U.S. patent application number 16/032752 was filed with the patent office on 2020-01-16 for common mode rejection in reservoir capacitor analog-to-digital converter.
The applicant listed for this patent is Analog Devices Global Unlimited Company. Invention is credited to Sandeep Monangi.
Application Number | 20200021305 16/032752 |
Document ID | / |
Family ID | 68979643 |
Filed Date | 2020-01-16 |
![](/patent/app/20200021305/US20200021305A1-20200116-D00000.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00001.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00002.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00003.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00004.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00005.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00006.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00007.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00008.png)
![](/patent/app/20200021305/US20200021305A1-20200116-D00009.png)
United States Patent
Application |
20200021305 |
Kind Code |
A1 |
Monangi; Sandeep |
January 16, 2020 |
COMMON MODE REJECTION IN RESERVOIR CAPACITOR ANALOG-TO-DIGITAL
CONVERTER
Abstract
A differential digital-to-analog (DAC) circuit that can include
a reservoir capacitor and various switches to couple the bottom
plates of the input capacitors, e.g., bit-trial capacitors, to
reference voltages, e.g., REF+ or REF-. In this manner, the
reservoir capacitor can be used to provide any differential charge
to the input capacitors, e.g., bit-trial capacitors, and the
reference voltages, e.g., REF+ and REF-, can be used to provide any
common mode charge to the input capacitors.
Inventors: |
Monangi; Sandeep;
(Srikakulam, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Analog Devices Global Unlimited Company |
Hamilton |
|
BM |
|
|
Family ID: |
68979643 |
Appl. No.: |
16/032752 |
Filed: |
July 11, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03M 1/804 20130101;
H03M 1/1245 20130101; H03M 1/68 20130101; H03M 1/10 20130101; H03M
1/462 20130101; G11C 27/026 20130101; G11C 27/02 20130101; H03M
1/002 20130101; H03M 1/06 20130101; H03M 1/0612 20130101; H03M
1/468 20130101; H03M 1/466 20130101; H03M 1/802 20130101; H03M
1/0607 20130101 |
International
Class: |
H03M 1/46 20060101
H03M001/46; H03M 1/06 20060101 H03M001/06; H03M 1/12 20060101
H03M001/12; H03M 1/68 20060101 H03M001/68 |
Claims
1. A differential digital-to-analog (DAC) circuit comprising: a
capacitor array including a number of DAC units, each DAC unit
including: a pair of input capacitors configured to couple to a
comparator; a dedicated reference capacitor associated with the
pair of input capacitors; and a control circuit configured to:
control operation of a first set of switches to transfer a
differential residue charge from the dedicated reference capacitor
to the pair of input capacitors when setting the pair of input
capacitors in a differential configuration based on a decision of
the comparator; and control operation of a second set of switches
to transfer a common-mode residue charge from a reference voltage
to set the pair of input capacitors when setting the pair of input
capacitors in a common-mode configuration based on a decision of
the comparator.
2. The differential DAC circuit of claim 1, wherein the input
capacitors are bit-trial capacitors, and wherein the control
circuit configured to control operation of the first set of
switches to transfer the differential residue charge from the
dedicated reference capacitor to the pair of input capacitors when
setting the pair of input capacitors in the differential
configuration based on the decision of the comparator is configured
to: control operation of the first set of switches to directly
couple or cross-couple plates of the reference capacitor to a first
plate of a first one of the pair of bit-trial capacitors and a
first plate of a second one of the pair of bit-trial capacitors
only when setting the pair of bit-trial capacitors in a
differential configuration based on a bit-trial result to transfer
the differential residue charge during the conversion phase, and
wherein a second plate of the first one of the pair of bit-trial
capacitors and a second plate of the second one of the pair of
bit-trial capacitors are configured to couple to inputs of the
comparator.
3. The differential DAC circuit of claim 1, wherein the input
capacitors are bit-trial capacitors, and wherein the control
circuit configured to control operation of the second set of
switches to transfer the common-mode residue charge from the
reference voltage to set the pair of input capacitors when setting
the pair of input capacitors in the common-mode configuration based
on the decision of the comparator is configured to: control
operation of the second set of switches to directly couple the
reference voltage to a first plate of a first one of the pair of
bit-trial capacitors and a first plate of a second one of the pair
of bit-trial capacitors only when setting the pair of bit-trial
capacitors in the common-mode configuration based on the bit-trial
result to transfer the common-mode residue charge during a
conversion phase, wherein a second plate of the first one of the
pair of bit-trial capacitors and a second plate of the second one
of the pair of bit-trial capacitors are configured to couple to
inputs of the comparator.
4. The differential DAC circuit of claim 1, wherein each of the
input capacitors includes a first plate and a second plate, wherein
setting the pair of input capacitors in the differential
configuration based on the decision of the comparator includes
setting the first plates of the input capacitors to one of two
states in a first set of states: 01 and 10, and wherein setting the
pair of input capacitors in the common-mode configuration based on
a decision of the comparator includes setting the first plates of
the input capacitors to one of two states in a second set of two
states: 11 and 00.
5. The differential DAC circuit of claim 1, wherein the control
circuit is configured to control a third set of switches to couple
the dedicated reference capacitor to the reference voltage during
the sampling phase.
6. The differential DAC circuit of claim 1, in combination with a
differential analog-to-digital converter (ADC) circuit for
converting an analog input to a digital output, the differential
ADC circuit comprising a first ADC circuit.
7. The differential DAC circuit of claim 6, wherein the input
capacitors are bit-trial capacitors, and wherein the first ADC
circuit is a successive approximation register ADC.
8. The differential DAC circuit of claim 6, w herein fife input
capacitors are bit-trial capacitors, wherein the differential ADC
circuit further comprises: an auxiliary ADC circuit having a
resolution less than a resolution of the first ADC circuit, wherein
the control circuit is further configured to: couple the analog
input signal onto the auxiliary ADC circuit; perform at least one
bit-trial using the auxiliary ADC circuit; and load an output of
the auxiliary ADC circuit onto at least one of the bit-trial
capacitors of the first ADC circuit.
