U.S. patent application number 15/890762 was filed with the patent office on 2019-08-08 for image sensor with dual conversion gain readout.
The applicant listed for this patent is OmniVision Technologies, Inc.. Invention is credited to Yandong Chen, Hiroaki Ebihara, Eiichi Funatsu, Rui Wang.
Application Number | 20190246057 15/890762 |
Document ID | / |
Family ID | 67220386 |
Filed Date | 2019-08-08 |
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United States Patent
Application |
20190246057 |
Kind Code |
A1 |
Ebihara; Hiroaki ; et
al. |
August 8, 2019 |
IMAGE SENSOR WITH DUAL CONVERSION GAIN READOUT
Abstract
A method includes reading a first analog reference signal from a
first storage node in a dual conversion gain pixel, and converting
the first analog reference signal to a first digital reference
signal using a comparator coupled to the dual conversion gain
pixel. The method also includes reading a first analog image signal
from the first storage node, and converting the first analog image
signal to a first digital image signal using the comparator. A
second analog image signal may be read from the first storage node
and a second storage node in the dual conversion gain pixel, and
the second analog image signal may be converted to a second digital
image signal. A second analog reference signal may be read from the
first storage node and the second storage node, and the second
analog reference signal may be converted to a second digital
reference signal using the comparator.
Inventors: |
Ebihara; Hiroaki; (Santa
Clara, CA) ; Wang; Rui; (San Jose, CA) ; Chen;
Yandong; (Santa Clara, CA) ; Funatsu; Eiichi;
(San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OmniVision Technologies, Inc. |
Santa Clara |
CA |
US |
|
|
Family ID: |
67220386 |
Appl. No.: |
15/890762 |
Filed: |
February 7, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04N 5/378 20130101;
H04N 5/3575 20130101; H01L 27/14605 20130101; H01L 27/14609
20130101; H01L 27/14643 20130101; H04N 5/2355 20130101; H04N
5/37452 20130101; H04N 5/37455 20130101; H04N 5/3559 20130101 |
International
Class: |
H04N 5/3745 20060101
H04N005/3745; H04N 5/378 20060101 H04N005/378; H01L 27/146 20060101
H01L027/146; H04N 5/355 20060101 H04N005/355; H04N 5/235 20060101
H04N005/235; H04N 5/357 20060101 H04N005/357 |
Claims
1. A method, comprising: reading a first analog reference signal
from a first storage node in a dual conversion gain pixel;
converting the first analog reference signal to a first digital
reference signal using a comparator coupled to the dual conversion
gain pixel; reading a first analog image signal from the first
storage node; converting the first analog image signal to a first
digital image signal using the comparator; reading a second analog
image signal from the first storage node and a second storage node
in the dual conversion gain pixel; converting the second analog
image signal to a second digital image signal using the comparator;
reading a second analog reference signal from the first storage
node and the second storage node; and converting the second analog
reference signal to a second digital reference signal using the
comparator.
2. The method of claim 1, wherein converting the first analog image
signal to the first digital image signal occurs between converting
the first analog reference signal to the first digital reference
signal and converting the second analog image signal to the second
digital image signal.
3. The method of claim 2, wherein converting the second analog
image signal to the second digital image signal occurs between
converting the first analog image signal to the first digital image
signal and converting the second analog reference signal to the
second digital reference signal.
4. The method of claim 1, further comprising outputting a ramp
signal to the comparator from a ramp circuit coupled to the dual
conversion gain pixel, wherein the ramp signal is used by the
comparator to convert the first analog reference signal to the
first digital reference signal, convert the first analog image
signal to the first digital image signal, convert the second analog
image signal to the second digital image signal, and convert the
second analog reference signal to the second digital reference
signal.
5. The method of claim 4, further comprising outputting an offset
signal to the comparator with the ramp circuit between converting
the first analog image signal to the first digital image signal and
converting the second analog image signal to the second digital
image signal.
6. The method of claim 5, wherein outputting the offset signal
includes adjusting an auto-zero voltage level output from the ramp
circuit.
7. The method of claim 4, wherein outputting the ramp signal
includes using a current source array coupled to a decoder to
generate the ramp signal.
8. The method of claim 4, wherein the decoder is coupled to gate
terminals of a plurality of switches to individually activate each
of the plurality of switches, and wherein the plurality of switches
is coupled between the current source array and an output of the
ramp circuit.
