output current (1/2) is
determined by the potential input into the gate of the NMOS
transistor 512 and is controlled by the voltage amplitude
controller 360, as stated above.
[0231] Two cells similar to the lower-bit current source cell 353_1
are disposed in parallel so that the higher-bit current source cell
355 having a current value 1 is formed. The i (or i+y) higher-bit
current source cells 355 are prepared so that the higher-bit
current source cell portion 354 is formed.
[0232] To form the lower-bit current source cells 353_2, . . . ,
353.sub.--j weighted with 1/4, . . . , 1/2.sup.j, respectively, the
output current (1/2) of the lower-bit current source cell 353_1 is
branched into 1/2.sup.j-1 based on the concept of a current mirror
so that a current source 530 that outputs currents weighted with
1/2.sup.j is formed. Then, the same number of basic current source
cells 500 shown in FIG. 6 operating with the current value
1/2.sup.j as the predetermined number of weights are disposed in
parallel so that the lower-bit current source cells 353_2, . . . ,
353.sub.--j for generating constant currents weighted with 1/4, . .
. , 1/2.sup.j are formed. With this configuration, by using the
same size of elements that can exhibit high relative ratio
precision, current source cells that can output currents having
different weights can be formed with high precision.
[0233] The current source 530 includes a transistor 532 having a
gate to which the same potential is supplied for the lower-bit
current source cell 353_1 and the higher-bit current source cell
355, which is a basic current source, two PMOS transistors 534 and
536, which are connected to each other based on the concept of a
current mirror, disposed at the output side (drain terminal) of the
transistor 532, and 2.sup.(j-1) NMOS transistors 538 disposed at
the output side (drain terminal) of the transistor 534 and having a
gate and a drain connected to each other.
[0234] The source terminals of the transistors 532 and 538 are
connected to an analog ground line 590, and the source terminals of
the transistors 534 and 536 are connected to a power supply line
591. With this configuration, one transistor 538 can serve as a
current source for outputting a current weighted with 1/2.sup.j.
The gates of the parallel-connected MOS transistors 538 are
controlled with the same reference voltage so that the MOS
transistors 538 can be operated with a constant current, and the
plurality of MOS transistors 538 having the same characteristics
can form a branched path for the current source cells. Accordingly,
a high-precision current branched path can be formed.
[0235] One transistor 538 and each of the transistors 512 of the
lower-bit current source cells 353_2, . . . , 353.sub.--j form a
current mirror circuit, and the same number of basic current source
cells 500 configured as shown in FIG. 6 having the transistors 512
as the number of weighted current values are disposed in parallel.
Then, the lower-bit current source cell 353_2 outputs a current
having a weight 1/4 (=1/2.sup.2), the lower-bit current source cell
353_3 outputs a current having a weight 1/8 (=1/2.sup.3), the
lower-bit current source cell 353_4 outputs a current having a
weight 1/16 (=1/2.sup.4), and the lower-bit current source cell
353.sub.--j (j=5) outputs a current having a weight 1/32
(=1/2.sup.5).
[0236] With this configuration, without the need to use elements of
extremely deformed sizes or shapes or to significantly increase the
number of elements in the higher-bit current source cell portion
354 corresponding to the higher bits of a digital input signal,
current source cells having different weights can be formed with
high precision by using the same size of elements that can exhibit
high relative ratio precision. The resolution of DA conversion can
be enhanced without the need to increase the circuit scale.
[0237] In the higher-bit current source cell portion 354, the
number associated with the data value (decimal) of the higher bits
of a digital input signal is selected, and in the lower-bit current
source cell portion 352, the number associated with the lower bit
value (bit data itself) of the input signal is selected.
Accordingly, the bit resolution can be enhanced with the addition
of the lower-bit current source cells.
[0238] Additionally, a current value having a 1/2 weight of a basic
current corresponding to the digit number (the weight "1" of the
sixth bit in the previous example) of predetermined digits is
generated by the lower-bit current source cell 3531. Then, by
branching the output current 1/2 to
1/two-to-the-power-of-certain-numbers (1/2.sup.j-1) to form a
branched path (corresponding to the transistor 532),
1/two-to-the-power-of-certain-numbers (1/2.sup.j) currents of the
basic current can be extracted. Thus, high-precision current source
cells can be formed with the same size of elements.
[0239] Current source cells are formed of constant current circuits
using MOS transistors, and the current weights of the constant
current circuits are determined by the number of parallel-connected
MOS transistors having the equal characteristics. Then, the
resulting current source cells are not influenced by manufacturing
variations and thus exhibit high relative ratio precision.
[0240] In this example, the higher-bit current source cells 355 and
the lower-bit current source cell 353_1 are formed by using a
current source cell that outputs a 1/2 weighted current value as
the basic element, and then, the operation current of the lower-bit
current source cell 353_1 is branched to 1/2.sup.(j-1) to form the
current source 530 that outputs the weighted current value
1/2.sup.j ( 1/32 since j=5). Then, the remaining lower-bit current
source cells 353_2 through 353.sub.--j in the lower-bit current
source cell portion 352 are formed by using the current source cell
that outputs the 1/2.sup.j-weighted current value as the basic
element. However, this is an example only, and the current source
530 may be configured in any manner as long as it can output a
current with a weight 1/2.sup.j with high precision. It should be
noted, however, that the current source 530 should be configured so
that currents having different weights can be output with high
precision, as in the configuration shown in FIG. 7.
Details of Scaling Processor
[0241] FIG. 8 illustrates the detailed configuration of the scaling
processor 332. It is now assumed that the j lower bits are 5 and
the scaling processor 332 performs a down-counting operation and
outputs scaled-down clocks in synchronization with the internal
count clock CKcnt supplied from the block controller 320.
[0242] More specifically, as shown in FIG. 8, the scaling processor
332 includes two cascade-connected inverters 602 and 604 for
outputting a .times.1-scaled-down clock, and also includes four
cascade-connected D flip-flops (hereinafter sometimes collectively
referred to as the "D-FF") 610 for outputting 1/2, . . . ,
1/2.sup.j-scaled-down clocks. The output of the inverter 604 is
input into the glitch suppression processor 336.sub.--j as the
.times.1-scaled-down clock, and the output of the non-inverted
output terminal Q of each D-FF 610 is input into the corresponding
glitch suppression processor 336_j-k as the 1/2.sup.k-scaled-down
clocks (k is from 1 to j-1).
[0243] Between adjacent D-FFs 610, a cascade-connected circuit
composed of a gate (inverter or NAND) 620 for receiving the output
at least from the non-inverted output terminal Q of the D-FF 610
immediately before and an inverter 622, and a switch circuit 630
combined from transfer gate circuits (one-input one-output
switches) 632 and 634 using complementary transistors having
opposite polarities are provided. An inverter 621 is connected to
the non-inverted output terminal Q of the D-FF 610, and a
scaled-down clock is output via the inverter 621. An output signal
of the non-inverted output terminal Q of the final-stage D-FF 610_4
is also supplied to the shift controller 333.
[0244] The internal count clock CKcnt1 is supplied to the clock
input terminals CK of the D-FF 610_1 through 610_4 via a clock gate
circuit 680 after it is gated by the clock gate circuit 680, and
also, a reset signal (active low) contained in the control signals
J330 is supplied to the input terminals R of the D-FF 610_1 through
610_4 from the block controller 320. The clock gate circuit 680
includes various gates, such as inverters, NANDs, and transfer
gates. The clock gate circuit 680 gates the internal clock CKcnt
based on the reset signal and enable signal and then supplies the
internal clock CKcnt1 to the D-FF 610.
[0245] The enable signal controls the input of the clock into the
D-FF 610, and only when the enable signal is active, the D-FF 610
becomes enabled. As a result, the D-FF 610 operates in
synchronization with the internal count clock CKcnt only when the
enable signal is active, and the output of the D-FF 610 is
maintained unless the D-FF 610 is enabled by the enable signal.