9. The differential DAC circuit of claim 1, wherein the number of
DAC units includes most significant bit (MSB) DAC units and least
significant bit (LSB) DAC units, wherein the LSB DAC units do not
include the second set of switches to transfer the common-mode
residue charge from the reference voltage to set the pair of input
capacitors when setting the pair of input capacitors in the
common-mode configuration based on the decision of the
comparator.
10. The differential DAC circuit of claim 1, wherein setting the
pair of input capacitors in the differential configuration based on
the decision of the comparator includes transferring a first
charge, the control circuit is further configured to: control
operation of the second set of switches to couple the reference
voltage to the previously set bit-trial capacitor to transfer a
second charge.
11. A method of operating a differential analog-to-digital
converter (ADC) circuit to convert an analog input to a digital
output, the method comprising: coupling the analog input onto a
capacitor array of a first ADC circuit, the capacitor array
including a number of DAC units, each DAC unit including: a pair of
input capacitors; and a dedicated reference capacitor associated
with the pair of input capacitors; and transferring a differential
residue charge from reservoir the dedicated reference capacitor to
the pair of input capacitors when setting the pair of input
capacitors in a differential configuration based on a decision of a
comparator; and transferring a common-mode residue charge from a
reference voltage to set the pair of bit-trial capacitors when
setting the pair of bit-trial capacitors in a common-mode
configuration based on a decision of the comparator.
12. The method of claim 11, wherein the input capacitors are
bit-trial capacitors, and wherein transferring the differential
residue charge from the dedicated reference capacitor to the pair
of input capacitors when setting the pair of input capacitors in
the differential configuration based on the decision of the
comparator includes: controlling operation of a first set of
switches to directly couple or cross-couple plates of the reference
capacitor to a first plate of a first one of the pair of bit-trial
capacitors and a first plate of a second one of the pair of
bit-trial capacitors only when setting the pair of bit-trial
capacitors in the differential configuration based on the bit-trial
result to transfer the differential residue charge during the
conversion phase; and coupling a second plate of the first one of
the pair of bit-trial capacitors and a second plate of the second
one of the pair of bit-trial capacitors to inputs of the
comparator.
13. The method of claim 11, wherein the input capacitors are
bit-trial capacitors, and wherein transferring the common-mode
residue charge from the reference voltage to set the pair of input
capacitors when setting the pair of input capacitors in the
common-mode configuration based on the decision of the comparator
includes: controlling operation of a second set of switches to
directly couple the reference voltage to a first plate of a first
one of the pair of bit-trial capacitors and a first plate of a
second one of the pair of bit-trial capacitors only when setting
the pair of bit-trial capacitors in the common-mode configuration
based on the bit-trial result to transfer the common-mode residue
charge during a conversion phase; and coupling a second plate of
the first one of the pair of bit-trial capacitors and a second
plate of the second one of the pair of bit-trial capacitors to
inputs of the comparator.
14. The method of claim 11, wherein each of the input capacitors
includes a first plate and a second plate, wherein setting the pair
of input capacitors in the differential configuration based on the
decision of the comparator includes setting the first plates of the
input capacitors to one of two states in a first set of states: 01
and 10, and wherein setting the pair of input capacitors in the
common-mode configuration based on the decision of the comparator
includes setting the first plates of the input capacitors to one of
two states in a second set of two states: 11 and 00.
15. The method of claim 11, further comprising: controlling a third
set of switches to couple the reference capacitor to the reference
voltage during the sampling phase.
16. The method of claim 11, wherein the input capacitors are
bit-trial capacitors, wherein the differential ADC circuit further
comprises an auxiliary ADC circuit having a resolution less than a
resolution of the first ADC circuit, the method further comprising:
coupling the analog input signal onto the auxiliary ADC circuit;
performing at least one bit-trial using the auxiliary ADC circuit;
and loading an output of the auxiliary ADC circuit onto at least
one of the bit-trial capacitors of the first ADC circuit.
17. The method of claim 11, wherein setting the pair of input
capacitors in the differential configuration based on the decision
of the comparator includes transferring a first charge, the method
further comprising: coupling the reference voltage to the
previously set bit-trial capacitor to transfer a second charge.
18. A differential digital-to-analog (DAC) circuit comprising:
means for coupling an analog input onto a capacitor array of a
first ADC circuit, the capacitor array including a number of DAC
units, each DAC unit including: a pair of input capacitors; and a
dedicated reference capacitor associated with the pair of input
capacitors; and means for transferring a differential residue
charge from the dedicated reference capacitor to the pair of input
capacitors when setting the pair of input capacitors in a
differential configuration based on a decision of a comparator; and
means for transferring a common-mode residue charge from a
reference voltage to set the pair of bit-trial capacitors when
setting the pair of bit-trial capacitors in a common-mode
configuration based on a decision of the comparator.
19. The differential DAC circuit of claim 18, wherein the input
capacitors are bit-trial capacitors, and wherein the means for
transferring the differential residue charge from the dedicated
reference capacitor to the pair of input capacitors when setting
the pair of input capacitors in the differential configuration
based on the decision of the comparator includes: means for
controlling operation of a first set of switches to directly couple
or cross-couple plates of the reference capacitor to a first plate
of a first one of the pair of bit-trial capacitors and a first
plate of a second one of the pair of bit-trial capacitors only when
setting the pair of bit-trial capacitors in the differential
configuration based on a bit-trial result to transfer the
differential residue charge during the conversion phase; and means
for coupling a second plate of the first one of the pair of
bit-trial capacitors and a second plate of the second one of the
pair of bit-trial capacitors to inputs of the comparator.
20. The differential DAC circuit of claim 18, wherein the input
capacitors are bit-trial capacitors, and wherein transferring the
common-mode residue charge from a reference voltage to set the pair
of input capacitors when setting the pair of input capacitors in
the common-mode configuration based on the decision of the
comparator includes: means for controlling operation of a second
set of switches to directly couple the reference voltage to a first
plate of a first one of the pair of bit-trial capacitors and a
first plate of a second one of the pair of bit-trial capacitors
only when setting the pair of bit-trial capacitors in the
common-mode configuration based on a bit-trial result to transfer
the common-mode residue charge during a conversion phase; and means
for coupling a second plate of the first one of the pair of
bit-trial capacitors and a second plate of the second one of the
pair of bit-trial capacitors to inputs of the comparator.
