Abstract:
A method of compensating for finite common mode rejection in a switched capacitor circuit including arrays of input capacitors coupled to first and second differential nodes, includes the step of sampling a common mode voltage onto the differential nodes during a sampling phase. The input capacitors are then coupled to a ground node against which the common mode voltage is referenced to capture an offset voltage between the first and second differential nodes. The voltage offset is then subtracted out.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application for patent is related to the following applications for patent: 
     U.S. patent application Ser. No. 09/919,181, filed Jul. 30, 2001, entitled “CIRCUITS AND METHODS FOR OFFSET VOLTAGE COMPENSATION IN A CHARGE REDISTRIBUTION DIGITAL TO ANALOG CONVERTER” by inventor Shyam S. Somayajula currently pending; 
     U.S. patent application Ser. No. 09/919,014, filed Jul. 30, 2001 by inventor Somayajula, entitled “A HIGH SPEED SUCCESSIVE APPROXIMATION RETURN PATH AND DATA CONVERSION METHODS AND CIRCUITS USING THE SAME” currently pending 
     U.S. patent application Ser. No. 09/919,411, filed Jul. 30, 2001 by inventor Somayajula, entitled “CIRCUITS AND METHODS FOR LATCH METASTABILITY DETECTION AND COMPENSATION AND SYSTEMS USING THE SAME” currently pending 
     U.S. patent application Ser. No. 09/919,410, filed Jul. 30, 2000 by inventor Somayajula, entitled “ANALOG TO DIGITAL CONVERSION CIRCUITS, SYSTEMS AND METHODS WITH GAIN SCALING SWITCHED-CAPACITOR ARRAY” currently pending 
     U.S. patent application Ser. No. 09/918,852, filed Jul. 30, 2001 by inventor Somayajula, entitled “CIRCUITS AND METHODS FOR LINEARIZING CAPACITOR CALIBRATION AND SYSTEMS USING THE SAME”, currently pending; and 
     U.S. patent application Ser. No. 09/918,616, filed Jul. 30, 2001 by inventor Somayajula, entitled “HIGH SPEED, LOW-POWER SHIFT REGISTER AND CIRCUITS AND METHODS USING THE SAME” currently pending. 
    
    
     FIELD OF INVENTION 
     The present invention relates in general to switched capacitor circuits and in particular to methods and circuits for compensating for finite common mode rejection in switched capacitor circuits. 
     BACKGROUND OF INVENTION 
     One particular technique for performing analog to digital (A/D) conversion is through successive approximation. The basic successive approximation A/D converter (ADC) includes an analog comparator and a clocked feedback loop having a successive approximation register (SAR) and a digital to analog converter (DAC). 
     Generally, the analog input signal voltage is sampled onto an array of weighted capacitors, during the sampling phase, the top plates of which are coupled to one comparator input. The other comparator input is coupled to a comparison voltage, which could be a fixed reference voltage in a single-ended system or the voltage at the top plates of second capacitor array in a differential system. 
     During the first clock cycle of the subsequent conversion phase, the bottom plate of the capacitor representing the digital MSB is coupled to a reference voltage while the bottom plates of the remaining capacitors in the array are coupled to ground or a second reference voltage (ground will be assumed here). The new top plate voltage appears at the input of the comparator and is compared against the comparison voltage. If the new top plate voltage is below the comparison voltage, then the MSB is “kept” by the SAR in the feedback loop by maintaining its bottom plate coupled to the reference voltage. On the other hand, if the top plate voltage is above the comparison voltage, the SAR couples and the bottom plate of the MSB capacitor to ground. The state of the MSB capacitor represent the MSB of the digital output word as a Logic 1. The bottom plate of the second MSB is then coupled to the reference voltage and the same test is performed to determine the state of the next digital code bit. The successive approximation algorithm continues by repeating this procedure for the remaining capacitors in the array such that the voltage difference at the inputs to the comparator converge to zero. At the end of this bit cycling process, the configuration of the switches coupling the bottom plates either to Vref or Gnd represents the input sample in digital form. 