That is, the enable signal can control the activation of the D-FF
610, which serves as a scaler.
[0246] The number of inputs of the gate 620 is different depending
on the stage between the D-FFs 620. The gate 620 between the first
stage and the second stage has one input and is actually an
inverter. The gate 620 between the second stage and the third stage
is a NAND having two inputs, and the non-inverted output Q of the
first-stage D-FF 610_1 is input into the second input terminal of
the NAND. The gate 620 between the third stage and the fourth stage
is a NAND having three inputs, and the non-inverted output Q of the
second-stage D-FF 610_2 is input into the second input terminal of
the NAND, and the non-inverted output Q of the first-stage D-FF
610_1 is input into the third input terminal of the NAND.
[0247] The transfer gate 632 receives the inverted output xQ
(indicated by Q with an overbar in FIG. 8) of the D-FF 610
immediately before the transfer gate 632, and the transfer gate 634
receives the non-inverted output Q of the D-FF 610 immediately
before the transfer gate 634. The outputs of the transfer gates 632
and 634 are supplied to the D input terminal of the D-FF 610
immediately after the transfer gates 632 and 634.
[0248] The transfer gate circuits 632 and 634 are each formed of a
CMOS switch formed by connecting the sources or the drains of a
P-channel MOS transistor p1 and an N-channel MOS transistor n1. The
gates of the MOS transistor p1 and the MOS transistor n1 are
simultaneously switched ON or OFF by complementary signals supplied
via the inverter 622. The complementary signals are supplied to the
transfer gate circuits 632 and 634 in opposite phases so that the
transfer gate circuits 632 and 634 are controlled to be ON or OFF
in opposite phases.
[0249] As the CMOS switch, only one type of transistors, i.e.,
P-channel MOS transistors p1 or N-channel transistors n1, may be
used. In this case, however, the threshold voltage Vth may
influence the switching performance. Accordingly, in this
embodiment, a CMOS switch using both MOS transistors p1 and n1 is
employed so that complementary signals can simultaneously switch ON
the transfer gate circuits 632 and 634.
[0250] By simultaneously switching ON the MOS transistors p1 and
n1, the resistance generated when the MOS transistors are turned ON
can be reduced to be smaller than that when only one type of MOS
transistors is used. Additionally, even if an open circuit failure
occurs in one of the transistors, the CMOS switch still functions
as a switch.
[0251] With this configuration, when the .times.1-scaled-down clock
(equal to the internal count clock CKcnt) is shifted to the H
level, the switching operation of the 1/2-scaled-down clock is
performed in the D-FF 610_1. Concerning each of the D-FF 610_2
through D-FF 610_4 corresponding to the 1/4-scaled-down clock
through the 1/16-scaled-down clock, respectively, when the
non-inverted outputs Q of all the previous-stage D-FFs 610 are at
the H level, the transfer gate 632 is switched ON and the transfer
gate 634 is switched OFF, and the inverted output xQ of the D-FF
610 is supplied to the D input terminal of the same D-FF 610. At
this time, if the internal count clock CKcnt is input into the D-FF
610, the logical values of the inverted output Q and the
non-inverted output xQ are inverted in synchronization with the
rising edge of the internal count clock CKcnt (i.e., when the
internal count clock CKcnt is shifted to the H level).
[0252] Then, one cycle of the 1/2-scaled-down clock is completed
with two clock pulses of the .times.1-scaled-down clock, and the
internal count clock CKcnt can be scaled down to 1/2. Thereafter,
similarly, one cycle of the 1/2.sup.k-scaled-down clock is
completed with 2.sup.k clock pulses of the .times.1-scaled-down
clock, and the internal count clock CKcnt can be scaled down to
1/2.sup.k. As a result, the lower-bit current source cells 353_1
through 353_5 are switched ON or OFF so that the j lower bits can
be DA converted.
[0253] For example, in the initial state, the supply of the
internal count clock CKcnt from the block controller 320 is
stopped, and then, the clock input terminal CK is shifted to the L
level, and also, the reset signal (active low) contained in the
control signals J340 from the block controller 320 is supplied to
the reset input terminal R of each D-FF 610. Then, each D-FF 610 is
reset so that the non-inverted output terminal Q is set to be the L
level and the inverted output terminal xQ is set to be the H
level.
[0254] Accordingly, in the initial state, the outputs of all the
scalers are shifted to the H level and are supplied to the
lower-bit current source cells 353 via the glitch suppression
processor 336, and then, all the lower-bit current source cells 353
are switched OFF. More specifically, the reset level (the output of
the inverter 604 and the L level of the non-inverted output of the
D-FF 610) of the 1/2.sup.k-scaled-down clock (k is from 0 to j-1)
is supplied to the lower-bit current source cells 353 via the
corresponding glitch suppression processor 336_j-k so that all the
lower-bit current source cells 353 are switched OFF.
[0255] Then, the .times.1-scaled-down clock is output from the
inverter 604 in synchronization with the internal count clock
CKcnt, and is input into the lower-bit current source cell 353_5
via the glitch suppression processor 336_5. That is, the internal
count clock CKcnt itself is supplied to the lower-bit current
source cell 3535, and the lower-bit current source cell 3535
generates a 1/2.sup.j (in this embodiment, 1/32)-weighted current
so that DA conversion for the LSB digital signal can be
executed.
[0256] Every time the .times.1-scaled-down clock is shifted to the
H level, the logical level of the output of the first-stage D-FF
610_1 is inverted, and the 1/2-scaled-down clock output from the
non-inverted output terminal Q is input into the lower-bit current
source cell 353_4 via the glitch suppression processor 336_4. The
lower-bit current cell source 353_4 generates 1/2.sup.(j-1) (in
this embodiment, 1/16)-weighted current so that DA conversion for
the second digital signal can be executed.
[0257] The non-inverted output Q of the D-FF 610_1 and the
non-inverted output Q and the inverted output xQ of the D-FF 610_2
define the logical state of the D input terminal of the D-FF 610_2,
and every time the 1/2-scaled-down clock is shifted to the H level,
the logical level of the output of the second-stage D_FF610_2 is
inverted. As a result, since an edge is generated in the D-FF 610_2
in every two cycles of the D-FF 610_1, one higher-bit control can
be implemented, and the ON/OFF state of the lower-bit current
source cell 353_3 is controlled. That is, the 1/4-scaled-down clock
output from the non-inverted output terminal Q is input into the
lower-bit current source cell 353_3 via the glitch suppression
processor 336_3. The lower-bit current cell source 353_3 generates
1/2.sup.(j-2) (in this embodiment, 1/8)-weighted current so that DA
conversion for the third digital signal can be executed.
[0258] The non-inverted output Q of the D-FF 610_1, the
non-inverted output Q of the D-FF 610_2, and the non-inverted
output Q and the inverted output xQ of the D-FF 610_3 define the
logical state of the D input terminal of the D-FF 610_3, and every
time the 1/4-scaled-down clock is shifted to the H level, the
logical level of the output of the third-stage D_FF 610_3 is
inverted. As a result, since an edge is generated in the D-FF 610_3
in every two cycles of the D-FF 610_2, one higher-bit control can
be implemented, and the ON/OFF state of the lower-bit current
source cell 353_2 is controlled. That is, the 1/8-scaled-down clock
output from the non-inverted output terminal Q is input into the
lower-bit current source cell 353_2 via the glitch suppression
processor 336_2. The lower-bit current cell source 353_2 generates
1/2.sup.(j-3) (in this embodiment, 1/4)-weighted current so that DA
conversion for the fourth digital signal can be executed.