Description
FIELD OF THE DISCLOSURE
[0001] This document pertains generally, but not by way of
limitation, to integrated circuits, and more particularly, to
analog-to-digital converter circuits and systems.
BACKGROUND
[0002] Successive approximation routine (SAR) analog-to-digital
converters (ADCs) convert an analog input to a digital value.
Typically, the analog input is held while the SAR ADC circuit
converges to a solution after a number of bit trials. Some SAR ADC
circuits convert a differential analog input to a digital value. A
differential SAR ADC may require that the common mode of the input
signal to be at a fixed value; e.g., Vref/2. This can be
accomplished by additional circuitry to translate the input common
mode to the common mode required by the SAR ADC, However; this
additional circuitry can result in additional space needed for an
SAR ADC circuit, additional power consumption, and can introduce
additional sources of noise in the signal chain.
SUMMARY OF THE DISCLOSURE
[0003] This disclosure describes, among other things, a
differential digital-to-analog (DAC) circuit that can include a
reservoir capacitor and various switches to couple the bottom
plates of the input capacitors; e.g., bit-trial capacitors, to
reference voltages, e.g., REF+ or REF-. In this manner, the
reservoir capacitor can be used to provide any differential charge
to the input capacitors, e.g., bit-trial capacitors, and the
reference voltages, e.g., REF+ and REF-, can be used to provide any
common mode charge to the input capacitors.
[0004] In some aspects, this disclosure is directed to a
differential digital-to-analog (DAC) circuit comprising a capacitor
array including a number of DAC units, each DAC unit including: a
pair of input capacitors configured to couple to a comparator; a
dedicated reference capacitor associated with the pair of input
capacitors; and a control circuit configured to: control operation
of a first set of switches to transfer a differential residue
charge from the reservoir capacitor to the pair of input capacitors
when setting the pair of input capacitors in a differential
configuration based on a decision of the comparator; and control
operation of a second set of switches to transfer a common-mode
residue charge from a reference voltage to set the pair of input
capacitors when setting the pair of input capacitors in a
common-mode configuration based on a decision of the
comparator.
[0005] In some aspects, this disclosure is directed to a method of
operating a differential analog-to-digital converter (ADC) circuit
to convert an analog input to a digital output. The method
comprises coupling the analog input onto a capacitor array of a
first ADC circuit, the capacitor array including a number of DAC
units, each DAC unit including: a pair of input capacitors; and a
dedicated reference capacitor associated with the pair of input
capacitors. The method further comprises transferring a
differential residue charge from the reservoir capacitor to the
pair of input capacitors when setting the pair of input capacitors
in a differential configuration based on a decision of a
comparator; and transferring a common-mode residue charge from a
reference voltage to set the pair of bit-trial capacitors when
setting the pair of bit-trial capacitors in a common-mode
configuration based on a decision of the comparator.
[0006] In some aspects, this disclosure is directed to a
differential digital-to-analog (DAC) circuit comprising means for
coupling the analog input onto a capacitor array of a first ADC
circuit, the capacitor array including a number of DAC units, each
DAC unit including: a pair of input capacitors; and a dedicated
reference capacitor associated with the pair of input capacitors;
and means for transferring a differential residue charge from the
reservoir capacitor to the pair of input capacitors when setting
the pair of input capacitors in a differential configuration based
on a decision of a comparator; and means for transferring a
common-mode residue charge from a reference voltage to set the pair
of bit-trial capacitors when setting the pair of bit-trial
capacitors in a common-mode configuration based on a decision of
the comparator.
[0007] This overview is intended to provide an overview of subject
matter of the present patent application. It is not intended to
provide an exclusive or exhaustive explanation of the invention.
The detailed description is included to provide further information
about the present patent application.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] In the drawings, which are not necessarily drawn to scale,
like numerals may describe similar components in different views.
Like numerals having different letter suffixes may represent
different instances of similar components. The drawings illustrate
generally, by way of example, but not by way of limitation, various
embodiments discussed in the present document.
[0009] FIG. 1 is a functional block diagram of an example of a
differential SAR ADC.
[0010] FIG. 2 is a functional block diagram of portions of an
example of a reservoir-capacitor SAR ADC.
[0011] FIG. 3 is circuit diagram showing an example of a portion of
a SAR ADC circuit including the circuit block of FIG. 2, in
accordance with this disclosure.
[0012] FIG. 4 is an example of a comparator circuit that can
implement various techniques of this disclosure.
[0013] FIG. 5 is circuit diagram showing an example of a circuit
that can include the main ADC circuit of FIG. 3 in combination with
an auxiliary ADC circuit.
[0014] FIG. 6 is a schematic diagram of the circuit block of FIG. 5
in a sampling phase.
[0015] FIG. 7 is a schematic diagram of the circuit block of FIG. 5
in a positive differential residue charge phase.
[0016] FIG. 8 is a schematic diagram of the circuit block of FIG. 5
in a negative differential residue charge phase.
[0017] FIG. 9 is a schematic diagram of the circuit block of FIG. 5
in a positive common mode residue charge phase.
[0018] FIG. 10 is a schematic diagram of the circuit block of FIG.
5 in a negative common mode residue charge phase.
DETAILED DESCRIPTION
[0019] Successive approximation routine (SAR) analog-to-digital
converters (ADCs) convert an analog input to a digital value.
Typically, the analog input is held while the SAR ADC circuit
converges to a solution after a number of bit trials. Some SAR. ADC
circuits convert a differential analog input to a digital value. A
differential SAR ADC may require that the common mode of the input
signal to be at a fixed value, e.g., Vref/2.
[0020] If the input common mode voltage varies from the fixed
value, e.g., Vref/2, the common mode voltage of the differential
digital-to-analog (DAC) top plates can vary during the hit trials
and the comparator in the ADC will have to manage this varying
common mode voltage. Moreover, if the final common mode voltage
during a conversion differs from conversion to conversion,
nonlinear errors can be introduced. Therefore, it is desirable for
a SAR ADC structure that accommodates variation of the input common
mode voltage.