9. The method of claim 4, wherein outputting the ramp signal
includes using a current integrator coupled between a current
source, a current sink, and an output of the ramp circuit.
10. The method of claim 7, wherein outputting the ramp signal
includes controlling a direction of the ramp signal by turning on
one of a first switch coupled between the current source and the
current integrator or a second switch coupled between the current
sink and the current integrator.
11. An image sensor apparatus including an analog to digital
converter, comprising: a dual conversion gain pixel including a
photodiode coupled to a first storage node and a second storage
node; a ramp circuit coupled to the dual conversion gain pixel to
output a ramp signal; and a comparator coupled to the dual
conversion gain photodiode and the ramp circuit to receive the ramp
signal, wherein the comparator is configured to: convert a first
analog reference signal, output from the first storage node, to a
first digital reference signal; convert a first analog image
signal, output from the first storage node, to a first digital
image signal after converting the first analog reference signal;
convert a second analog image signal, output from the first storage
node and the second storage node, to a second digital image signal
after converting the first analog image signal; and convert a
second analog reference signal, output from the first storage node
and the second storage node, to a second digital reference signal
after converting the second analog image signal.
12. The apparatus of claim 11, wherein the first storage node
includes a floating diffusion, and the second storage node includes
at least one of a floating diffusion or a capacitor.
13. The apparatus of claim 11, wherein the ramp circuit is coupled
to output an offset signal to the comparator after converting the
first analog image signal to a first digital image signal.
14. The apparatus of claim 13, wherein the offset signal adjusts an
auto-zero voltage level output from the ramp circuit.
15. The apparatus of claim 11, wherein the ramp circuit includes a
current source array coupled to a decoder to generate the ramp
signal.
16. The apparatus of claim 15, wherein the decoder is coupled to
gate terminals of a plurality of switches to activate the plurality
of switches, and wherein the plurality of switches is disposed
between the current source array and an output of the ramp
circuit.
17. The apparatus of claim 11, wherein the ramp circuit includes a
current integrator coupled between a current source, a current
sink, and an output of the ramp circuit.
18. The apparatus of claim 17, wherein the ramp circuit includes a
first switch coupled between the current source and the current
integrator, and a second switch coupled between the current sink
and the current integrator, and wherein a direction of the ramp
signal is controlled by turning on the first switch or the second
switch.
19. The apparatus of claim 11, wherein the comparator includes: a
first PMOS transistor and a second PMOS transistor coupled in
parallel to a supply voltage, wherein the gate terminals of the
first PMOS transistor and the second PMOS transistor are coupled to
a drain terminal of the first PMOS transistor to form a current
mirror; a first NMOS transistor and a second NMOS transistor
coupled in parallel, wherein the a first NMOS transistor and a
second NMOS transistor are coupled to the first PMOS transistor and
the second PMOS transistor respectively; and an NMOS tail
transistor coupled to the first NMOS transistor, second NMOS
transistor, and ground.
20. The apparatus of claim 11, further comprising a plurality of
the dual conversion gain pixels including the dual conversion gain
pixel disposed, at least in part, in a semiconductor material and
arranged into an array.
Description
TECHNICAL FIELD
[0001] This disclosure relates generally to image sensors, and in
particular but not exclusively, relates to analog to digital
conversion (ADC).
BACKGROUND INFORMATION
[0002] Image sensors have become ubiquitous. They are widely used
in digital still cameras, cellular phones, security cameras, as
well as, medical, automobile, and other applications. The
technology used to manufacture image sensors has continued to
advance at a great pace. For example, the demands of higher
resolution and lower power consumption have encouraged the further
miniaturization and integration of these devices.
[0003] The typical image sensor operates as follows. Image light
from an external scene is incident on the image sensor. The image
sensor includes a plurality of photosensitive elements such that
each photosensitive element absorbs a portion of incident image
light. Photosensitive elements included in the image sensor, such
as photodiodes, each generate image charge upon absorption of the
image light. The amount of image charge generated is proportional
to the intensity of the image light. The generated image charge may
be used to produce an image representing the external scene.
[0004] The charge may be converted into a digital representation of
the charge by analog-to-digital converter (ADC) circuits in the
image sensor based on a comparison to a reference voltage signal.
However, if the amount of generated charge is more than the voltage
range of the ADC circuits, the digital representation of the charge
may not be correct.