[0259] The non-inverted output Q of the D-FF 610_1, the
non-inverted output Q of the D-FF 610_2, the non-inverted output Q
of the D-FF 610_3, and the non-inverted output Q and the inverted
output xQ of the D-FF 610_4 define the logical state of the D input
terminal of the D-FF 610_4, and every time the 1/8-scaled-down
clock is shifted to the H level, the logical level of the output of
the fourth-stage D_FF 610_4 is inverted. As a result, since an edge
is generated in the D-FF 610_4 in every two cycles of the D-FF
610_3, one higher-bit control can be implemented, and the ON/OFF
state of the lower-bit current source cell 353_1 is controlled.
That is, the 1/8-scaled-down clock output from the non-inverted
output terminal Q is input into the lower-bit current source cell
353_1 via the glitch suppression processor 336_1. The lower-bit
current cell source 353_1 generates 1/2.sup.(j-4) (in this
embodiment, 1/2)-weighted current so that DA conversion for the
fifth digital signal can be executed.
Details of Shift Register Unit
[0260] FIG. 9 illustrates the detailed configuration of the shift
register unit 342 when the higher-bit current source cells 355 for
the i higher bits (2.sup.i) for outputting a current value of 1 are
disposed in the higher-bit current source cell portion 354.
[0261] The shift register unit 342 executes DA conversion on each
bit of the i higher bits. More specifically, the shift register
unit 342 includes cascade-connected shift registers 343
sequentially corresponding to the i higher bits of the digital
signal, and sequentially sets the shift output terminals of the
shift registers 343 to be active H based on the shift clock CKsr
supplied from the lower-bit controller 330. That is, the shift
register unit 342 outputs the data values decoded from the i higher
bits of the input digital signal to the shift output terminals of
the shift registers 343. The shift outputs of the shift registers
343 are supplied to the corresponding glitch suppression circuits
of the glitch suppression processor 346. The glitch suppression
processor 346 performs glitch suppression processing on the shift
outputs by using the corresponding glitch suppression circuits, and
then supplies the resulting outputs to the higher-bit current
source cells 355 of the higher-bit current source cell portion
354.
[0262] More specifically, as shown in FIG. 9, the shift register
unit 342 includes the cascade-connected shift registers 343
provided with the same number of D-FFs as the number of
i-higher-bit (2.sup.i) current source cells 355. The D input
terminal of the first-stage shift register 343_1 is pulled up to
the power supply, and the non-inverted output Q of the k-th shift
register 343 is supplied to the D input terminal of the (k+1)-th
shift register 343.
[0263] The shift clock CKsr, for example, the clock inverted from
the 1/2.sup.(j-1)-scaled-down clock by the inverter 334, is
supplied from the shift controller 333 to the clock input terminals
CK Of the shift registers 343, and the shift registers 343 are
operated in synchronization with the shift clock CKsr. The reset
signal (active low) contained in the control signals J340 is
supplied from the block controller 320 to the reset input terminals
R of the shift registers 343. The set signal (active low) contained
in the control signals J340 is also supplied from the block
controller 320 to the set input terminals S of the shift registers
343.
[0264] The set signal is a signal supplied only to the shift
registers 343. In this embodiment, the set signal is effective only
when it is desired that the current source cell connected to the
set signal is switched ON in the reset state. For example, the set
signal is effective when an offset is provided for the output
reference signal.
[0265] With this configuration, the shift register unit 342 is
reset by the reset signal in the initial state, and is then set by
the set signal. Then, a predetermined number of shift registers 343
are activated by the enable signal, and the first through
2.sup.i-th shift registers 343 are sequentially activated in
synchronization with the shift clock CKsr so that the higher-bit
current source cells 355 weighted with 1 are sequentially switched
ON.
[0266] Accordingly, the .times.1 (weighted with 1) higher-bit
current source cells 355 are activated reliably one by one in
synchronization with the shift clock CKsr. Since the shift clock
CKsr is information indicating a carry bit or a borrow bit in the
lower-bit controller 330, DA conversion performed by the shift
register unit 342 and the higher-bit current source cell portion
354 under the control of the shift register unit 342 in
synchronization with the shift clock CKsr is started from the
(j+1)-th bit.
[0267] To define the digital resolution ("x" of x bits) of DA
conversion by specifying the number of higher-bit current source
cells 355 to be used by the control signals J342, the number of
shift registers 343 to be activated is defined. That is, only a
predetermined number of shift registers 343 corresponding to the
desired digital resolution has been determined so that the only the
higher-bit current source cells 355 corresponding to the desired
digital resolution can be used.
[0268] For example, when an active edge of the shift clock CKsr is
input into the clock input terminal CK of the shift register 343,
the first shift register 343_1 outputs the H level indicating the
state of the D input terminal to the non-inverted output terminal
Q, and also sets the inverted output terminal xQ to the L level.
The L level indicating the state of the inverted output terminal xQ
is supplied to the higher-bit current source cell 355_1 via the
corresponding glitch suppression processor 346_1 and switches ON
the higher-bit current source cell 355_1.
[0269] When an active edge of the subsequent shift clock CKsr is
input into the clock input terminal CK of the shift register 343,
the second shift register 343_2 outputs the H level indicating the
state of the D input terminal (i.e., the state of the non-inverted
output terminal Q of the first shift register 343_1) to the
non-inverted output terminal Q, and also sets the inverted output
terminal xQ to the L level. The L level indicating the state of the
inverted output terminal xQ is supplied to the higher-bit current
source cell 355_2 via the corresponding glitch suppression
processor 346_2 and switches ON the higher-bit current source cell
355_2.
[0270] Thereafter, similarly, when an active edge of the subsequent
shift clock CKsr is input into the clock input terminal CK of the
shift register 343, the k-th shift register 343.sub.--k outputs the
H level indicating the state of the D input terminal (i.e., the
state of the non-inverted output terminal of the (j-1)-th shift
register 343_k-1) to the non-inverted output terminal Q, and also
sets the inverted output terminal xQ to the L level. The L level
indicating the state of the inverted output terminal xQ is supplied
to the higher-bit current source cell 355.sub.--k via the
corresponding glitch suppression processor 346.sub.--k and switches
ON the higher-bit current source cell 355.sub.--k.
[0271] The above-described operation is sequentially performed on
the activated shift registers 343 so that an analog voltage
corresponding to the desired digital resolution can be generated.
For the inactivated shift registers 343, even if an active edge of
the shift clock CKsr is input into the clock input terminal CK, the
non-inverted output terminal Q is maintained at the L level and the
inverted output terminal xQ is maintained at the H level regardless
of the state of the non-inverted output terminal Q of the previous
shift register 343. Accordingly, after the target digital
resolution has reached, DA conversion can be terminated.
[0272] If the digital resolution is 7 bits, when the 2.sup.7-th
shift register 343 is switched ON, DA conversion performed by the
higher-bit controller 340 and the higher-bit current source cell
portion 354 is terminated. Alternatively, when the higher bit data
of the desired input digital value (digital code) has reached, the
supply of the shift clock CKsr from the lower-bit controller 330
may be stopped so that DA conversion for higher bits can be
terminated.
[0273] Then, when DA conversion is terminated, the output currents
of the current source cells 353 and 355 output to the selection
output line 396 so far are combined, and the combined current is
converted into a voltage by the reference resistor 398. Thus, a
reference signal whose voltage level of the DAC output terminal
DACout sequentially varies monotonously can be generated. That is,
it is possible to provide a DA converter suitable for generating
reference signal voltages used for so-called
"single-slope-integrating" or (ramp-signal-comparison) AD
conversion.
[0274] The DA converter 300 of this embodiment can be used, not
only as a DA converter for generating reference signal voltages for
single-slope-integrating AD conversion, but also as a general DA
converter. For example, when a value corresponding to an input
digital signal is reached, the voltage level of the DAC output
terminal DACout when DA conversion is terminated can be used so
that an analog voltage corresponding to the input digital signal
can be obtained. That is, DA conversion can be conducted on a
multi-bit input digital signal.