[0021] This disclosure describes, among other things, a
differential digital-to-analog (DAC) circuit that can include a
reservoir capacitor and various switches to couple the bottom
plates of the input capacitors, e.g., bit-trial capacitors, to
reference voltages, e.g., REF+ or REF-. In this manner, the
reservoir capacitor can be used to provide any differential charge
to the input capacitors, e.g., bit-trial capacitors, and the
reference voltages, e.g., REF+ and REF-, can be used to provide any
common mode charge to the input capacitors.
[0022] FIG. 1 is a functional block diagram of an example of a
differential SAR ADC. The SAR ADC 100 includes a positive
digital-to-analog converter (DAC) circuit 105, a negative DAC
circuit 110, and a comparator circuit 115. Each DAC circuit (or
"DAC" in this disclosure) can include weighted bit-trial capacitors
120. In the example, the capacitors are weighted as C/2, C/4 . . .
C/(2.sup.N), where N is the number of bits in the DACs and C is the
total capacitance of the bit-trial capacitors added together. A
differential analog input voltage (IN+, IN-) can be sampled onto
the bit-trial capacitors with respect to the common mode of the
comparator (CompCM) by closing switches 125 and 130. The input
voltage can be held on the capacitors by opening switches 130, then
opening switches 125. The top plates of the capacitors can be at
the CompCM voltage.
[0023] The positive DAC 105 and the negative DAC 110 can also be
connected to positive and negative reference voltage (REF+, REF-).
As part of the successive approximation routine, bit trials for
each of the bit-trial capacitors can be performed iteratively. In a
bit trial, the output of the positive DAC 105 and the output of the
negative DAC 110 can be applied to the inputs of the comparator
circuit 115. Based on the output of the comparator circuit, a bit
capacitor can be connected to either REF+ or REF- using switches
135. If the bit capacitor is connected to REF+ the bit of the
digital value corresponding to the bit-trial capacitors is assigned
a logic value `1`, and if the bit capacitor is connected to REF+
the bit of the digital value corresponding to the bit-trial
capacitors is assigned a logic value `0`. Conversion can then
proceed to the next bit capacitor until all bits of the digital
value are determined.
[0024] FIG. 2 is a functional block diagram of portions of an
example of a reservoir-capacitor based SAR ADC 200. Similar to the
SAR ADC of FIG. 1, the reservoir-capacitor based SAR ADC 200
includes two DAC circuits, a positive DAC circuit 205 and a
negative circuit DAC circuit 210, and a comparator circuit 215.
Each DAC circuit 205, 210 can form a capacitor array and can
include weighted bit-trial capacitors 220 shown attached to circuit
blocks 222 to simplify the figure. Some examples of the DACs
include 8, 12, or 16-bit DACs. There can be a circuit block for
each bit weight (C/2, C/4 . . . C/(2.sup.N)) and a circuit block
can include an arrangement of electronic switches and a reservoir
capacitor. Each circuit block 222 can be a unit element or "DAC
unit" of a respective DAC capacitor array.
[0025] The reservoir-capacitor SAR ADC 200 can include logic
circuitry 250 (also referred to as a "control circuit"). In certain
examples, the logic circuitry 250 can be included in an SAR ADC
controller. In certain examples, the logic circuitry 250 can
include a sequencer to advance the SAR ADC 200 through multiple
circuit states to perform the SAR.
[0026] FIG. 3 is circuit diagram showing an example of a portion of
a SAR ADC circuit 300 including the circuit block 222 of FIG. 2, in
accordance with this disclosure. The circuit block 222 can be a
unit element or "DAC unit" of a capacitor array of a DAC circuit
such that a DAC circuit, e.g., DAC circuit 205 of FIG. 2, can
include a number of circuit blocks 222. For purposes of clarity,
only one DAC unit is depicted in FIG. 3. Each circuit block 222 can
be coupled to the differential analog input voltage (IN+, IN-),
differential reference voltage (REF+, REF-), and comparator circuit
215.
[0027] The circuit block 222 can include a reservoir capacitor 301.
In some example configurations, the reservoir capacitor 301 can be
a dedicated reference capacitor associated with the pair of input
capacitors 220A, 220B of a DAC unit element 222. The value of
capacitance of a reservoir capacitor can be larger than the
capacitance value of its corresponding bit capacitor. For example,
the value of capacitance of a reservoir capacitor can be five to
twenty times the capacitance of the bit-trial capacitors.
[0028] The differential analog input voltage (IN+, IN-) can sampled
and stored onto input capacitors 220A, 220B, e.g., bit-trial
capacitors, with respect to the comparator common mode (CompCM)
using switches 302A, 302B (collectively "switches 302") and 304A,
304B (collectively "switches 304"). The reference voltage can be
sampled on the reservoir capacitor 301 when the input voltage is
sampled on the bit-trial capacitors 220 using switches 306A, 306B
(collectively "switches 306"). Switches 308A-308D (collectively
"switches 308") can isolate the reservoir capacitor 301 from the
input capacitors 220A, 220B, e.g., bit-trial capacitors, during the
sampling.
[0029] Logic circuitry, e.g., logic circuitry 250 of FIG. 2, can
open switches 302, 304 and 306, and close switch 310 for a bit
trial. Depending on the result of the hit trial at the comparator
output, the voltage of the reservoir capacitor 301 can be added or
subtracted from the voltage of the input capacitors 220A, 220B,
e.g., bit-trial capacitors. The bit of the digital value
corresponding to the trial can be set to a logical `1` or `0`
depending on the comparator result and therefore whether the
voltage of the reservoir capacitor 301 is added or subtracted from
the bit-trial capacitors using switches 308.
[0030] Logic circuitry, e.g., logic circuitry 250 of FIG. 2, can
progress the conversion of the differential analog input signal
through all of the bit-trial capacitors until all bits of the
digital value are determined. In certain examples, the logic
circuitry can include a SAR controller (e.g., a processor) that
progresses the SAR ADC through a conversion according to outcomes
determined at the comparator circuit 215. In certain examples, the
logic circuitry can include a logic sequencer that progresses the
conversion through a series of logic states that correspond to
steps of the bit trials. Using reservoir capacitors can speed up
the bit trials from the embodiment of FIG. 1 because resampling
does not need to take place during every bit trial.