     Successive approximation A/D converters are useful a wide range of applications, including data acquisition, test equipment, instrumentation, cellular communications, among others. Notwithstanding, in order to improve and broaden the utility of this type of A/D converter, significant challenges remain to be addressed. These challenges include improving the device speed given a set of process constraints, reducing the coding error rate, handling metastable states and device calibration 
     SUMMARY OF INVENTION 
     In accordance with the inventive concepts, circuits and methods are disclosed for compensating for a finite common mode rejection in switch capacitor circuitry. Such circuits and methods are particularly useful in circuitry such as charge redistribution analog to digital converters, although not necessarily limited thereto. 
     According to one embodiment of the inventive concepts, a method is disclosed for compensating for finite common mode rejection in a switched capacitor circuit including arrays of input capacitors coupled to first and second differential nodes. A common mode voltage is sampled onto the differential nodes during a sampling phase. Next, the input capacitors are coupled to a ground node against which the common mode voltage is referenced to capture an offset voltage between the first and second differential nodes. The voltage offset is then subtracted out. 
     Advantageously, any voltage offset caused by the finite common mode rejection of the input capacitor arrays at a given common mode voltage in a switch capacitor circuitry can easily be compensated for. With respects to circuits and systems including a comparator or similar operational amplifier circuit also introducing an input offset voltage, both the offset caused by the finite common mode rejection and that caused by the operational amplifier circuit can be subtracted out simultaneously. This feature is particularly useful in a charge redistribution analog to digital converters where an input voltage offset, no matter the source, can cause errors in the digital encoding. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a high level block diagram of a digital receiver suitable for describing the principles of the present invention; 
     FIG. 2 is a high level functional block diagram of a single-chip analog to digital converter suitable for describing the present inventive concepts; 
     FIG. 3 is a high level functional diagram of one offset compensation circuit according to the principles of the present invention; 
     FIG. 4A is a functional block diagram of a switched-capacitor charge redistribution ADC embodying the present principles; 
     FIG. 4B illustrates one pair of the capacitor arrays of FIG. 4A, in further detail; 
     FIG. 5A is a conceptual schematic diagram illustrating a typical configuration during the offset voltage compensation sampling phase; and 
     FIG. 5B is a conceptual schematic diagram illustrating a typical configuration during the offset voltage compensation conversion phase. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-5B of the drawings, in which like numbers designate like parts. 
     FIG. 1 is a high level block diagram of a digital receiver  100  suitable for describing the principles of the present invention. Generally, receiver  100  includes an RF input port or antenna  101  for receiving an RF signal, such as those used in telecommunications, and radio and television transmission. The desired RF signal is then extracted from the received signal by a bandpass filter  102 . A low noise amplifier (LNA)  103  sets the system noise figure before the RF signal is downconverted in frequency by a mixer stage  104  operating in response to a local oscillator (LO). The output of mixer stage  104  may be passed through a second low noise amplifier (LNA)  105  before conversion into the digital domain. 
     In system  100 , analog to digital conversion is performed using analog to digital converter (ADC)  200  discussed in detail below. Once converted into the digital domain, the received data can be processed using a digital signal processor (DSP) subsystem  106 . 
     FIG. 2 is a high level functional block diagram of a single-chip analog to digital converter  200  suitable for describing the present inventive concepts. A/D converter  200  is based on 4 interleaved successive-approximation A/D converters (ADCs)  201 , which will be discussed further below. The digitally coded samples are de-interleaved in block  202  and output through output register  203 . A/D converter  200  also includes an on-chip reference voltage generator  204 , clock generator  205  and clock generation circuitry  206 . 
     Each of the ADCs  202  includes a fully differential capacitor array  206  under control of switch control circuitry  207 . During the tracking mode, capacitor array  206  samples the input voltage at the differential analog inputs AIN+ and AIN−. During the hold mode, the sampled charge is held while the successive approximation algorithm generates the digital bits. 
     A gain stage  208  with minimal gain provides signal isolation in front of latch (comparator)  209 . SAR return path  210  and FSM  211  feedback the reference voltage to capacitor array  206 . 