[0275] Since the timing for terminating DA conversion has been set
only for the higher bits, it is difficult to guarantee the
precision of the lower bits. For the lower bits, however, when the
target data of the higher bits is reached, the supply of the
internal count clock CKcnt can also be stopped when the j lower
bits of the input digital signal are reached, and then, DA
conversion on the lower bits can also be terminated. In this case,
by using the voltage level of the DAC output terminal DACout when
DA conversion is terminated at the point in which the value
corresponding to the multi-bit input digital signal is reached, an
analog voltage precisely reflecting the multi-bit input digital
signal can be obtained. As a result, high-precision DA conversion
on multi-bit input digital signals can be implemented.
Details of Glitch Suppression Processor
[0276] FIGS. 10A and 10B illustrate the detailed configuration of
the glitch suppression processors 336 and 346 and the operation
thereof. The glitch suppression processors 336 and 346 each
include, as shown in FIG. 10A, cascade-connected inverters 382 and
384. The first-stage inverter 382 has a function of inverting the
logical state (phase) of an input signal. The second-stage inverter
384 is a delay unit provided with a logical-state (phase) inversion
function and a delay function for a delay .DELTA.t0.
[0277] The glitch suppression processors 336 and 346 configured as
described above each supply, as complementary signals, a selection
control signal for the transistor 524 (differential switch 1) which
is not delayed by the inverter 384 and an inverted selection
control signal for the transistor 526 (differential switch 2) which
is delayed by the inverter 384 to the transistors 524 and 526,
which form differential switches.
[0278] For example, as indicated by (A1) in FIG. 10A, in the glitch
suppression processor 336, the output of the inverter 604 of the
scaling processor 332 and the non-inverted output Q of the D-FF 610
of the scaling processor 332 are supplied to the inverter 382.
Then, the inverted output xQ, which is the output of the inverter
382, is supplied to the gate of the transistor 524 operating as the
differential switch 1 of the current source cell 353, and the
non-inverted output Q, which is the output of the inverter 384, is
supplied to the gate of the transistor 526 operating as the
differential switch 2 of the current source cell 353.
[0279] As indicated by (A2) in FIG. 10A, in the glitch suppression
processor 346, the inverted output xQ of each shift register 343 of
the shift register unit 342 of the higher-bit controller 340 is
supplied to the inverter 382. Then, the non-inverted output Q,
which is the output of the inverter 382, is supplied to the gate of
the transistor 524 operating as the differential switch 1 of the
current source cell 355, and the inverted output xQ, which is the
output of the inverter 384, is supplied to the gate of the
transistor 526 operating as the differential switch 2 of the
current source cell 355.
[0280] Glitch components appearing in the selection output line 396
are likely to occur in the following situation. Due to temporal
variations (delay .DELTA.t1) of complementary signals input into
the differential switches, as indicated by (B1) in FIG. 10B, the
inputs into the transistors 524 and 526 operating as differential
switches forming the current source cells 353 and 355 become at the
L level and in the OFF state, and then, the transistor 524
subjected to DA conversion in the selection output line 396 is
switched ON from the OFF state. In this state, glitch components
are likely to occur. More specifically, when both the transistors
524 and 526 are in the OFF state, the output current from the
current source cell is zero, and in this state, if the current
source cell is activated to suddenly generate an output current,
glitch components occur.
[0281] In contrast, if the transistors 524 and 526 operating as the
differential switches of the current source cell 353 or 355 are
controlled via the glitch suppression processor 336 or 346,
respectively, as shown in FIG. 10A, the gate input of the
transistor 526, which is the output of the inverter 384, is
activated (H level) or inactivated (L level), as indicated by (B2)
in FIG. 10B, after the delay .DELTA.t0 from the output of the
inverter 382. Accordingly, in the process of inactivating the
transistor 526 after the delay .DELTA.t0 after activating the
transistor 524, when the transistors 524 and 526 are in the OFF
state, it is possible to prevent the transistor 524 from being
switched ON. Thus, the occurrence of glitch can be suppressed. This
is because the current source cell outputs a current always via one
of the switches (transistors 524 and 526).
Summary of Overall Operation
[0282] The overall operation of the DA converter 300 is summarized
as follows. The DA converter 300 is operated under the control of
the count clock CKdac, the input digital signal Din, and the
control signals J320 supplied from the communication/timing
controller 20 to the block controller 320. The block controller 320
first decodes the input digital signal Din and stops supplying the
internal count clock CKcnt to the lower-bit controller 330. The
block controller 320 controls DA conversion by manipulating the set
signal or the reset signal supplied to the higher-bit controller
340 or the lower-bit controller 330.
[0283] More specifically, in the initial state, the block
controller 320 inactivates the enable signal to gate the internal
count clock CKcnt, and stops the supply of the internal count clock
Ckcnt1 to the lower-bit controller 330 so that it can reset all of
the D-FFs 610, which serve as scalers, and all of the shift
registers 343.
[0284] Then, the communication/timing controller 20 switches OFF
the reset signal and switches ON the set signal. Then, the block
controller 320 activates the enable signal and supplies the
internal count clock CKcnt1 to the lower-bit controller 330. As a
result, the scaling processor 332 starts counting in
synchronization with the internal count clock CKcnt (more
specifically, the internal count clock CKcnt1 after being gated by
the clock gate circuit 680) so that the outputs (non-inverted
outputs Q of the D-FFs 610) of the scalers are supplied to the
glitch suppression processor 336. The inverter 382 of the glitch
suppression processor 336 inverts the phases and the inverter 384,
which serves as a delay unit, controls the timing. Then, two clocks
Q and xQ, which are complementary signals, are input into the
built-in differential switches (transistors 524 and 526) of the
corresponding lower-bit current source cell 353, and the lower-bit
current source cell 353 weighted with 1/2.sup.k (k is 1 to j) is
turned ON.
[0285] Simultaneously, after the 1/2.sup.(j-1)-scaled-down clock is
shaped into the shift clock CKsr by the shift controller 333, the
shift clock CKsr is supplied to the shift register unit 342 of the
higher-bit controller 340 and is used for switching the uniformly
weighted .times.1 higher-bit current source cells 355. For example,
when the first shift register 343 is switched ON, the first
higher-bit current source cell 355_1 is switched ON, and also, the
D input terminal of the second shift register 343 is activated (H
level). Then, the second shift register 343 is switched ON in
response to the rising edge of the 1/2.sup.(j-1)-scaled-down clock,
and thus, the second higher-bit current source cell 355_2 is
switched ON.
[0286] In this manner, because of the cooperative operation of the
lower-bit controller 330 and the higher-bit controller 340 based on
the internal count clock CKcnt, the DA converter 300 can function
as a DA conversion circuit reliably producing grayscale levels in
synchronization with the internal count clock CKcnt, and generates
progressively changing reference voltages with high precision.
[0287] In the above-described embodiment, to generate progressively
increasing reference voltages, the selection of the current source
cells 353 and 355 is controlled so that the added currents of the
selected current source cells 353 and 355 are increased. However,
if the selection of the current source cells 353 and 355 is
controlled so that the added currents of the selected current
source cells 353 and 355 are decreased, progressively decreasing
reference voltages can be generated.
Layout
[0288] FIGS. 11A and 11B illustrate a first example of the layout
of the DA converter 300. When disposing the scaling processor 332,
the shift register unit 342, and the current source cells 353 and
355 on a semiconductor substrate to form the DA converter 300,
clock supply delay and skewing between the shift registers 343
should be considered.
[0289] For example, if skewing occurs in the internal count clock
CKcnt supplied to the scalers (in this embodiment, D-FFs 610 and
the peripheral circuits thereof) of the scaling processor 332, a
timing difference is generated between the scalers, and thus, the
differential non linearity (DNL) characteristics are decreased. If
skewing occurs in the shift clock CKsr supplied to the 2.sup.i
shift registers 343, the DNL exceeds one LSB and the precision of
the reference voltage is deteriorated.