[0031] The differential SAR ADCs of FIG. 1 and FIG. 2 can require
that the common mode of the input signal be at Vref/2, for example.
If the common mode of input signal is maintained at a fixed value,
e.g., Vref/2, the common mode of the DAC top plates during the bit
trials can be a fixed value V.sub.CM, COMP. At the end of the
conversion, the bits of the negative DAC are complementary to the
bits of the positive DAC, e.g., if a bit of the positive DAC is a
"1", a bit of the negative DAC can be a "0".
[0032] If the input common mode varies from the fixed value, e.g.,
Vref/2, the common mode of the DAC top plates can vary during the
bit trials and the comparator in the ADC will have to manage this
varying common mode voltage. Moreover, if the final common mode
voltage during a conversion differs from conversion to conversion,
nonlinear errors can be introduced. As explained above, additional
circuit components may be needed to provide an input to the ADC
with a fixed common mode voltage. Therefore, it is desirable for a
SAR ADC structure that accommodates variation of the input common
mode voltage.
[0033] Using the techniques of this disclosure, additional switches
can be included in the circuit block 222 to couple the bottom
plates of the input capacitors, e.g., bit-trial capacitors 220, to
reference voltages REF+ or REF-. As seen in FIG. 3, switches 312A,
312B (collectively "switches 312") can couple the bottom plates of
the input capacitors, e.g., bit-trial capacitors 220A, 220B, to
REF+ and switches 314A, 314B (collectively "switches 314") can
couple the bottom plates of the input capacitors, e.g., bit-trial
capacitors 220A, 220B, to REF-. In this manner, the reservoir
capacitor 301 can be used to provide any differential charge to the
capacitors 220A, 220B, and the reference voltages REF+ and REF- can
be used to provide any common mode charge to the bit-trial
capacitors 220A, 220B.
[0034] In this disclosure, the terms "top plate" and "bottom plate"
are used for convenience to describe the capacitors and are not
meant to imply that there is any, required spatial orientation for
the capacitors. Further, the switches referred to in this
disclosure can include transistors and, in particular,
complementary metal-oxide-semiconductor (CMOS) transistors due to
their high performance and yield.
[0035] For an N-bit SAR ADC circuit, without an auxiliary ADC
circuit, e.g., flash ADC or SAR ADC circuit, there can be `N`
circuit blocks 222 of FIG. 3, e.g., unit elements or "units" of a
capacitor array of a DAC circuit, connected in parallel and coupled
to the comparator 215. In some example configurations, all of the
bit-trial capacitors 220 can be associated with a respective
circuit block 222 similar to the example shown in FIG. 3. In other
example configurations, only some of the bit-trial capacitors 220,
e.g., at least some of the most significant bits (MSBs), can be
associated with a respective circuit block 222 similar to the
example shown in FIG. 3. For example, in some implementations, the
least significant bit (LSB) bit-trial capacitors do not need to
couple to reference voltages REF+ or REF- because they only resolve
the differential signal information and, as such, switches 312 and
314 are not needed for the circuit blocks 222 associated with the
LSBs. As such, in some examples, the circuit 300 associated with
the LSBs can exclude switches 312 and 314.
[0036] FIG. 4 is an example of a comparator circuit that can
implement various techniques of this disclosure. The comparator
circuit 215 of FIG. 4 is an example of the comparator circuit 215
of FIG. 3.
[0037] The comparator circuit 215 can include a non-inverting input
(+) and an inverting input (-) coupled to the top plate nodes
"TOP-P" and "TOP-N" that are coupled to the top plates of the
bit-trial capacitors, e.g., top plates 212A and 212B of FIG. 2. The
comparator 215 can include an additional input 420 that can be the
intended DAC common mode output reference voltage for the
comparator's differential inputs. The input 420 can be connected to
a bias voltage (which could be any suitable voltage, including
ground), although it could also be connected to other sources, for
example a variable voltage that asymptotes to the bias voltage. The
comparator circuit 415 can include an additional output OUT2, such
that the outputs OUT1 and OUT2 are responsive to both the
differential and common mode components of its inputs.
[0038] The logic circuitry, e.g., the logic circuitry 250 of FIG.
2, can accommodate the additional comparator outputs and produce
independent bus outputs to control the positive DAC and negative
DAC circuits.
[0039] The comparator circuit 215 of FIG. 4 is shown and described
in commonly assigned U.S. Pat. No. 7,432,844 to Mueck et al.,
titled "Differential Input Successive Approximation Analog to
Digital Converter with Common Mode Rejection," the entire contents
of which being incorporated herein by reference, including FIGS.
7A-7E and the associated description of those figures.
[0040] The circuit 300 of FIG. 3 can form part of a main SAR ADC
circuit. In some example configurations, both a main SAR ADC and an
auxiliary ADC, e.g., a flash or SAR. ADC, can be used in
conjunction with the techniques described above with respect to
FIG. 3. The auxiliary ADC can have a resolution less than the main
ADC circuit.
[0041] FIG. 5 is circuit diagram showing an example of a circuit
500 that can include the main ADC circuit of FIG. 3 in combination
with an auxiliary ADC circuit. The circuit block 222 can be a unit
element or "DAC unit" of a capacitor array of a DAC circuit such
that a DAC circuit, e.g., DAC circuit 205 of FIG. 2, can include a
number of circuit blocks 222. For purposes of clarity, only one DAC
unit is depicted in FIG. 3. The circuit 500 can include a first
auxiliary ADC circuit 502A to convert a voltage referenced from the
input of the positive DAC circuit to the comparator common mode,
and a second auxiliary ADC circuit 502B to convert a voltage
referenced from the input of the negative DAC circuit to the
comparator common mode.
[0042] The auxiliary ADC circuits 502A, 502B, e.g., a flash/SAR.
ADC circuits, can provide low resolution using a smaller sampling
capacitor and a low power, relatively noisy comparator, and a main
ADC, e.g., main ADC circuit 200 of FIG. 2, can provide for high
resolution. An auxiliary ADC circuit can help convert higher input
voltage ranges using a low voltage supply and can allow the main
ADC to power down its comparator during the acquisition phase. An
auxiliary ADC can convert the input voltage to, for example, 2-bit
accuracy using a binary search algorithm. An auxiliary ADC can
perform this conversion before the main ADC takes any action. Then,
the auxiliary ADC circuit can transfer the results of the
low-resolution conversion to the main ADC circuit, including to
bit-trial capacitors 220, which can then resolve the remaining
bits.
