# Patent

## Title
Systems and methods for providing a pipelined analog-to-digital converter

## Abstract

Systems comprising: a first MDAC stage comprising: a sub-ADC that outputs a value based on an input signal; at least two reference capacitors that are charged to a Vref; at least two sampling capacitors that are charged to a Vin; and a plurality of switches that couple the at least two reference capacitors so that they are charged during a sampling phase, that couple the at least two sampling capacitors so that they are charged during the sampling phase, that couple at least one of the reference capacitors so that it is parallel to one of the at least two sampling capacitors during a hold phase, and that couple the other of the at least two sampling capacitors so that it couples the at least one of the reference capacitors and the one of the at least two sampling capacitors to a reference capacitor of a second MDAC stage.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 14/005,887, filed Feb. 27, 2014, which is the U.S. National Phase Application under 35 U.S.C. §371 of International Patent Application No. PCT/US2012/029654, filed Mar. 19, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/454,217, filed Mar. 18, 2011. Each of the above-referenced patent applications is hereby incorporated by reference herein in its entirety.


STATEMENT REGARDING GOVERNMENT FUNDED RESEARCH
This invention was made with government support under grant PHY 0612811 awarded by the National Science Foundation. The government has certain rights in the invention.


BACKGROUND
Power hungry buffers and/or large decoupling capacitors have long been the solution to providing accurate reference voltages in pipelined Analog-to-Digital Converters (ADCs). Zero-crossing based pipelined ADCs offer a low power alternative to traditional op-amp based solutions, but their reference buffer design is exacerbated since the reference voltage has only a fraction of the hold phase (known as the pre-charge phase) to settle to the desired accuracy.
Accordingly, new systems and methods for providing a pipelined Analog-to-Digital Converter (ADC) are desirable.
SUMMARY
Systems and methods for providing a pipelined Analog-to-Digital Converter (ADC) are provided. In some embodiments, systems for providing a pipelined Analog-to-Digital Converter are provided, the systems comprising: a first multiplying Digital-to-Analog Converter (MDAC) stage comprising: a sub-Analog-to-Digital Converter (ADC) that outputs a value based on an input signal; at least two reference capacitors that are charged to a reference voltage; at least two sampling capacitors that are charged to a sampling voltage; and a plurality of switches that couple the at least two reference capacitors so that they are charged during a sampling phase, that couple the at least two sampling capacitors so that they are charged during the sampling phase, that couple at least one of the reference capacitors so that it is parallel to one of the at least two sampling capacitors during a hold phase, and that couple the other of the at least two sampling capacitors so that it couples the at least one of the reference capacitors and the one of the at least two sampling capacitors to a reference capacitor of a second MDAC stage.
In some embodiments, methods for providing a pipelined Analog-to-Digital Converter are provided, the methods comprising: in a first multiplying Digital-to-Analog Converter (MDAC) stage: outputting from a sub-Analog-to-Digital Converter (ADC) a value based on an input signal; charging at least two reference capacitors to a reference voltage; charging at least two sampling capacitors to a sampling voltage; and using a plurality of switches to couple the at least two reference capacitors so that they are charged during a sampling phase, to couple the at least two sampling capacitors so that they are charged during the sampling phase, to couple at least one of the reference capacitors so that it is parallel to one of the at least two sampling capacitors during a hold phase, and to couple the other of the at least two sampling capacitors so that it couples the at least one of the reference capacitors and the one of the at least two sampling capacitors to a reference capacitor of a second MDAC stage.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a pipelined Analog-to-Digital Converter (ADC) in accordance with some embodiments.
FIGS. 2a-2c are block diagrams of a Multiplying Digital-to-Analog Converter (MDAC) stage in accordance with some embodiments.
FIG. 3 is a timing diagram of clock signals in accordance with some embodiments.
FIGS. 4a-4h are circuit diagrams showing a pipelined ADC stage in accordance with some embodiments.
FIG. 5 is a circuit diagram of another pipelined ADC stage in accordance with some embodiments.
FIG. 6 is another timing diagram of clock signals in accordance with some embodiments.
FIG. 7 is a circuit diagram of a final ADC stage in accordance with some embodiments.