[0290] The occurrence of clock skewing originates from a wiring
delay caused by the routing of clock signal lines. Accordingly, a
wiring delay of clock signals in a digital signal processing
circuit determines the operation limit of the DA converter 300
which operates in synchronization with the internal count clock
CKcnt.
[0291] If there is a timing displacement in switching the current
source cells 353 and 355, the DNL characteristics also decreased.
The switching timing displacement originates from a wiring delay
caused by the routing of signal lines of signals (scaled-down
clocks and shift outputs) for switching the current source cells
353 and 355. Accordingly, as in clock skewing, a wiring delay of
the switching signals in a digital signal processing circuit
determines the operation limit of the DA converter 300.
[0292] Accordingly, it is necessary to consider a layout of the DA
converter 300 by giving priority to the wiring of digital signals
(scaled-down clocks and shift outputs) when disposing the analog
current source cells 353 and 355. More specifically, to increase
the speed, the first-stage shift register 343 is disposed at the
position closest to the position at which the shift clock CKsr is
output from the shift controller 333 of the lower-bit controller
330, and the remaining shift registers 343 are sequentially
disposed in a two-dimensional matrix such that they turn around
from one line to another line. As a result, clock skewing does not
occur between the shift registers 343.
[0293] The shift clock CKsr is supplied to the shift registers 343
according to a tree wiring structure. Selection control signals are
supplied to the glitch suppression processor 346 and the higher-bit
current source cells 355 corresponding to the shift registers 343
according to an isometric wiring pattern. In the tree wiring
structure, it is desirable that the lines for the shift registers
343 are routed in the same direction from the output position of
the shift clock CKsr.
[0294] For example, as shown in FIG. 11A, the scaling processor 332
assigned to the lower bits, the glitch suppression processor 336,
and the lower-bit current source portion 352 (lower-bit current
source cells 353) are disposed at the bottom right. Then, the shift
registers 343 of the shift register unit 342 assigned to the higher
bits are sequentially disposed from the right to the left in the
bottom line in the order of digital codes (i higher bits), and then
turn around to the top line from the left to the right in the order
of digital codes (i higher bits). In this manner, the shift
registers 343 are disposed in the two lines in the order of digital
codes.
[0295] The shift registers 343 are disposed so that the shift clock
CKsr (in this embodiment, 2.sup.(j-1)-scaled-down clock) generated
in the shift controller 333 of the lower-bit controller 330 can be
supplied to all of the shift registers 343 in the shortest possible
path. More specifically, the shift registers 343 in the bottom line
are sequentially disposed from the right to the left in the order
of digital codes, while the shift registers 343 in the top line are
sequentially disposed from the right to the left in the order
reverse to the order of the digital codes in the bottom line. If
necessary, the internal count clock CKcnt may be supplied to the
shift registers 343 in the bottom line and in the top line via
buffers, as indicated by the broken lines in FIG. 11A. In this
manner, the delay occurring in the corresponding functions can be
substantially the same between the top and bottom lines. The
maximum clock delay is defined by the left edge and the right edge
of the same line.
[0296] In contrast, if the signal lines for the internal count
clock CKcnt are routed so that the shift registers 343 are disposed
in the order of digital codes both in the bottom line and in the
top line, as shown in FIG. 11B, the wiring length between the
wiring for supplying the internal count clock CKcnt to the
first-stage shift register 343_1 and the wiring for supplying the
internal count clock CKcnt to the final-stage shift register
343_2.sup.j is significantly increased. Accordingly, the delay
difference between the two shift registers 343_1 and 343_2.sup.j is
considerably increased to such a degree that the maximum clock
delay is increased substantially to twice the layout shown in FIG.
11A.
[0297] The glitch suppression processor 336 and the lower-bit
current source cells 353 are disposed at the bottom right at the
positions closer to the corresponding scalers (in this embodiment,
D-FFs 610 and the peripheral circuits thereof) of the scaling
processor 332 so that the output of the scaling processor 332 can
be supplied to the glitch suppression processor 336 in the shortest
possible path and so that the output of the glitch suppression
processor 336 can be supplied to the corresponding lower-bit
current source cells 353 in the shortest possible path.
[0298] The glitch suppression processor 346 and the higher-bit
current source cells 355 are disposed close to the corresponding
shift registers 343 disposed in two lines so that the output of
each shift register 343 of the shift register unit 342 can be
supplied to the glitch suppression processor 346 in the shortest
possible path and so that the output of the glitch suppression
processor 346 can be supplied to the corresponding higher-bit
current source cells 355 in the shortest possible path. With this
arrangement, all of the current source cells 353 and 355 can be
disposed in a single well.
[0299] It is not essential that the shift registers 343 are
disposed in two lines, and they may be disposed in four lines.
FIGS. 12A and 12B illustrate a second example of the layout of the
DA converter 300. As shown in FIG. 12A, the scaling processor 332
assigned to the lower bits, and the glitch suppression processor
336 and the lower-bit current source cell portion 352 (lower-bit
current source cells 353) are disposed at the bottom right. Then,
the shift registers 343 of the shift register unit 342 assigned to
the higher bits are disposed from the right to the left in the
first line from the bottom in the order of digital codes (k=1 to
p), and the shift registers 343 then turn around to the second line
from the bottom from the left to the right in the order of digital
codes (k=p+1 to q). Then, the shift registers 343 turn around to
the third line from the right to the left in the order of digital
codes (k=q+1 to r). The shift registers 343 then turn around to the
fourth line from the left to the right in the order of digital
codes (k=r+1 to 2.sup.i). In this manner, the shift registers 343
are sequentially disposed in the four lines in the order of digital
codes.
[0300] With this arrangement, the shift clock CKsr (in this
embodiment, 2.sup.(j-1)-scaled-down clock) generated in the shift
controller 333 of the lower-bit controller 330 is supplied to all
the shift registers 343 in the shortest possible path. More
specifically, the internal count clock CKcnt is supplied from the
right to the left in each line of the shift registers 343. If
necessary, the internal count clock CKcnt may be supplied to each
line via a corresponding buffer, as indicated by the broken lines
in FIG. 12A. Accordingly, the internal count clock CKcnt can be
input into each shift register 343 with the smallest delay. In this
manner, the delay occurring in the corresponding functions can be
substantially the same between the individual lines. The maximum
clock delay is defined by the left edge and the right edge of the
same line.
[0301] In contrast, if the signal lines for the internal count
clock CKcnt are routed so that the shift registers are disposed in
the same direction in the order of digital codes in each line, as
shown in FIG. 12B, the wiring length between the wiring for
supplying the internal count clock CKcnt to the first-stage shift
register 343_1 and the wiring for supplying the internal count
clock CKcnt to the final-stage shift register 343_2.sup.j is
significantly increased. Accordingly, the delay difference between
the two shift registers 343_1 and 343_2.sup.j is considerably
increased to such a degree that the maximum clock delay is
increased substantially to four times the layout shown in FIG.
12A.
[0302] In this manner, because of the cooperative operation of the
lower-bit controller 330 and the higher-bit controller 340 based on
the internal count clock CKcnt, the desired current source cells
353 and 355 can be selected speedily and reliably. It is thus
possible to provide a DA conversion circuit that can reliably
produce grayscale levels in synchronization with the internal count
clock CKcnt and can generate progressively changing reference
voltages with high precision.
[0303] Thus, the occurrence of glitch or erroneous coding, which is
caused by the independent operation for lower bits and higher bits,
can be significantly reduced, and stable DA conversion can be
implemented.
[0304] As a result, high-precision reference voltages, which are
necessary for solid-state imaging elements, can be generated
speedily. It is possible to provide a DA conversion circuit
operating based on a high clock, for example, 200 MHz or 300 MHz.
Although the circuit scale is similar to that of the mechanism
disclosed in Japanese Unexamined Patent Application Publication No.
11-17545, a DA conversion circuit exhibiting excellent high speed
performance (in this case, the reference signal is speedily
changed) and DNL characteristics can be implemented.