[0043] In some examples, auxiliary ADC circuits can be used in the
comparison of the output of the positive DAC, the output of the
negative DAC, and the common mode voltage of the comparator
circuit. In the non-limiting example of FIG. 5, the auxiliary ADC
circuit is a flash ADC circuit (also referred to as "a
direct-conversion ADC").
[0044] In contrast to a SAR ADC circuit, the result of the flash
ADC circuit can be available after one conversion cycle. In the
example of FIG. 9, the flash ADC circuit 502A can convert a voltage
referenced from the input of the positive DAC circuit to the
comparator common mode and flash ADC circuit 502B can convert a
voltage referenced from the input of the negative DAC circuit to
the comparator common mode. Before a conversion phase using the
first ADC circuit, the logic circuit 250 can control couple the
analog input signal onto the flash ADC circuits 502A, 502B, perform
at least one bit-trial using the flash ADC circuits 502A, 502B, and
load an output of the flash ADC circuits 502A, 502B onto at least
one of the bit-trial capacitors of the first ADC circuit.
[0045] There are four possible outcomes or "states" (states "10",
"01", "11", and "00") to the conversion of the input are shown in
Table 1 below:
TABLE-US-00001 TABLE 1 Case No. FlashP FlashN 1 1 0 2 0 1 3 1 1 4 0
0
[0046] The states can be determined by the comparators of auxiliary
ADC circuits, e.g., flash ADC circuits 502A, 502B of FIG. 5. In
configurations that do not include auxiliary ADC circuits, such as
shown in FIG. 3, the states can be determined by a comparator such
as comparator 215 in FIG. 4.
[0047] In Case Nos. 1 and 2, the auxiliary ADC circuits for the bit
position have different outcomes. For example, in Case No. 1,
FlashP is 1 and FlashN is 0. For Case Nos. 1 and 2, there is no
input common mode issue because the input voltage is intermediate
the output voltage of the positive DAC circuit and the negative DAC
circuit. For Case No. 1 capacitors 220A and 220B can resolve a
positive differential residue. For Case No. 2, capacitors 220A and
220B can resolve a negative differential residue.
[0048] For case No. 3, both the input at the positive side and the
input at the negative side are greater than the comparator common
mode. Thus, the input voltage has a common mode different from the
comparator common mode and the bit-trial capacitors 220A and 220B
can resolve positive common mode voltage. For case No. 4, both the
input at the positive side and the input at the negative side are
less than the comparator common mode. Thus, capacitors 220A and
220B can resolve a negative common mode voltage.
[0049] To reduce a difference between the input common mode voltage
and Vref/2, for example, the logic circuitry 250 can apply a
correction voltage to one or more of the bit-trial capacitors 220A,
220B. In particular, using the techniques of this disclosure, the
logic circuitry 250 can couple the reference voltages REF+ and REF-
to the bit-trial capacitors 220A, 220B to provide any common mode
residue charge. This can cause the common mode voltage of top
plates to converge to a desired value. In this manner, the
reservoir capacitor 301 can be used to provide any differential
residue charge to the bit-trial capacitors 220A, 220B, as described
below with respect to FIGS. 7 and 8, and the reference voltages
REF+ and REF- can be used to provide any common mode residue charge
to the bit-trial capacitors 220A, 220B, as described below with
FIGS. 9 and 10.
[0050] The techniques of this disclosure are applicable to circuits
without auxiliary ADC circuits, such as shown in FIG. 3. In such
configurations, the comparator 215, e.g., as shown in FIG. 4, can
determine the states as shown in Table 1. However, for purposes of
conciseness, the techniques will be described with respect to the
circuit of FIG. 5, which can include auxiliary ADC circuits in
combination with a main ADC circuit.
[0051] As mentioned above, the circuit block 222 of FIGS. 3 and 5
can be a unit element or "DAC unit" of a capacitor array of a DAC
circuit such that a DAC circuit, e.g., DAC circuit 205 and/or DAC
circuit 210 of FIG. 2, can include a number of circuit blocks 222.
The DAC units can include most significant bit (MSB) DAC units and
least significant bit (LSB) DAC units. In some example
configurations, all of the DAC units can be similar such that the
MSB and LSB units include the set of switches 312A, 312B, 314A,
314E to transfer a common-mode residue charge from a reference
voltage to set the pair of input capacitors when setting the pair
of input capacitors in a common-mode configuration based on a
decision of the comparator.
[0052] In other example configurations, only some of the DAC units
include the set of switches 312A, 312B, 314A, 314B. For example,
the LSB DAC units in some configurations do not include the
switches 312A, 312B, 314A, 314B. In configurations in which the LSB
DAC units do not include the switches 312A, 312B, 314A, 314B,
either some or all of the MSB DAC units the switches 312A, 312B,
314A, 314B.
[0053] FIG. 6 is a schematic diagram of the circuit block 222 of
FIG. 5 in a sampling phase. During a sampling phase, the top plate
switches 304A, 304B can be closed to couple the top plates of
bit-trial capacitors 220A, 220B to the bias voltage, e.g., shown as
common mode voltage CompCM. In addition, switches 302A, 302B can be
closed to couple the bottom plates of bit-trial capacitors 220A,
220B to analog input signal IN+, IN-, respectively. During the
sampling phase, the reservoir capacitor 301 can be charged by
coupling to the main reference voltage sources REF+ and REF- using
switches 306A, 306B.
[0054] The bit-trial capacitors of the DAC circuits can store the
input signal as sampled charge and deliver a differential input
voltage to the inputs of the comparator 215. The comparator
differential input voltage can represent the imperfection between
the original input signal and an estimate of the input signal
developed by the DAC circuits. The comparator differential input
voltage can include an input common mode offset, and the comparator
circuit 215 can include a comparator common mode offset.