     In a conventional charge redistribution DAC, the bottom plate of a graduated filter array is coupled to the input during the sampling phase and all capacitors are charged to the input signal voltage. After the capacitors are charged, their bottom plates are coupled to ground during hold phase such that the top plates of the capacitors are at a voltage V X  which is approximately equal to −A IN , where A IN  is the input sample voltage. During bit cycling, the bottom plate of the capacitor representing the MSB is coupled to the reference and the voltage of the top plate compared against the reference. The bottom capacitors of the remaining capacitors remain coupled to ground to form a voltage divider. If upper plate voltage is below the reference, then the capacitor continues to be coupled to the reference (i.e. a logic 1 for the bit), otherwise the bottom plate of the capacitor is recoupled to ground (i.e. reset to 0). 
     One problem encountered in the design of comparators is the input offset voltage between its differential inputs. The effect of this offset is a shift, either up or down, in the input threshold voltage above which the comparator output swings towards the high output rail and below which the comparator output swings towards the low voltage rail. In the particular case of a successive approximation circuit, a shifting in the threshold voltage can cause the comparator to swing in the wrong direction, especially when the feedback loop voltage closely approaches the input sample voltage during the generation of the least significant bits. The comparator input offset voltage thus can result in the mis-coding of one or more bits in the digital output word. Specifically, the offset must be kept below the voltage corresponding to ½ LSB to avoid coding errors. 
     Common mode rejection (CMR) is another problem which much be addressed in differential systems, such as ADCs  201 . In the ideal case, the sampled analog input signal A IN  will be symmetric around the common mode voltage V CM , when seen at the differential inputs to the comparator. (For purposes of the present discussion, it will be assumed that V CM  is a stable voltage of approximately            V   REF     2     ,                          
     although this voltage may change depending on the specific design). With respects to actual circuits however, the impedances of the capacitor arrays at the comparator inputs will not match. The resulting non-infinite CMR appears as an additional offset voltage at the comparator inputs which contribute to the error into the SAR bit conversion process. The state of the input arrays after the normal mode sampling phase is represented by Equation (1):          VaP   -   VaN     =         [       (         C   INPVrf       C   TOTP       +       C   INNSgnd       C   TOTN         )     -     (         C   InnVref       C   TOTN       +       C   inpSgnd       C   TOTP         )       ]     ·     V   REF       -         A   IN     2     ·     [         C   INTOTP       C   TOTP       +       C   INTOTN       C   TOTN         ]       -       V   cm     ·     [         C   INTOTP       C   TOTP       -       C   INTOTN       C   TOTN         ]                                
     where: 
     VaP=voltage at the positive comparator input; 
     VaN=voltage at the negative comparator input; 
     C TOTP =total capacitance coupled to the positive input node; 
     C INPVRF =total input capacitance at the positive comparator input coupled to V REF  after bit-cycling; 
     C TOTN =total capacitance coupled to the negative input node; 
     C INNVRF =total input capacitance at the negative comparator input coupled to V REF  after bit-cycling; 
     C INTOTP =total input capacitance coupled to the positive input node; and 
     C INTOTN =total input capacitance coupled to the negative input node. 
     The last term of this equation represents the offset component at the comparator inputs caused by a non-zero common mode voltage and a capacitance mismatch between the input capacitors of the positive and negative paths. Depending on the size of this offset and the bit being generated, this offset can contribute, at least in part, to the comparator output swinging in the wrong direction. 
     One circuit embodiment of the principles of the present invention is shown in FIG.  3 . In this case, an error cancellation (compensation) stage  301 , which is preferrably a subtractor, is placed in front of the comparator. This cancellation stage receives feedback from the comparator output and cancels out the input voltage offset caused by the comparator and the finite CMR of the capacitor arrays at a given input CM voltage. The feedback loop allows the input offset to be dynamically cancelled as the input sample and reference voltages change and/or if the sampling or clock frequencies change. Cancellation stage  301  performs the cancellation operation in the analog domain, although its control is performed in the digital domain. A preferred charge redistribution DAC suitable with an integral offset compensation (cancellation) circuitry is discussed below in conjunction with FIGS. 4A-4C. 