[0305] The occurrence of glitch or a decrease in DNL
characteristics presents a problem, in particular, when generating
fast reference signals used for single-slope-integrating (or
ramp-signal-comparison) AD conversion, which require monotonous
changing characteristics (for example, a monotonous increase or a
monotonous decrease). In this embodiment, the layout of the DA
converter 300 that can suppress a wiring delay is employed, and the
glitch suppression processors 336 and 346 are provided.
Accordingly, the occurrence of glitch or a decrease in DNL
characteristics can be suppressed. For example, a fast DA
conversion circuit having 300 MHz can be reliably implemented.
[0306] The occurrence of erroneous coding also causes a problem. In
this embodiment, however, the higher-bit current source cells 355
generating the same weighted current as the same number as digital
codes are provided, and the higher-bit current source cells 355
corresponding to the higher-bit digital data of the input digital
signal Din are sequentially switched ON by using the shift
registers 343. Accordingly, the output voltage is sequentially
changed in accordance with the bit count value, and thus, the
occurrence of erroneous coding or glitch can be reliably
prevented.
[0307] The lower bits of the input digital signal Din are
controlled by the binary mode. The same number of current source
cells 353 generating the 1/2.sup.j-weighted current as the lower
bits are provided, and the current source cells 353 corresponding
to the lower bits of the input digital signal Din are switched ON
by using the binary outputs. Accordingly, the output voltage can be
sequentially changed in accordance with the binary value, and as in
the higher bits, the occurrence of erroneous coding or glitch can
be reliably prevented.
[0308] That is, the DA converter 300 of this embodiment includes
digital blocks using scalers and shift registers, and thus, a
high-speed DA conversion circuit that can suppress the occurrence
of glitch can be implemented. As a result, a high-speed
high-precision operation, such as 300 MHz/12 bits output, can be
implemented. The DA converter 300 is very suitable for generating
reference signals used for single-slope-integrating AD conversion,
which requires high-precision DNL rather than integrating non
linearity (INL).
Application to Gamma Correction
[0309] FIG. 13 illustrates the principle that the slope of a
reference signal is dynamically changed. As briefly discussed with
reference to FIG. 5, by changing the clock frequency for operating
the DA converter 300, a change characteristic (slope) of the DA
output can be adjusted. By utilizing this characteristic, the slope
of the DA output is dynamically changed while conducting AD
conversion by using a reference signal generated in the DA
converter 300, and then, the AD conversion result reflecting gamma
(.gamma.) correction can be obtained.
[0310] For example, based on the principle shown in FIG. 13, when
the DAC mode switch signal contained in the control signal J320
supplied from the communication/timing controller 20 to the block
controller 320 is at the L level, the block controller 320
constantly supplies the internal count clock CKcnt (indicated by
10b in FIG. 13) having frequency f1 to the lower-bit controller
330. In contrast, when the DAC mode switch signal is at the H
level, the block controller 320 initially supplies the internal
count clock CKcnt (indicated by 11b in FIG. 13) having a frequency
(for example, f1/2) lower than frequency f1 to the lower-bit
controller 330. Accordingly, the DAC output changes more gently
(smaller slope) than that when the DAC mode switch signal is at the
L level. When a DA output having a smaller slope is used as a
reference signal for AD conversion, the resulting AD output data is
increased. This means that AD conversion is conducted by increasing
the analog gain.
[0311] Then, when the DAC code reaches a predetermined value Da,
the block controller 320 supplies the internal count clock CKcnt
(indicated by 10b in FIG. 13) having frequency f1 to the lower-bit
controller 330. In this case, the DA output is changed with the
same slope as that when the DAC mode switch signal is at the L
level.
[0312] Then, when the DAC code reaches a predetermined value Db,
the block controller 320 supplies the internal count clock CKcnt
(indicated by 9b in FIG. 13) having a frequency (for example,
2.times.f1) higher than frequency f1 to the lower-bit controller
330. Accordingly, the DA output changes faster (with a larger
slope) than that when the DAC mode switch signal is at the L level.
When a DA output having a larger slope is used as a reference
signal for AD conversion, the resulting AD output data is
decreased. This means that AD conversion is conducted by decreasing
the analog gain.
[0313] In this example, the DA output is progressively changed with
linear characteristics. However, the frequency of the internal
count clock CKcnt may be changed in more levels (more precisely) so
that the DA output can be gradually and continuously changed by
using a high-order function, such as a quadratic function.
[0314] FIGS. 14 and 15 illustrate examples of mechanisms based on
the principle shown in FIG. 13 in the context that the dynamic
range is increased by performing temporal addition processing in
the column AD circuits 25 of the solid-state imaging device 1 shown
in FIG. 1.
[0315] The temporal addition processing described below is
characterized in that the dynamic range is increased by performing,
as the product-sum operation function, the temporal addition
processing. In this case, the temporal addition processing is
performed for a plurality of pixel signals captured by different
charge storage periods. Accordingly, a composite image as a
combination of images having different charge storage periods can
be obtained as a processed image. This processed image (composite
image) has a wide dynamic range.
[0316] For the temporal addition processing, when images having
different charge storage periods are handled, pixels subjected to
addition processing should be those disposed at the same position.
For a charge-storage period range, a long charge storage period is
set to be generally one frame period. If an electronic shutter
function is used, the long charge storage period can be decreased.
In contrast, a short charge storage period is set to be one
horizontal period (for example, 64 microseconds) or shorter. The
reason for this is as follows. Due to the property unique to the
CMOS sensors in which the charge storage period is different
depending on the horizontal line due to the scanning, when reading
out the pixel signals by scanning each horizontal line (scanning
line), after reading the pixel signal stored for the long storage
period in the same horizontal period, charge is stored for the
shorter charge storage period, and then, the pixel signal stored
for the shorter storage period is immediately read.
[0317] Even after reading out the n-bit digital value, the counter
254 can retain the digital value therein. In the temporal addition
processing, digital addition processing is executed on a plurality
of pixel signals by utilizing the retaining characteristic of the
counter 254.
[0318] When performing the addition computation by using the
counter 254, the combination of the counting modes for the reset
components .DELTA.V and the signal components Vsig in AD conversion
for signals to be processed is set to be the same between read-out
operations. That is, when performing the temporal addition
processing, after completing the AD conversion for first pixels
stored for a comparatively long storage period, without resetting
the counter 254, the AD conversion for second pixel signals is
performed for the reset components .DELTA.V and the signal
components Vsig by the same combination of the counting modes as
that for the AD conversion for the first pixels stored for a
comparatively long storage period. This is specifically discussed
below.
[0319] As shown in FIG. 14, for the pixel signal V1 of the first
pixel stored for a comparatively long period, by performing
down-counting during the first read-out operation and by performing
up-counting during the second read-out operation, only the electric
signal components Vsig1 in accordance with the quantity of incident
light on the unit pixel 3 can be extracted by the difference
processing within the counter 254 (t10 to t24). The count value
expressed by equation (2) retained in the counter 254 is an n-bit
digital value representing the positive signal voltage Vsig1.
[0320] Also for the pixel signal V2 of the second pixel stored for
a comparatively short period, down-counting is performed during the
first read-out operation, and up-counting is performed during the
second read-out operation. That is, AD conversion is performed by
the same combination of counting modes as that for the first pixel
signal V1 (t30 to t44). Accordingly, the subtraction processing
expressed by equation (1) is automatically performed in the counter
254, and the resulting count value is retained in the counter
254.
[0321] AD conversion for the pixel signal V2 of the second pixel
stored for a comparatively short period is performed by the same
combination of the counting modes as that for the pixel signal V1
of the first pixel stored for a comparatively long period.
Accordingly, the count value retained in the counter 254 becomes
the n-bit digital value representing a positive voltage (Vsig2), as
expressed by equation (3). Black reference components are omitted
in equation (3).