[0005] Many techniques have been employed to increase the voltage
range of the ADC circuits. However, some of these techniques may
not provide the desired range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Non-limiting and non-exhaustive examples of the invention
are described with reference to the following figures, wherein like
reference numerals refer to like parts throughout the various views
unless otherwise specified.
[0007] FIG. 1A illustrates one example of an imaging system, in
accordance with the teachings of the present disclosure.
[0008] FIG. 1B illustrates a schematic of an example dual
conversion gain pixel, which may be included in the system of FIG.
1A, in accordance with the teachings of the present disclosure.
[0009] FIG. 2 illustrates a schematic of a comparator, in
accordance with the teachings of the present disclosure.
[0010] FIG. 3 illustrates relative voltage ranges of the comparator
in FIG. 2, in accordance with the teachings of the present
disclosure.
[0011] FIG. 4 illustrates a schematic of a binary ramp circuit
which may be coupled to the comparator of FIG. 2, in accordance
with the teachings of the present disclosure.
[0012] FIG. 5 illustrates a schematic of a current integration ramp
circuit which may be coupled to the comparator of FIG. 2, in
accordance with the teachings of the present disclosure.
[0013] FIGS. 6A-6F illustrate timing diagrams for electronics in,
and coupled to, the comparator depicted in FIG. 2, in accordance
with the teachings of the present disclosure.
[0014] FIG. 7 illustrates a method of operating an image sensor
with dual conversion gain readout, in accordance with the teachings
of the present disclosure.
[0015] Corresponding reference characters indicate corresponding
components throughout the several views of the drawings. Skilled
artisans will appreciate that elements in the figures are
illustrated for simplicity and clarity and have not necessarily
been drawn to scale. For example, the dimensions of some of the
elements in the figures may be exaggerated relative to other
elements to help to improve understanding of various embodiments of
the present invention. Also, common but well-understood elements
that are useful or necessary in a commercially feasible embodiment
are often not depicted in order to facilitate a less obstructed
view of these various embodiments of the present invention.
DETAILED DESCRIPTION
[0016] Examples of an apparatus, system and method relating an
image sensor with dual conversion gain readout are described
herein. In the following description, numerous specific details are
set forth to provide a thorough understanding of the examples. One
skilled in the relevant art will recognize, however, that the
techniques described herein can be practiced without one or more of
the specific details, or with other methods, components, materials,
etc. In other instances, well-known structures, materials, or
operations are not shown or described in detail to avoid obscuring
certain aspects.
[0017] Reference throughout this specification to "one example" or
"one embodiment" means that a particular feature, structure, or
characteristic described in connection with the example is included
in at least one example of the present invention. Thus, the
appearances of the phrases "in one example" or "in one embodiment"
in various places throughout this specification are not necessarily
all referring to the same example. Furthermore, the particular
features, structures, or characteristics may be combined in any
suitable manner in one or more examples.
[0018] With dual conversion gain (DCG) pixels, wider dynamic range
can be achieved by reading out one signal at low conversion gain
(LCG) and one at high conversion gain (HCG), and then combining the
signals together (resulting in high dynamic range (HDR) readout).
In DCG readouts, there is generally a large difference in the
bitline reset level between LCG readout and HCG readout. In some
solutions to this problem, comparators having two input stages
could be used, but the size of the comparator becomes larger than a
conventional comparator. This consumes additional space on the
integrated circuit chip. In this disclosure, DCG readout methods
with small size comparators are introduced. Additionally, a new
pixel readout scheme is proposed: HCG reset, followed by readout of
the HCG image signal, followed by readout of the LCG image signal,
and then LCG reset.
[0019] FIG. 1A illustrates one example of an imaging system 100 in
accordance with an embodiment of the present disclosure. Imaging
system 100 includes pixel array 102, control circuitry 104, readout
circuitry 108, and function logic 106. In one example, pixel array
102 is a two-dimensional (2D) array of photodiodes, or image sensor
pixels (e.g., pixels P1, P2 . . . , Pn). As illustrated,
photodiodes are arranged into rows (e.g., rows R1 to Ry) and
columns (e.g., column C1 to Cx) to acquire image data of a person,
place, object, etc., which can then be used to render a 2D image of
the person, place, object, etc. However, photodiodes do not have to
be arranged into rows and columns and may take other
configurations.