DETAILED DESCRIPTION
Systems and methods for providing a pipelined Analog-to-Digital Converter (ADC) are provided. In accordance with some embodiments, a zero-crossing pipelined ADC that uses a reference pre-charge technique is provided.
Turning to FIG. 1, an example of a general architecture 100 that can be used for a pipelined ADC in some embodiments is shown. As illustrated, architecture 100 can include a clock generator 102, non-final ADC stages 104, 106, and 108, and a final ADC stage 110. Although three non-final ADC stages are illustrated in FIG. 1, any suitable number of non-final ADC stages can be used in some embodiments.
Clock generator 102 can be any suitable circuit for generating clock signals for controlling the pipelined ADC in some embodiments. Examples of clock signals that can be generated by generator 102 are described below in accordance with some embodiments.
Non-final ADC stages 104, 106, and 108 can be any suitable non-final ADC stages, and may include a sub-ADC and a Multiplying Digital-to-Analog Converter (MDAC) such as the MDAC described below in accordance with some embodiments. Final ADC stages 110 can be any suitable final ADC stages, such as those described below in accordance with some embodiments.
FIGS. 2a, 2b, and 2c illustrate example architectures 202, 216, and 222 of a 1-bit MDAC with a charge based reference that can be used in a non-final ADC stage in accordance with some embodiments. More particularly, FIG. 2a illustrates an example 1-bit MDAC architecture during a sample phase, FIG. 2b illustrates an example 1-bit MDAC architecture during a hold phase, and FIG. 2c illustrates an alternative 1-bit MDAC architecture to that in FIG. 2a during a sample phase.
As shown in FIG. 2a, architecture 202 can include reference capacitors Crefp 204 and Crefn 206, sampling capacitors C1 208 and C2 210, a comparator 212, and a load capacitance 214.
As shown, in the sample phase, the reference voltages Vrefp and Vrefn can be sampled onto the reference capacitors and input voltage Vin can be sampled onto the sampling capacitors.
During a hold phase, as shown in FIG. 2b, one of two switches 218 and 220 can be closed based on whether a sub-ADC output bit associated with the stage is a zero or a one. That is, if the bit is a zero, then switch 218 will be closed, and if the bit is a one, then switch 220 will be closed. Charge stored on the corresponding capacitor 204 (which corresponds to switch 218) or capacitor 206 (which corresponds to switch 220) can be added to parallel sampling capacitor C2 210 and transferred along with charge from sampling capacitor C1 208 to load capacitance 214.
As shown in FIG. 2c, in some embodiments, the charge that is supplied to the reference capacitors need not come from a voltage source. Instead, currents Irefp and Irefn from current sources 224 and 226 can be used to charge capacitors Crefp and Crefn for a period of time Tref and thereby provide the same amount of charge. The value of the reference voltage can be given by Vref=(Iref/Cref)Tref. When using a current source to charge a reference capacitor, the exact value of the reference voltage sampled may not be known beforehand, as the parameters Iref, Cref and Tref can be process dependent. But because this value is input signal independent, the error can be combined with stage gain error and corrected with Digital Gain Error Correction.
In some embodiments, each MDAC can use a 1 bit/stage architecture with a nominal gain of 1.75.
Turning to FIG. 3, a timing diagram of clock signals that can be generated by a clock generator for controlling the operation of a pipelined ADC in accordance with some embodiments is illustrated. As shown, each stage of the pipelined ADC has a sample phase and a hold phase represented by Φs and Φh, respectively. During the sample phase, the stage input Vin is sampled onto the sampling capacitors. During the hold phase, charge from the stage is transferred to the next stage.
During a pre-charge phase of the reference phase, represented by Φrpe for even numbered stages and Φrpo for odd numbered stages, the reference capacitors can be discharged.