     A switched-capacitor charge redistribution DAC  400  embodying the present principles is shown in FIG.  4 A. In this embodiment, two capacitor arrays are provided for each of the paths into the positive and negative differential inputs into comparator  404 , namely, Input Capacitor Array  401  and Offset Compensation Capacitor Array  402 . One pair of arrays  401  and  402  is shown in further detail in FIG.  4 B. Input Capacitor Array  401  comprises a termination capacitor C T  and an array of capacitors C INi , i=1 to m graduated in the sequence C, 2C, 4C, . . . 2 m−2 C, 2 m−1 C(ff). In the following discussion, input capacitors  401  coupled to the positive comparator input are designated C INPi  and those coupled to the negative comparator input designated C INNi . Generally, the termination capacitors do not participate in the bit-cycling process, except during capacitance calibration (trim) operations. 
     Offset Compensation Capacitor Array  402 , comprises a termination capacitor C T  and an array of capacitors C OFFj , j=1 to n, graduated in the sequence C, 2C, 4C, . . . 2 n−2 C, 2 n−1 C (ff). Offset capacitors coupled to the positive comparator input are designated C OFFPj  and those coupled to the negative comparator input C OFFNj . 
     The bottom plate of each capacitor in array  401  is associated with a set of switches  403 , one of which is shown in further detail. Specifically, each input array switch set includes a switch S AINi  for coupling the plate to the analog input voltage A IN+  or A IN− , a switch S VRi+  for coupling to the positive reference voltage rail V REF+ , a switch S GNDi  for coupling to the negative reference rail V REF− , in this case GND, and a switch S CMi  for coupling to the input common mode voltage V CMI  referenced to the differential inputs A IN+  and A IN− . 
     A similar set of switches  404 , indexed j=1 to m, selectively couple the bottom plates of the capacitors of offset array  402  to the input signal (A IN+  or A IN− ) and the reference voltage (V REF+ ) and GND. After the SAR bit cycling process, the state of switches  403  represent the digitally coded input sample and is stored in registers  406  for output. 
     According the inventive principles, the offset voltage caused by the non-infinite CMR is captured on the top plates of the input capacitor arrays. This offset is then compensated using calibration arrays  402 . This process is generally illustrated in FIGS. 5A and 5B. 
     During the offset compensation sampling phase shown in FIG. 5A, the common mode voltage V CM  is sampled onto all the input capacitors C INTOTP  and C INTOTN , of the positive and negative paths. At the same time, the bottom plates of the positive path offset capacitors C OFFMSBP  and C OFFLSBP  are coupled to V REF  and Gnd, respectively. The complementary negative path offset capacitors C OFFMSBN  and C OFFLSBN  are coupled to Gnd and V REF , respectively. (“Complementary” sets of capacitors are those coupled by the appropriate switches at a given instant to opposing reference voltages. For example, if capacitor C INPi  in input array  401   a  at the positive comparator input is coupled to V REF , then the corresponding capacitor in C INNi  in input array  401   b  at the negative comparator input is coupled to GND, and vice versa. Similarly, if a capacitor C OFFPj  in offset array  402   a  is coupled to V REF , capacitor C OFFNj  in offset array  402   b  is coupled to GND, and vice versa.) 
     At the start of the calibration conversion phase, the bottom plates of the input capacitors C INTOTP  and C INTOTN  are coupled to GND VCM , which is the ground against which the common mode voltage Vcm is referenced. A SAR bit cycling process is then performed using only the offset capacitors C OFFPj  and C OFFNj . The bits on the offset capacitors after bit cycling approximate the inverse of the offset voltage V OFF , such that the bit positions of the corresponding switches represent the offset compensation code. This code encompasses not only the voltage offset introduced by the comparator, but also that caused by the finite CMR of the capacitor arrays. 
     During subsequent normal mode operation, the input capacitors C INTOTP  and C INTOTN  are coupled to the system inputs to sample the differential analog inputs A INP  and A INN . During normal mode sampling, the MSB offset capacitor of the positive path and the LSB offset capacitors of the negative path are coupled to V REF  while the LSB offset capacitors of the positive path and the MSB capacitor of the negative path are coupled to ground. A SAR bit cycling operation is then performed through the input capacitors C INPi  and C INNi . During the normal operation conversion phase, the offset capacitors C OFFPj  and C OFFNj  are selectively coupled to V REF  and ground to reflect the offset compensation codes determined during the calibration operations. 
     While a particular embodiment of the invention has been shown and described, changes and modifications may be made therein without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.