[0020] In one example, after each image sensor photodiode/pixel in
pixel array 102 has acquired its image data or image charge, the
image data is readout by readout circuitry 108 and then transferred
to function logic 106. Readout circuitry 108 may be coupled to
readout image data from the plurality of photodiodes in pixel array
102. In various examples, readout circuitry 108 may include
amplification circuitry, analog-to digital (ADC) conversion
circuitry, or otherwise. In some examples, one or more comparators
112 may be included for each of the readout columns. Function logic
106 may simply store the image data or even manipulate the image
data by applying post image effects (e.g., crop, rotate, remove red
eye, adjust brightness, adjust contrast, or otherwise). In one
example, readout circuitry 108 may readout a row of image data at a
time along readout column lines (illustrated) or may readout the
image data using a variety of other techniques (not illustrated),
such as a serial readout or a full parallel readout of all pixels
simultaneously.
[0021] To perform ADC, for example, the readout circuitry 108 may
receive a reference voltage VRAMP from a ramp circuit 110. VRAMP
may be received by the comparator 112, which may also receive image
charge from a pixel of the pixel array 102. The comparator 112 may
determine a digital representation of the image charge based on a
comparison of VRAMP to the image charge voltage level. The signal
VRAMP may be at various voltage levels depending on the timing of
the ADC operation, and may be used to auto-zero the inputs of the
comparator 112 before image charge is received, e.g., read, by the
readout circuitry 108. In some examples, VRAMP may be increased
when the inputs are auto-zeroed, which may provide an increase to
the voltage range used during signal comparison, for example.
Although ramp circuit 110 is shown as a separate block of the
imaging system 100, ramp circuit 110 may be included in other
blocks, such as the column readout circuit 108, or a voltage
generator block (not shown).
[0022] In one example, control circuitry 104 is coupled to pixel
array 102 to control operation of the plurality of photodiodes in
pixel array 102. For example, control circuitry 104 may generate a
shutter signal for controlling image acquisition. In one example,
the shutter signal is a global shutter signal for simultaneously
enabling all pixels within pixel array 102 to simultaneously
capture their respective image data during a single acquisition
window. In another example, the shutter signal is a rolling shutter
signal such that each row, column, or group of pixels is
sequentially enabled during consecutive acquisition windows. In
another example, image acquisition is synchronized with lighting
effects such as a flash.
[0023] In one example, imaging system 100 may be included in a
digital camera, cell phone, laptop computer, or the like.
Additionally, imaging system 100 may be coupled to other pieces of
hardware such as a processor (general purpose or otherwise), memory
elements, output (USB port, wireless transmitter, HDMI port, etc.),
lighting/flash, electrical input (keyboard, touch display, track
pad, mouse, microphone, etc.), and/or display. Other pieces of
hardware may deliver instructions to imaging system 100, extract
image data from imaging system 100, or manipulate image data
supplied by imaging system 100.
[0024] FIG. 1B illustrates a schematic of an example dual
conversion gain pixel (e.g., PD1), which may be included in the
system in FIG. 1A. Illustrated are photodiode 101, transfer
transistor 103, first storage node 105 (e.g., a floating
diffusion), dual floating diffusion transistor 107, second storage
node 109 (e.g., including a floating diffusion and capacitor 111),
reset transistor 113, select transistor 115, and source follower
transistor 117.
[0025] As shown, all components may be disposed in a semiconductor
material or substrate. Photodiode 101 transfers first analog image
charge (generated in response to incident light on photodiode 101)
to first storage node 105 when a transfer signal is applied to the
gate terminal of transfer transistor 103. Source follower
transistor 117 may then amplify the first analog signal on first
storage node 105, and that signal may be read out to the bitline
when select transistor 115 is turned on. Reading out the charge on
first storage node 105 may be used to form an image with high
conversion gain. Similarly, charge may be read out of photodiode
101 to first storage node 105, and, if additional charge needs to
be absorbed, dual floating diffusion gate 107 may be turned on to
store charge in second storage node 109 (and capacitor 111). Source
follower transistor 117 may then be used to amplify the second
analog image charge contained in first storage node 105 and second
storage node 107 which is readout to the bitline. This image data
may be used to form an image with low conversion gain. Image charge
in the dual conversion gain pixel may be reset by turning on reset
gate (and other transistors).