( Count value in the second comparison period ) - ( count value in
the first comparison period ) = - ( signal components Vsig 2 +
reset components .DELTA. V 2 + offset components in the column AD
circuit 25 ) + ( reset components .DELTA. V 2 + offset components
in the column AD circuit 25 ) = - ( signal components Vsig 2 ) ( 3
) ##EQU00002##
[0322] Upon completing the second counting operation on the pixel
signal V1 of the first pixel stored for a comparatively long
period, the first up-counting operation on the pixel signal V2 of
the second pixel stored for a comparatively short period is started
without resetting the count value retained in the counter 254.
Then, the count value expressed by equation (2) is added to the
count value expressed by equation (3).
[0323] Accordingly, the count value retained in the counter 254
after completing the second counting operation on the pixel signal
V2 of the second pixel stored for a comparatively short period is
the n-bit digital value representing the addition computation
result (Vsig1+Vsig2) between the two pixel signals V1 and V2, as
expressed by equation (4).
The count value for the first pixel signal + ( second comparison
period ) - ( first comparison period ) = ( signal components Vsig +
reset components .DELTA. V + offset components in the column AD
circuit 25 ) + ( reset components .DELTA. V + offset components in
the column AD circuit 25 ) = ( signal components Vsig 1 ) + (
signal components Vsig 2 ) ( 4 ) ##EQU00003##
[0324] At a predetermined timing (t48) after the completion of the
AD conversion for a plurality of pixel signals having different
charge storage periods, the communication/timing controller 20
instructs the horizontal scanning circuit 12 to read out the pixel
data. In response to this instruction, the horizontal scanning
circuit 12 sequentially shifts the horizontal selection signals
CH(i) to be supplied to the counters 254 via the control lines
12c.
[0325] Then, the count value expressed by equation (4)
stored/retained in the counter 254, i.e., the n-bit digital data
representing the addition result (Vsig1+Vsig2) between the two
pixel signals V1 and V2, can be sequentially output to the outside
the column processor 26 or the outside the chip having the pixel
portion 10 from the output terminal 5c via the n horizontal signal
lines 18. Thereafter, the operation is similarly repeated for pixel
signals of another row. As a result, computed data D2 representing
the two-dimensional addition image indicating the addition
computation result between a plurality of pixel signals having
different storage periods can be obtained.
[0326] It is now assumed that the dynamic range of the image sensor
is 60 dB and that the long charge storage period is set to be about
one frame period, for example, about 1/15 milliseconds, and the
short charge storage period is set to be one horizontal period or
shorter, about 1/15 microseconds. In this case, the sensor output
with respect to the light quantity for the long storage period
changes up to three orders of magnitudes in accordance with a
change in the light quantity. The sensor output with respect to the
light quantity for the short storage period also changes up to
three orders of magnitudes. However, the light quantity detected by
the pixels stored for the short storage period differs from that by
the pixels stored for the long storage period by three orders of
magnitudes.
[0327] According to the addition processing result (Vsig1+Vsig2)
obtained by adding the outputs in the first and second different
storage periods, the dynamic range of six orders of magnitudes,
i.e., 120 dB, can be implemented. For example, an image having
saturated portions detected by the long storage period can be
compensated for by an image detected by the short storage period.
Thus, the image portions at the saturation level or higher that are
difficult to output only by one storage period can be
reproduced.
[0328] The "one field period" is a period in which an image is read
by two-dimensionally scanning the imaging plane (more specifically,
one vertical scanning period), and the "one frame period" is a
period necessary for forming an image by all pixels on the imaging
plane. When sequential scanning (progressive scanning) for scanning
all rows is performed in the vertical direction, the "one field
period" is equal to the "one frame period". In contrast, when
interlaced scanning is performed in which some rows are
sequentially scanned in the vertical direction in the first
vertical scanning operation and the remaining rows are scanned in
the vertical direction in the second vertical scanning operation,
"k fields" are equal to "one frame". "k" indicates the number of
scanning operations, and normally, k is 2. Regardless of the
progressive scanning or the interlaced scanning, one vertical
scanning period for reading an image by two-dimensionally scanning
the imaging plane may be referred to as "one frame" in a broad
sense. In this specification, the frame in the following
description means the frame in a broad sense.
[0329] In actuality, if addition processing, such as that shown in
FIG. 14, is simply performed, it is difficult to achieve the ideal
knee characteristic in which the sensor output with respect to the
light quantity matches the luminosity. That is, the sensor output
does not match the human visual characteristic in which the
brightness is identified in proportion to the logarithm of the
light quantity.
[0330] To solve this problem, it is preferable that, by considering
the luminosity, the amount by which the reference signal used for
comparison processing is changed over time is adjusted so that the
coefficient for image signals subjected to addition processing is
set. Particularly, it is preferable that the amount by which the
reference signal is changed over time is adjusted for images
obtained under a comparatively short storage period. Then, the
luminosity can be adjusted without causing saturation of high level
signals that would be saturated under the condition of a normal
storage period.
[0331] More specifically, when conducting AD conversion for pixel
signals stored for the short storage period, as shown in FIG. 15,
the slope of the reference signal RAMP generated by the reference
signal generator 27 is preferably changed in several levels instead
of being changed linearly. Alternatively, instead of changing the
slope of the reference signal RAMP in different levels while
maintaining the linearity, it may be continuously and progressively
changed by using a high-order function, such as a quadric
function.
[0332] In this case, in accordance with the logarithmic
characteristic of the human eye, the grayscale precision in a dark
place is maintained so that the human eye is sensitive to a change
in the brightness in a dark place, and the grayscale precision in a
bright place is decreased so that the human eye is dull to a change
in the brightness in a bright place. More specifically, it is
preferable that the slope of the reference potential RAMP is set to
be smaller at the initial stage of the AD conversion so that the
factor can be large (higher gain), and as the AD conversion
proceeds, the slope of the reference potential RAMP is increased.
With this arrangement, the knee characteristic in which a high
luminance range is compressed in accordance with the luminous
characteristic of the human can be achieved.
[0333] To achieve this characteristic, for example, the cycle of
the count clock CKdac to be supplied to the DA conversion circuit
27a of the reference signal generator 27 is progressively
facilitated while the DA conversion circuit 27a reduces the voltage
for each clock pulse by .DELTA.RAMP. For example, the
communication/timing controller 20 supplies a clock which is scaled
down from the reference clock CKdac by the factor of 1/ma (for
example, 1/2) to the reference signal generator 27 from time t40 to
time Ta so that a potential having a straight line y1
(corresponding to 11b in FIG. 13) with a factor ma (for example, 2)
is supplied. Then, from time Ta to time Tb, the
communication/timing controller 20 supplies the reference count
clock CKdac so that a potential having a straight line y2
(corresponding to 10b in FIG. 13) is supplied, and after time Tb,
the communication/timing controller 20 supplies a clock which is
scaled up from the reference count clock CKdac by the factor of mb
(for example, 2) so that a potential having a straight line y3
(corresponding to 9b in FIG. 13) with a factor 1/mb (for example,
1/2) is supplied.
[0334] Alternatively, while setting the cycle of the count clock
CKdac supplied to the DA conversion circuit 27a of the reference
signal generator 27 to be constant, the counter output value x and
the slope (rate of change) .beta. contained in the control data CN4
are progressively adjusted. For example, from time t40 to time Ta,
the potential calculated by y1=.alpha.1 (initial value)-.beta.1*x
is output. From time Ta to time Tb, the potential calculated by
y2=.alpha.2 (initial value)-.beta.2*x is output. After time Tb, the
potential calculated by y3=.alpha.3 (initial value)-.beta.3*x is
output. It should be noted that .alpha.1<.alpha.2<.alpha.3,
for example, 1/2:1:2, and .beta.1<.beta.2<.beta.3.
[0335] With this arrangement, .gamma. correction is conducted by
providing high gain components in the short storage period and by
providing low gain components in the high luminance range, that is,
the knee characteristic in which the high luminance range can be
compressed in accordance with the luminous characteristic of the
human can be achieved.