During a reference charge phase, represented by Φre for even numbered stages and Φro for odd numbered stages, reference current sources Irefp and Irefn can charge the reference capacitors Crefp1, Crefp2 and Crefn1, Crefn2 to a nominal reference voltage Vrefp and Vrefn, respectively
During a pre-charge phase for the sampling phase, represented by Φpo for odd numbered stages and Φpe for even numbered stages, the sampling capacitors can be discharged.
FIGS. 4a-4h illustrate an example of a design for a pipelined ADC stage of a circuit 400 that uses a 1-bit MDAC architecture as described above in FIGS. 2a-2C in accordance with some embodiments. As shown, the ADC stage is a differential stage and receives input voltages Vinp and Vinn. Charge from these input voltages is stored in sampling capacitors C1p, C2p and C1n, C2n, respectively. Input voltages Vinp and Vinn are also compared by comparator 402 (which acts as a 1-bit sub-ADC) to provide an output bit bN. Reference capacitors Crefp1, Crefp2, Crefn1, and Crefn2 are charged by current sources Irefp and Irefn. At the end of the sample phase, based on the value of bN output by comparator 402, the charge from one pair of reference capacitors Crefp1, Crefn1 or Crefp2, Crefn2 is transferred to the output capacitors C1p, C1n during the hold phase, by shorting one of their plates to Vcm.
In circuit 400, the sampling of the input voltages, the charging of the reference capacitors, and the transfer of charge from these capacitors to the next stage is controlled by the illustrated switches that open or close based on the value of Φh, Φrpe, Φrpo, Φre, Φro, Φpe, Φpo, ΦN−1, ΦdN−1, ΦN, ΦdN, ΦN+1, ΦdN+1, bN, and bN+1.
As described above, Φh, Φrpe, Φrpo, Φre, Φro, Φpe, Φpo can be generated by a clock generator in some embodiments. The eight FIGS. 4a-4h show the states of switches for Φh, Φrpe, Φrpo, Φre, Φro, Φpe, Φpo at the eight sub-stage periods 1-8, respectively, shown in FIG. 3, in some embodiments.
ΦN−1, ΦdN−1, ΦN, ΦdN, ΦN+1, ΦdN+1 can be generated by the ZCDs and delay circuits shown in some embodiments. bN and bN+1 can be generated by the comparators shown in some embodiments. Although specific states for the switches for ΦN−1, ΦdN−1, ΦN, ΦdN, ΦN+1, ΦdN+1, bN, and bN+1 are shown, these switches can be opened or closed in some of the sub-stage periods based on the operation of the circuit and the stage input voltage in some embodiments. bN and bN+1 can correspond to switches that close when these signals are high and bN* and bN+1* can correspond to switches that close when the signals bN and bN+1 are low.
Any suitable zero-crossing detector (ZCD) can be used in circuit 400 in some embodiments. For example, a ZCD similar to the ZCD described in L. Brooks et al., “A 12b, 50 MS/s, Fully Differential Zero-Crossing Based Pipelined ADC,” IEEE Journal of Solid-State Circuits, December 2009, which is hereby incorporated by reference herein in its entirety, can be used in some embodiments. In some embodiments, this ZCD can use static inverters at its output to generate the necessary logic levels. Overshoot can be corrected by adjusting the relative strengths of the PMOS load pair of the ZCD in some embodiments.
Although not shown, additional calibration logic for gain calibration can be implemented in some embodiments.
In some embodiments, the output current sources Ip, In that perform the charge transfer can employ local negative feedback to provide a constant ramp rate for good linearity.
In some embodiments, the bias currents for a first set of stages (e.g., stages 1-3) can be analog and digitally programmable, while those of the rest of the stages can share a common digital control.
In some embodiments, pipelined ADC 100 of FIG. 1 can be implemented so as to provide a multi-bit output at each ADC stage. Multi-bit non-final ADC stages 104, 106, and 108 in such a pipelined ADC can be any suitable multi-bit ADC stages, and can include a sub-ADC path and a multi-bit MDAC stage in some embodiments.
In some embodiments, the sub-ADC path can include a set of signal and reference capacitors and a sequential search a successive approximation register (SAR) ADC that can obtain its thresholds by capacitive division between its input and its reference, and hence may not require a reference buffer.