[0026] In some examples, a first analog reference signal may be
read out of first storage node 105 when no image charge is in first
storage node 105. A second analog reference signal may be read out
of both first storage node 105 and second storage node 109 when no
image charge is stored in nodes 105/109. It is appreciated that the
first and second reference signals may be subtracted from the image
signals and may be used to correct for noise. Moreover, the low
conversion gain and high conversion gain image signals may be
combined to form a high dynamic range image.
[0027] FIG. 2 is a schematic of comparator 212 in accordance with
an embodiment of the present disclosure. Comparator 212 may be an
example of comparator 112 of FIG. 1A. Comparator 212 may provide an
output signal in response to a comparison of an image charge signal
received on the bitline input to a reference voltage signal VRAMP
received on the VRAMP input.
[0028] In one example, a first analog reference signal may be read
from a first storage node in a dual conversion gain pixel (see
e.g., PD1 in FIG. 1A) and comparator 212 may be used, at least in
part, to convert the first analog reference signal to a first
digital reference signal. Then, a first analog image signal may be
read from the first storage node in the dual conversion gain pixel,
and comparator 212 may convert the first analog image signal to a
first digital image signal. Following this, a second analog image
signal may be read from the first storage node and a second storage
node in the dual conversion gain pixel, and comparator 212 may be
used to convert the second analog image signal to a second digital
image signal. Next, a second analog reference signal may be read
from the first storage node and the second storage node in the dual
conversion gain pixel, and comparator 212 may convert the second
analog reference signal to a second digital reference signal.
[0029] The illustrated embodiment of comparator 212 includes an
NMOS tail transistor coupled between a reference voltage, ground in
some examples, and two (first and second) NMOS input transistors.
The NMOS tail transistor may be coupled to receive a TAIL BIAS
signal at a gate, which may enable/disable the NMOS tail
transistor, and in turn comparator 212. The first and second NMOS
input transistors may be coupled in parallel between the NMOS tail
transistor and first and second PMOS transistors. The two PMOS
transistors may be coupled in parallel between the two NMOS input
transistors and a reference voltage, VDD for example. The two PMOS
transistors may further be coupled together by their gates.
Additionally, the comparator 212 may include two auto-zero
switches, AZQ1 and AZQ2, coupled between gates of respective ones
of the two NMOS input transistors and the drain sides of the same
NMOS input transistors. In some examples, two noise filtering
capacitors, may be included to capacitvely couple their respective
inputs to the gates of the two NMOS input transistors.
[0030] The comparator 212 may be described as having a reference
voltage input side and a bitline, e.g., image charge, input side.
The reference voltage input side may receive a reference voltage
VRAMP, and the bitline input side may receive an image charge
voltage signal. The reference voltage input side may include one of
the NMOS input transistors, auto-zero switch AZQ1, and one of the
PMOS transistors. The gate of the PMOS transistor of the reference
voltage input side may be coupled to a drain of the same
transistor, which may result in the gate both PMOS transistors
coupled to the drain forming a current mirror. The image charge
input side may include one of the NMOS transistors, auto-zero
switch AZQ2, and one of the PMOS transistors.
[0031] In operation, comparator 212 may compare a voltage level of
an image charge signal on the bitline input to a voltage level of
the VRAMP reference signal during an ADC operation. The voltage
range of the VRAMP signal during ADC, may be used to determine the
image charge level, for example. However, prior to ADC, the inputs
to the comparator, e.g., VRAMP and bitline, may be normalized,
e.g., auto-zeroed, to a base voltage level of of the PMOS
transistors, which may be referred to herein as the auto-zero
voltage level. Control signals A and B provided to the gates of
AZQ1 and AZQ2, respectively, may couple/decouple the gates of the
NMOS input transistors from their respective drains before an ADC
operation is performed. Normalizing the voltages on the gates of
the NMOS input transistors may force the inputs to the same voltage
level so to remove any error or noise from the ADC operation.
[0032] In some examples, increasing the auto-zero voltage level may
allow comparator 212 to determine larger image charge values. One
technique to increase the auto-zero voltage level may be to
increase the voltage of VRAMP during an auto-zero operation of the
comparator 212. For example, a voltage level of VRAMP may be
increased while the comparator 212 is normalized, such as after
AZQ1 is disabled but before AZQ2 is disabled. Increasing the
auto-zero voltage may result in an increase in the voltage range of
the comparator 212 because the voltage difference between the
auto-zero voltage level and a minimum input voltage level may be
increased, which may result in the increase in the voltage range of
the comparator 212.