[0336] Although in any of the above-described examples the
reference signal RAMP is changed linearly or by polygonal lines, it
may be changed non-linearly, such as exponentially or by using a
quadric function.
[0337] As described above, as the application of the temporal
addition processing, the following operation is performed. When
conducting AD conversion by performing the counting operation for a
plurality of pixel signals having different charge storage periods
at the same position, digital data obtained for one of the pixel
signals is used as the initial value for the counting operation for
the other pixel signal (pixel signal to be subjected to the second
counting operation). Accordingly, after conducting AD conversion by
performing the counting operation for the other pixel signal, n-bit
digital data indicating the addition computation result for all the
pixel signals can be automatically obtained and output as the
computed data D2. In this embodiment, as the computed data D2, a
signal output which is not easily saturated for a larger light
quantity can be obtained, and data that can increase the dynamic
range can be obtained. The bit width of the digital image data can
be maintained to n bits while implementing a wide dynamic range, in
other words, the bit width can be compressed. Accordingly, an image
having a wide dynamic range in accordance with the light quantity
while suppressing the overexposed highlight or the underexposed
shadow can be obtained.
[0338] Without additional circuits, such as an adder and a line
memory device, digital-value addition processing for a plurality of
pixel signals having different charge storage periods at the same
position can be performed according to the on-chip method. Since
images having different storage periods can be combined as a
digital value, the wide dynamic range can be implemented with a
simple structure without the need to add external circuits, such as
a frame memory, or internal circuits.
[0339] In the same horizontal period, after reading out a pixel
signal stored for a long storage period, charge is stored for a
short storage period, and then, the pixel signal stored for the
short storage period is read. Accordingly, the long-period storage
signal and the short-period storage signal are sequentially output
substantially at the same timing, thereby eliminating the need to
provide a memory for synchronizing the two signals with each
other.
[0340] Since the dynamic range is increased by combining two pixel
signals having different storage periods (if necessary, the number
of pixel signals having different storage periods may be
increased), a device having a normal pixel structure can be used
without the need for a dedicated pixel structure, such as an
inter-pixel memory. Thus, the type of sensor device is not
restricted.
[0341] As described above, if the slope of the reference signal
RAMP generated by the reference signal generator 27 is
progressively changed when single-slope-integrating AD conversion
is conducted, a wide dynamic range can be implemented by combining
different storage periods, and also, .gamma. correction can be made
to the sensitivity characteristic, thereby implementing a more
natural composite image. The difference in the sensitivity caused
by the different storage periods can be naturally compensated for,
and a more natural image can be synthesized.
[0342] It should be understood by those skilled in the art that
various modifications, combinations, sub-combinations and
alterations may occur depending on design requirements and other
factors insofar as they are within the scope of the appended claims
or the equivalents thereof.
[0343] For example, in the foregoing embodiment, although AD
conversion functions are disposed in the column regions located at
positions to read out signals from the pixel portion 10, they may
be provided at another portion. For example, analog pixel signals
may be output to the horizontal signal line 18, and then, they are
converted into digital signals and are delivered to the output
circuit 28.
[0344] In the aforementioned embodiment, the counting operation
after switching the counting modes is started from the final count
value before switching the counting modes. In this case, a special
arrangement is not necessary for switching the modes if an up/down
counter for outputting the count value in synchronization with the
count clock CK0 is used.
[0345] However, if an asynchronous up/down counter suitable for a
fast operation since the operation limit frequency is determined by
only the limit frequency of the first flip-flop (counter basic
element) is used, the count value is lost when switching the
counting modes, and thus, it is difficult to perform the correct
counting operation while maintaining the same value before and
after switching the counting modes. Accordingly, an adjustment
processor for allowing the counting operation after switching the
counting modes to start from the count value before switching the
counting modes is preferably disposed. Details of the adjustment
processor are not given here. If addition processing between a
plurality of signals is performed, the counting modes are the same
for the first and second counting operations, and thus, the
above-described arrangement is not necessary.
[0346] In the above-described embodiment, concerning a pixel in
which the signal components Vsig appear after the reset components
.DELTA.V (reference components) in the time sequential order, and
the second-stage processor processes a signal having a positive
sign (a signal having a higher level has a greater positive value),
and then, true components of the pixel are determined. In this
case, as the first processing operation, the comparison processing
and the down-counting operation are performed for the reset
components .DELTA.V (reference components), and as the second
processing operation, the comparison processing and the up-counting
operation are performed for the signal components Vsig. However,
regardless of the time-sequential order in which the reference
components and the signal components appear, the combination of the
signal components and the counting mode and the processing order
are arbitrary. Depending on the processing order, the digital data
obtained as a result of the second operation may be a negative
value. In this case, sign inversion or correction computation can
be performed.
[0347] According to the device architecture of the pixel portion 10
in which the reset components .DELTA.V (reference components)
should be read after the signal components Vsig and in which the
second-stage processor processes signals having positive signs, it
is more efficient to perform the comparison processing and the
down-counting operation for the signal components Vsig as the first
operation and to perform the comparison processing and the
up-counting operation for the reset components .DELTA.V (reference
components) as the second operation.
[0348] In the foregoing embodiment, assuming that signal components
Vsig appear after reset components .DELTA.V (reference components)
for the same pixel, difference processing for finding true signal
components is performed when performing the addition computation
between a plurality of pixel signals having different storage
periods. If only signal components Vsig can be subjected to the
processing without considering the reset components .DELTA.V
(reference components), the difference processing for finding the
true signal components can be omitted.
[0349] In the aforementioned embodiments, regardless of the
operation mode, the up/down counter is used in common to perform
the counting operation by switching the operation modes. The
counter is not restricted to an up/down counter that can switch the
modes as long as it performs the counting operation by a
combination of the down-counting mode and the up-counting mode.
[0350] In the foregoing embodiments, NMOS or PMOS unit pixels
disposed in a matrix are used for a sensor. However, the sensor is
not restricted to this type, and a line sensor may be used, in
which case, operations and advantages similar to those of the
above-described embodiments can be achieved.
[0351] In the above-described embodiments, as a solid-state imaging
device that can selectively read signals from desired unit pixels
by address control, a CMOS sensor provided with a pixel portion
that generates signal charge by receiving light is used. The
generation of signal charge is not restricted to light, but may be
to electromagnetic waves in general, such as infrared, ultraviolet,
or X rays. The above-described features disclosed in the foregoing
embodiment can be applied to a semiconductor device having unit
elements in which many devices for outputting an analog signal in
accordance with the quantity of electromagnetic waves are
disposed.
[0352] The solid-state imaging device described above may be a
one-chip type or a module type obtained by a combination of a
plurality of chips and having an imaging function. In addition, the
solid-state imaging device may be used for an electronic apparatus,
such as a camera or a cellular telephone, having an imaging
function.
[0353] By using a one-chip type or a module type, when implementing
a function of generating image signals, the functions necessary for
controlling the operations of the comparators and the counters can
be collectively handled, thereby facilitating the handling or
management of the components. Additionally, since the elements
necessary for AD conversion are integrated as an IC or a module,
the manufacturing of electronic apparatuses as finished products
can be facilitated.
[0354] A CMOS-type solid-state imaging device which is responsive
to electromagnetic waves, such as light or radiation, input from an
external source, has been discussed by way of example. However, the
mechanism discussed in the foregoing embodiment can be applied to
any type of device that detects a physical quantity distribution.
The mechanism for converting analog signals into digital signals
can be used for a device for detecting a physical quantity
distribution other than light, for example, for a fingerprint
authentication device (see, for example, Japanese Unexamined Patent
Application Publication Nos. 2002-7984 and 2001-125734) that
detects fingerprint images based on fingerprint information in
accordance with a change in electric characteristics or optical
characteristics caused by a pressure.
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