In some embodiments, the multi-bit MDAC stages can be implemented using a 2.5 bit zero-crossing MDAC stage, each with a gain of four, such as stage 500 of FIG. 5 in some embodiments.
Similarly to the circuits described above, in stage 500, during the sample phase Φs, the differential input Vinp, Vinn is sampled on the signal (or sampling) capacitors Csigp, Csign. Concurrently, during the reference pre-charge phase Φref, six reference current sources, Irefp<5:0> and Irefn<5:0>, pre-charge six reference capacitors, Crefp<5:0> and Crefn<5:0>, to the nominal reference voltages. At the end of the pre-charge phase Φp, based on outputs S<5:0> of a sub-ADC corresponding to the stage (not shown), the reference capacitors are connected appropriately to Vp and Vn by a switch 502. By the end of the hold phase Φh, the charge is transferred to the next stage capacitors C′sigp, C′sign. The value of the reference voltage in the case of reference pre-charging is given by Vref=(Irefp/Crefp−Irefn/Crefn)Tref, where Tref is the duration for which the reference current sources Irefp<5:0> and Iref<5:0> charge the reference capacitors, Crefp<5:0> and Cref<5:0>. The exact value of Vref may not be known beforehand in some embodiments, but, because Vref is input-signal independent, the variation can be combined with stage gain error and corrected with digital gain error correction.
The loading can be reduced in some embodiments by connecting the reference capacitors to Vp and Vn only when required.
In some embodiments, each Cref<5:0> can be chosen to be half the size of Csig as a tradeoff between the extra loading and power supply noise rejection. In some embodiments, the reference current sources Irefp<5:0> and Irefn<5:0> can be implemented as digitally programmable cascoded current sources. Switches (not shown) can be used to short the reference capacitors to the common-mode voltage Vcm during Φpr, and pre-charge the outputs Voutp and Voutn to GND and VDD respectively, during Φp, in a similar manner to that shown in FIGS. 4a-4h in some embodiments.
The output current sources, Ip and In, perform the actual charge transfer and can be implemented as regulated cascodes for good linearity performance in some embodiments. The zero-crossing detector (ZCD) can include a two-stage differential-to-single-ended amplifier for improved common-mode rejection. The overshoot at the output of the MDAC due to the finite delay of the ZCD can be compensated for by introducing an offset through a digitally programmable capacitor array connected to Vp and Vn as described in J. Chu et al., “A zero-crossing based 12b 100 MS/s pipelines ADC with decision boundary gap estimation calibration,” VLSI Circuits (VLSIC), 2010 IEEE Symposium on, June 2010, which is hereby incorporated by reference herein in its entirety.
A timing diagram for clock signals that can be used control stage 500 of FIG. 5 in accordance with some embodiments is shown in FIG. 6. The illustrated clock signals can be generated by any suitable clock generator in some embodiments.
An example of a circuit 700 that can be used to implement a final ADC stage 110 in accordance with some embodiments is shown in FIG. 7. As illustrated, reference pre-charging can be implemented in circuit 700 similarly to that shown and described above in connection with FIG. 5. Circuit 700 can also use a binary search SAR 702 to resolve the final eight bits of the pipelined ADC in some embodiments. Depending on the outputs D<7:1> of SAR 702, the reference capacitors are connected appropriately by switch 704 to implement a binary search in some embodiments. The SAR timing can be controlled using a delay locked loop in some embodiments.
In some embodiments, a pipelined ADC as described here can be implemented using only two external voltages: a VDD and a common-mode voltage Vcm, which can be nominally VDD/2 formed using a resistive divider from VDD.
Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of the invention, which is only limited by the claims which follow. Features of the disclosed embodiments can be combined and rearranged in various ways.