[0055] The logic circuitry 250 of FIG. 2 can be coupled to positive
and negative DAC circuits, the switches, and the comparator
circuit, such as shown in FIG. 1. As part of a SAR operation, the
logic circuitry 250 can initiate successive bit trials using the
weighted bit-trial capacitors to convert the input voltage to a
digital value by comparing the outputs of the positive DAC circuit
and the negative DAC circuit using the comparator circuit and
updating the DAC values. As the conversion progresses, the
comparator differential input voltage can be reduced towards zero
by transferring charge from the reference reservoir capacitor 301
to the bit-trial capacitors 220A, 220B. This can converge the
estimate of the input signal on the DAC circuits to the original
input signal.
[0056] FIG. 7 is a schematic diagram of the circuit block 222 of
FIG. 5 in a positive differential residue charge phase. In the
example configuration shown in FIG. 7, the logic circuitry 250 of
FIG. 2 determined that to converge the estimate of the input signal
on the DAC circuits to the original input signal, a positive
differential residue charge can be applied, e.g., based on flash
ADC results such as Case No. 1 in Table 1.
[0057] To reduce the comparator differential input voltage towards
zero, the logic circuit 250 (or "control circuit") can close
switches 308A, 308D to transfer charge from the reference reservoir
capacitor 301 to set the input capacitors 220A, 220B, e.g.,
bit-trial capacitors, in a "10" state. In other words, the control
circuit can control operation of a first set of switches to
transfer a differential residue charge from the reservoir capacitor
301 to the pair of input capacitors 220A, 220B when setting the
pair of input capacitors in a differential configuration based on a
decision of the comparator. Setting the pair of input capacitors in
a differential configuration based on a decision of the comparator
can include setting the first plates of the input capacitors to a
state "10", which is a first of two states in a first set of
states: 01 and 10.
[0058] For example, when setting the pair of bit-trial capacitors
in a differential configuration based on a bit-trial result to
transfer the differential residue charge during the conversion
phase, the control circuit 250 can control operation of a first set
of switches 308A, 308D to directly couple plates of the reference
capacitor 301 to a first plate of the bit-trial capacitor 220A and
a first plate of bit-trial capacitor 220B, where a second plate of
the bit-trial capacitor 220A and a second plate the bit-trial
capacitor 220Bs are configured to couple to inputs of the
comparator 215.
[0059] FIG. 8 is a schematic diagram of the circuit block 222 of
FIG. 5 in a negative differential residue charge phase. In the
example configuration shown in FIG. 8, the logic circuitry 250 of
FIG. 2 determined that to converge the estimate of the input signal
on the DAC circuits to the original input signal, a negative
differential residue charge can be applied, e.g., based on flash
ADC results such as Case No. 2 in Table 1.
[0060] To reduce the comparator differential input voltage towards
zero, the logic circuit 250 can close switches 308B, 308C to
transfer charge by cross-coupling from the reference reservoir
capacitor 301 to the input capacitors 220A, 220B, e.g., bit-trial
capacitors, to set to a "01" state. In other words, the control
circuit can control operation of a first set of switches to
transfer a differential residue charge from the reservoir capacitor
301 to the pair of input capacitors 220A, 220B when setting the
pair of input capacitors in a differential configuration based on a
decision of the comparator. Setting the pair of input capacitors in
a differential configuration based on a decision of the comparator
can include setting the first plates of the input capacitors to a
state "01", which is a second of two states in a first set of
states: 01 and 10.
[0061] For example, when setting the pair of bit-trial capacitors
in a differential configuration based on a bit-trial result to
transfer the differential residue charge during the conversion
phase, the control circuit 250 can control operation of a first set
of switches 308B, 308C to cross-couple plates of the reference
capacitor 301 to a first plate of the bit-trial capacitor 220A and
a first plate of bit-trial capacitor 220B, where a second plate of
the bit-trial capacitor 220A and a second plate the bit-trial
capacitor 220Bs are configured to couple to inputs of the
comparator 215.
[0062] In some example implementations, after coupling the
reservoir capacitor 301 to the input capacitors, e.g., bit-trial
capacitors, to transfer a first change such as described with
respect to FIGS. 7 and 8, the control circuit can couple a
reference voltage, e.g., REF+ or REF-, to the previously set input
capacitor, e.g., bit-trial capacitor, to transfer a second charge.
For example, during a conversion phase, the control circuit, e.g.,
logic circuit 250 of FIG. 2, can control transfer of a first charge
from the reservoir capacitor 301 to set at least one of the
bit-trial capacitors 220A, 220B based on a comparator decision.
Then, the control circuit can couple a reference voltage, e.g.,
REF+ or REF-, to the at least one previously set bit-trial
capacitors to transfer a second charge, where the reference voltage
is more accurate than the voltage of the reservoir, or reference,
capacitor.
[0063] In this manner, the reservoir capacitor 301 can supply most
of the charge to the bit-trial capacitors 220A, 220B as the
bit-trials are performed. The accurate reference voltage source,
e.g., an "external" reference buffer circuit, only needs to supply
the difference, e.g., an inaccuracy, in the charge supplied by the
reservoir capacitors. Instead of having to resettle for each
bit-trial, the accurate reference voltage source has only to
deliver the initial charge to the reservoir capacitors during
acquisition and once more when the ADC circuit is ready to sample
onto the residue amplifier.
[0064] FIG. 9 is a schematic diagram of the circuit block 222 of
FIG. 5 in a positive common mode residue charge phase. As explained
above, when the outputs of the auxiliary ADC circuits are both
`1s`, such as in Case No. 3 in Table 1, both the input at the
positive side and the input at the negative side are more than the
comparator common mode CompCM because of the positive common mode
residue. The logic circuitry 250 can apply a correction voltage to
the input capacitors 220A, 220B, e.g., bit-trial capacitors, based
upon the bit trial. The logic circuitry 250 (or "control circuit")
can close switches 3112A, 3112B to couple the bit-trial capacitors
220A, 220B to the positive reference voltage REF+ to transfer
positive charge to the bit-trial capacitors 220A, 220B.
[0065] In other words, the control circuit can control operation of
a second set of switches to transfer a common-mode residue charge
from a positive reference voltage REF+ to set the pair of input
capacitors 220A, 220B when setting the pair of input capacitors in
a common-mode configuration based on a decision of the comparator.