[0033] Comparator 212 can be reset by making XAZ_1 and XAZ_2 logic
low. The reset operation ends by turning both of XAZ_1 and XAZ_2
logic high. XAZ_1 may be turned to logic high prior to, or at the
same time as, XAZ_2 is turned logic high. Comparator 212 may be
reset before ADC for HCG Reset. This way, the operating point of
comparator 212 is adjusted to the bitline voltage at HCG readout.
When comparator 212 is reset, kT/C noise may be induced in the
floating nodes, Vinn and Vinp. But the reset noise of comparator
212 and the comparator offset can be cancelled by subtracting the
output at HCG reset from output at HCG signal because comparator
212 is reset only once during HCG readout.
[0034] Similarly, the comparator is reset before ADC for the LCG
signal. This way, the operating point of the comparator is adjusted
to the bitline voltage at LCG mode. Also the reset noise of the
comparator 212 and the comparator offset can be cancelled by
subtracting reset and signal outputs for LCG, because comparator
212 is reset only once during LCG readout.
[0035] FIG. 3 illustrates relative voltage ranges of the comparator
in FIG. 2, in accordance with the teachings of the present
disclosure. As stated above, one problem with designing devices for
this application is that the voltage range of the comparator may
not be adequate. For example, the comparator has larger voltage
range on the lower side but smaller voltage range for the higher
side. In examples of disclosure, the low conversion gain (LCG)
image signal is readout prior to LCG reset. The bitline voltage may
be higher after the comparator is reset for LCG signal readout.
This means that the initial voltage range of the comparator for
receiving the LCG signal to the reset process may be lowered. Using
the ramp and offset techniques discussed herein, this disclosure
keeps the voltage supplied to the comparator in an operational
range.
[0036] FIG. 4 illustrates a schematic of an example ramp circuit
450 which may be coupled to the comparator of FIG. 2 to generate a
ramp signal. By changing the output of decoder 451, the ramp
generator can generate the signal shown in FIGS. 6A-6B. As shown,
ramp circuit 450 includes using a current source array 453 coupled
to decoder 451 to generate the ramp signal. Decoder 451 is coupled
to gate terminals of a plurality of switches 455 to individually
activate each of the plurality of switches 455. Plurality of
switches 455 is coupled between the current source array 453 and an
output of the ramp circuit 450. In the depicted example, a driver
is also coupled between the current source array 453 and the output
of ramp circuit 450. A resistor ("Rout") is also coupled between
ground, current source array 453 and the output of ramp circuit
450.
[0037] FIG. 5 illustrates a schematic of an example current
integration ramp circuit 560 which may be coupled to the comparator
of FIG. 2 to generate a ramp signal. As shown, ramp circuit 560
includes a current integrator (e.g., an operational amplifier with
a capacitor and switch) coupled between current source 561, a
current sink 569, and an output of the ramp generator. As shown
first switch 563 is coupled between current source 561 and the
current integrator 565, and second switch 567 is coupled between
current sink 569 and current integrator 565. This configuration
allows for controlling a direction of the ramp signal (see e.g.,
FIG. 6D, ramp signal going down for HCG signals, and ramp signal
going up for LCG signals) by turning on first switch 563 or second
switch 567. Turning on first switch 563 or the second switch 567
connects an input of the amplifier to the current source 561 or the
current sink 569. This helps to keep the comparator within its
prescribed limits.
[0038] In current integration, the ramp generator 560 may output
the ramp voltage offset between: (1) the 1.sup.st comparator reset
and ramp start of HCG reset; (2) the 2.sup.nd comparator reset and
LCG signal (e.g., shown in FIGS. 6A-6B); (3) XAZ_1 to logic high
and XAZ_2 to logic high (for example in FIG. 6B). Voltage offsets
can be generated by changing "Vref" voltage (as shown in FIG. 6C),
or they can be generated by sinking or sourcing current to "integ."
node in FIG. 5. However, large voltage offsets between the LCG
signal and LCG reset may be generated by sinking current through
the current source, because the offset should match the ramp signal
accurately and not to change the operating point of the current
integrator too much. This offset may be added to the output of LCG
reset so that the error in the voltage offset will add up to the
final output.