## Claims

1. A system for providing a pipelined Analog-to-Digital Converter, comprising:
a first multiplying Digital-to-Analog Converter (MDAC) stage comprising:
a sub-Analog-to-Digital Converter (ADC) that outputs a value based on an input signal;
a first reference capacitor that is charged to a reference voltage;
a second reference capacitor that is charged to the reference voltage;
a first sampling capacitor that is charged to a sampling voltage;
a second sampling capacitor that is charged to the sampling voltage; and
a plurality of switches that couple the first reference capacitor and the second reference capacitor so that the first reference capacitor and the second reference capacitor are charged during a sampling phase, that couple the first sampling capacitor and the second sampling capacitor so that the first sampling capacitor and the second sampling capacitor are charged during the sampling phase, that couple the first reference capacitor so that the first reference capacitor is parallel to the second sampling capacitor during a hold phase in response to the value output by the sub-ADC having a first value, and that couple the first sampling capacitor so that the first sampling capacitor couples the first reference capacitor and the second sampling capacitor to a reference capacitor of a second MDAC stage during the hold phase in response to the value output by the sub-ADC has the first value.

2. The system of claim 1, wherein the first MDAC stage further comprises a first current source coupled to the first reference capacitor and a second current source coupled to the second reference capacitor.

3. The system of claim 2, wherein the first current source and the second current source are cascoded current sources.

4. The system of claim 2, wherein the first current source charges the first reference capacitor for a given period of time and the second current source charges the second reference capacitor for the given period of time.

5. The system of claim 1, wherein the first MDAC stage further comprises a zero-crossing detector that controls how long the first sampling capacitor and second sampling capacitor are coupled to an input voltage.

6. The system of claim 5, wherein the first MDAC stage further comprises a delay circuit that receives an output signal of the zero-crossing detector and provides a delayed signal that controls when the first sampling capacitor and second sampling capacitor are coupled to the input voltage.

7. The system of claim 1, wherein the sub-ADC is a comparator.

8. The system of claim 1, wherein the sub-ADC is a flash ADC.

9. The system of claim 1, wherein the first MDAC stage is a differential MDAC stage.

10. The system of claim 1, wherein the first reference capacitor and the second reference capacitor are each half the size of the first sampling capacitor and the second sampling capacitor.

11. A method for providing a pipelined Analog-to-Digital Converter, comprising:
in a first multiplying Digital-to-Analog Converter (MDAC) stage:
outputting from a sub-Analog-to-Digital Converter (ADC) a value based on an input signal;
charging a first reference capacitor and a second reference capacitor to a reference voltage;
charging a first sampling capacitor and a second sampling capacitor to a sampling voltage; and
using a plurality of switches to couple the first reference capacitor and the second reference capacitor so that the first reference capacitor and the second reference capacitor are charged during a sampling phase, to couple the first sampling capacitor and the second sampling capacitor so that the first sampling capacitor and the second sampling capacitor are charged during the sampling phase, to couple the first reference capacitor so that the first reference capacitor is parallel to the second sampling capacitor during a hold phase in response to the value output by the sub-ADC having a first value, and to couple the first sampling capacitor so that the first sampling capacitor couples the first reference capacitor and the second sampling capacitor to a reference capacitor of a second MDAC stage during the hold phase in response to the value output by the sub-ADC has the first value.

12. The method of claim 11, charging the first reference capacitor using a first current source and charging the second reference capacitor using a second current source.

13. The method of claim 12, wherein the first current source and the second current source are cascoded current sources.

14. The method of claim 12, wherein the first current source charges the first reference capacitor for a given period of time and the second current source charges the second reference capacitor for the given period of time.

15. The method of claim 11, wherein controlling how long the first sampling capacitor and second sampling capacitor are coupled to an input voltage using a zero-crossing detector.

16. The method of claim 15, further comprising providing a delayed signal that controls when the first sampling capacitor and second sampling capacitor are coupled to the input voltage in response to an output of the zero-crossing detector.

17. The method of claim 11, wherein the sub-ADC is a comparator.

18. The method of claim 11, wherein the sub-ADC is a flash ADC.

19. The method of claim 11, wherein the first MDAC stage is a differential MDAC stage.

20. The method of claim 11, wherein the first reference capacitor and the second reference capacitor are each half the size of the first sampling capacitor and the second sampling capacitor.