Setting the pair of input capacitors in a common-mode configuration
based on a decision of the comparator includes setting the first
plates of the input capacitors to a state "11", which a first of
two states in a second set of two states: 11 and 00.
[0066] For example, when setting the pair of bit-trial capacitors
in a common-mode configuration based on a bit-trial result to
transfer the common-mode residue charge during a conversion phase,
the control circuit 250 can control operation of a second set of
switches 312A, 312B to directly couple the positive reference
voltage REF+ to a first plate of the bit-trial capacitor 220A and a
first plate of bit-trial capacitor 220B, where a second plate of
the bit-trial capacitor 220A and a second plate the bit-trial
capacitor 220Bs are configured to couple to inputs of the
comparator 215. In this manner, the logic circuitry can correct the
difference in the common mode voltages such that the common mode of
the input signal matches the comparator common mode CompCM.
[0067] FIG. 10 is a schematic diagram of the circuit block 222 of
FIG. 6 in a negative common mode residue charge phase. As explained
above, when the outputs of the auxiliary ADC circuits are both
`0s`, such as in Case No. 3 in Table 1, both the input at the
positive side and the input at the negative side are less than the
comparator common mode CompCM because of the negative common mode
residue. The logic circuitry 250 can apply a correction voltage to
the input capacitors 220A, 220B, e.g., bit-trial capacitors, based
upon the bit trial. The logic circuitry 250 can close switches
314A, 314B to couple the bit-trial capacitors 220A, 220B to the
negative reference voltage REF- to transfer negative charge to the
bit-trial capacitors 220A, 220B.
[0068] In other words, the control circuit can control operation of
a second set of switches to transfer a common-mode residue charge
from a negative reference voltage REF- to set the pair of input
capacitors 220A, 220B when setting the pair of input capacitors in
a common-mode configuration based on a decision of the comparator.
Setting the pair of input capacitors in a common-mode configuration
based on a decision of the comparator includes setting the first
plates of the input capacitors to a state "00", which a second of
two states in a second set of two states: 11 and 00. In this
manner, the logic circuitry can correct the difference in the
common mode voltages such that the common mode of the input signal
matches the comparator common mode CompCM.
[0069] Although the process of correcting for common mode mismatch
has been described in terms of one bit of a DAC circuit at a time
with respect to FIGS. 9 and 10, the common mode estimation may be
performed for several bits of the DAC circuit at the same time. In
addition, the correction could be applied to several bits at the
same time. Thus, the correction may be applied to a bit capacitor
array of the first DAC circuit and a bit capacitor array of the
second DAC circuit, where a capacitor array includes one or more
bit-trial capacitors.
[0070] Various Notes
[0071] Each of the non-limiting aspects or examples described
herein may stand on its own, or may be combined in various
permutations or combinations with one or more of the other
examples.
[0072] The above detailed description includes references to the
accompanying drawings, which form a part of the detailed
description. The drawings show, by way of illustration, specific
embodiments in which the invention may be practiced. These
embodiments are also referred to herein as "examples." Such
examples may include elements in addition to those shown or
described. However, the present inventors also contemplate examples
in which only those elements shown or described are provided.
Moreover, the present inventors also contemplate examples using any
combination or permutation of those elements shown or described (or
one or more aspects thereof), either with respect to a particular
example (or one or more aspects thereof), or with respect to other
examples (or one or more aspects thereof) shown or described
herein.
[0073] In the event of inconsistent usages between this document
and any documents so incorporated by reference, the usage in this
document controls.
[0074] In this document, the terms "a" or "an" are used, as is
common in patent documents, to include one or more than one,
independent of any other instances or usages of "at least one" or
"one or more." In this document, the term "or" is used to refer to
a nonexclusive or, such that "A or B" includes "A but not B," "B
but not A," and "A and B," unless otherwise indicated. In this
document, the terms "including" and "in which" are used as the
plain-English equivalents of the respective terms "comprising" and
"wherein." Also, in the following claims, the terms "including" and
"comprising" are open-ended, that is, a system, device, article,
composition, formulation, or process that includes elements in
addition to those listed after such a term in a claim are still
deemed to fall within the scope of that claim. Moreover, in the
following claims, the terms "first," "second," and "third," etc.
are used merely as labels, and are not intended to impose numerical
requirements on their objects.
[0075] Method examples described herein may be machine or
computer-implemented at least in part. Some examples may include a
computer-readable medium or machine-readable medium encoded with
instructions operable to configure an electronic device to perform
methods as described in the above examples. An implementation of
such methods may include code, such as microcode, assembly language
code, a higher-level language code, or the like. Such code may
include computer readable instructions for performing various
methods. The code may form portions of computer program products.
Further, in an example, the code may be tangibly stored on one or
more volatile, non-transitory, or non-volatile tangible
computer-readable media, such as during execution or at other
times. Examples of these tangible computer-readable media may
include, but are not limited to, hard disks, removable magnetic
disks, removable optical disks (e.g., compact discs and digital
video discs), magnetic cassettes, memory cards or sticks, random
access memories (RAMs), read only memories (ROMs), and the
like.
[0076] The above description is intended to be illustrative, and
not restrictive. For example, the above-described examples (or one
or more aspects thereof) may be used in combination with each
other. Other embodiments may be used, such as by one of ordinary
skill in the art upon reviewing the above description. The Abstract
is provided to comply with 37 C.F.R. .sctn. 1.72(b), to allow the
reader to quickly ascertain the nature of the technical disclosure.
It is submitted with the understanding that it will not be used to
interpret or limit the scope or meaning of the claims. Also, in the
above Detailed Description, various features may be grouped
together to streamline the disclosure. This should not be
interpreted as intending that an unclaimed disclosed feature is
essential to any claim. Rather, inventive subject matter may lie in
less than all features of a particular disclosed embodiment. Thus,
the following claims are hereby incorporated into the Detailed
Description as examples or embodiments, with each claim standing on
its own as a separate embodiment, and it is contemplated that such
embodiments may be combined with each other in various combinations
or permutations. The scope of the invention should be determined
with reference to the appended claims, along with the full scope of
equivalents to which such claims are entitled.
* * * * *