[0039] FIGS. 6A-6F illustrate timing diagrams for electronics in,
and coupled to, the comparator depicted in FIG. 2, in accordance
with the teachings of the present disclosure. It is appreciated
that the timing diagrams depicted are just one of many ways to
control the circuit in FIG. 2.
[0040] FIG. 6A depicts operation of a comparator (e.g., comparator
212 in FIG. 2). As depicted the readout/ADC conversion sequence is:
(1) HCG reset; (2) HCG signal; (3) LCG signal; (4) LCG reset. The
length of the ramp is used to convert the image and reference
signals into the digital domain. However, as described in
connection with FIG. 3, a small comparator may not have the
requisite voltage range.
[0041] FIG. 6B depicts a similar timing diagram as FIG. 6A, however
FIG. 6B includes using a negative offset of the ramp signal to
decrease the reset level of the comparator. As shown, the ramp
voltage is increased after comparator is reset before starting ADC
for the LCG reset. If the voltage of the floating nodes, Vinn and
Vinp (see e.g., nodes in FIG. 2), are increased too much, the input
devices of the comparator can slip into the linear region at the
flipping point. In the linear region, the gain of the comparator
becomes small so that error can be large when the comparator makes
its match decision. In general, the voltage increment in the
floating nodes, Vinn and Vinp, between when the comparator is reset
and when the comparator makes a decision should be much smaller
than the threshold voltage of the input devices if the comparator
is reset in the manner shown in FIG. 6A.
[0042] In some examples, the voltage range of the ADC can be
increased by changing the reset sequence. As shown in FIG. 6B, the
voltage of the ramp signal is decreased after XAZ_1 is turned to
logic high. When XAZ_1 is logic high and XAZ_2 is logic low, the
comparator operates as voltage follower and "Vout" and "Vinn" are
almost same voltage as "Vinp". Accordingly, if the ramp voltage is
decreased, "Vinp", "Vout" and "Vinn" also decrease. This way,
"Vinp" and "Vinn" can be lowered when the comparator is reset. This
will put input devices in the saturation region, regardless of
whether the voltages of bitline and ramp signal are increased after
the reset or not.
[0043] FIG. 6C depicts a similar timing diagram as FIG. 6B, however
FIG. 6C includes using bidirectional ramp signals (e.g., signals
produced by the ramp circuit in FIG. 5). The voltage offset between
the LCG signal and the LCG reset are generated by sinking current
from "integ" node (see e.g., FIG. 5), and the other voltage offsets
are generated by "Vref". One of ordinary skill in the art will
appreciate that other offsets also can be generated by the current
sink or current source current sources in the same way.
[0044] FIG. 6D is similar to FIG. 6A, however, FIG. 6D employs a
bidirectional ramp signal during the LCG signal and the LCG reset
readouts. Put another way, the ramp signals are in opposite
directions. This way, there is no need to generate a voltage offset
between the LCG signal and the LCG reset, the device simply needs
to reset the integrator by turning "integ_en" signal (see e.g.,
FIG. 5) to logic low to short the capacitor.
[0045] FIG. 6E is similar to FIG. 6B, however, FIG. 6E employs both
an offset voltage and a bidirectional ramp signal for the readouts
of the LCG signal and LCG reset.
[0046] FIG. 6F depicts the current integration ADC control signal
for the timing diagram of FIG. 6E. It is appreciated that the ramp
circuit in FIG. 5 may be used to generate the ramp signal depicted.
It is also appreciated that while there is no express need to
generate voltage offset between LCG signal and LCG reset, the
offset voltage can be generated by using current source and current
sinks of the device depicted in FIG. 5. However, this may enhance
the efficiency of the comparator.
[0047] FIG. 7 illustrates a method 700 of operating an image sensor
with dual conversion gain readout. One of ordinary skill in the art
having the benefit of the present disclosure will appreciate that
some of blocks 701-715 in FIG. 7 may occur in a different order
than discussed and even in parallel. Moreover, blocks may be added
to, or removed from, method 700 in accordance with the teachings of
the present disclosure.
[0048] Block 701 shows reading a first analog reference signal
(e.g., no stored image charge) from a first storage node (e.g.,
first storage node 109 in FIG. 1B) in a dual conversion gain pixel.
In some examples, reading may include amplifying the voltage on the
storage node using a source follower transistor and reading the
amplified signal from a bitline.
[0049] Block 703 depicts converting the first analog reference
signal to a first digital reference signal using a comparator
(e.g., comparator 212 in FIG. 2) coupled to the dual conversion
gain pixel. The first digital reference signal may be used to
remove noise in the high conversion gain image data (see
below).
[0050] Block 705 illustrates reading a first analog image signal
(e.g., charge generated from a photodiode 101 of FIG. 1B absorbing
light) from the first storage node. As with reading out the first
analog reference signal, this may involve amplifying the signal on
the storage node with a source follower and reading the amplified
signal out to a bitline.
[0051] Block 707 shows converting the first analog image signal to
a first digital image signal using the comparator. In the depicted
example, converting the first analog image signal to the first
digital image signal occurs between converting the first analog
reference signal to the first digital reference signal and
converting the second analog image signal to the second digital
image signal. Put another way, block 707 occurs temporally after
block 703 and before block 711. The first analog image signal may
be used to form high conversion gain image data.
[0052] In some examples a ramp circuit (see e.g., FIGS. 4-5) is
coupled to the comparator to output an offset signal after
converting the first analog image signal to the first digital image
signal. Put another way, the offset signal may be received by the
comparator between blocks 707 and 711. In some examples, the offset
signal includes adjusting an auto-zero voltage level output from
the ramp circuit.
[0053] Block 709 depicts reading a second analog image signal from
the first storage node and a second storage node (e.g., second
storage node 109 including capacitor 111 in FIG. 1B) in the dual
conversion gain pixel.
[0054] Block 711 illustrates converting the second analog image
signal to a second digital image signal using the comparator. In
the depicted example, converting the second analog image signal to
the second digital image signal occurs between converting the first
analog image signal to the first digital image signal and
converting the second analog reference signal to the second digital
reference signal. Put another way block 711 occurs after block 707
but before block 715. The second analog reference signal may be
used to form low conversion gain image data because charge is read
out of two storage nodes (and, in some examples, a capacitor as
well).
[0055] Block 713 depicts reading a second analog reference signal
from the first storage node and the second storage node.
[0056] Block 715 illustrates converting the second analog reference
signal to a second digital reference signal using the comparator.
The second analog reference signal may be used to remove noise in
the low conversion gain image data.
[0057] It is appreciated that the converting steps shown in blocks
703, 707, 711, and 715 may be, at least in part, completed by
outputting a ramp signal to the comparator from a ramp circuit
coupled to the dual conversion gain pixel. As stated, the ramp
signal is used by the comparator to convert the first analog
reference signal to the first digital reference signal, convert the
first analog image signal to the first digital image signal,
convert the second analog image signal to the second digital image
signal, and convert the second analog reference signal to the
second digital reference signal. In some examples, outputting the
ramp signal includes using a current source array coupled to a
decoder to generate the ramp signal, and the decoder is coupled to
gate terminals of a plurality of switches to activate the plurality
of switches. In one example, the ramp signal is generated using a
current integrator coupled between a current source, a current
sink, and an output of the ramp circuit. And outputting the ramp
signal includes controlling a direction of the ramp signal by
turning on one of a first switch coupled between the current source
and the current integrator, or a second switch coupled between the
current sink and the current integrator.
[0058] In the above description, various pieces of circuitry are
described. One of ordinary skill in the art having the benefit of
the present disclosure will appreciate that the timing of
electrical signals applied to the various pieces of circuitry can
occur in any order and even in parallel. Moreover, pieces of
circuitry and timing events may be added or removed, in accordance
with the teachings of the present disclosure. It is appreciated
that transistors may be referred to as "switches" or "gates" in
accordance with the teachings of the present disclosure.
[0059] The above description of illustrated examples of the
invention, including what is described in the Abstract, is not
intended to be exhaustive or to limit the invention to the precise
forms disclosed. While specific examples of the invention are
described herein for illustrative purposes, various modifications
are possible within the scope of the invention, as those skilled in
the relevant art will recognize.
[0060] These modifications can be made to the invention in light of
the above detailed description. The terms used in the following
claims should not be construed to limit the invention to the
specific examples disclosed in the specification. Rather, the scope
of the invention is to be determined entirely by the following
claims, which are to be construed in accordance with established
doctrines of claim interpretation.
* * * * *