Abstract:
A switched capacitor circuit ( 300 ) including an operational amplifier ( 206 ) having an input and an output, a sampling capacitor ( 203 ) and a set of switches ( 204, 205, 301,  and  302 ) are disclosed. During a first phase, switches ( 201, 204 ) sample an input voltage by charging sampling capacitor ( 203 ). During a first portion of a second phase, the operational amplifier input is electrically coupled to sampling capacitor ( 203 ) through a first path including switch ( 301 ) having a first time constant. During a second portion of the second phase, the operational amplifier input is electrically coupled with sampling capacitor ( 203 ) through a second path including switch ( 302 ) having a second time constant, the second time constant being smaller than the first time constant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application for patent is related to the following applications for patent: 
     Pending U.S. patent application Ser. No. 09/904,940, filed Jul. 12, 2001 by inventor Yu Qing YANG and entitled “SWITCHED-CAPACITOR SUMMER CIRCUITS AND METHODS AND SYSTEMS USING THE SAME”; and 
     Pending U.S. patent application Ser. No. 09/870,900, filed May 30, 2001 by inventor Yu Qing YANG and entitled “SWITCHED-CAPACITOR SUMMER CIRCUITS AND METHODS AND SYSTEMS USING THE SAME”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to switched-capacitor circuitry and methods and in particular to switched-capacitor circuits and methods with improved settling time and systems using the same. 
     2. Description of the Related Art 
     Switched-capacitor circuits are used in a wide range of applications, including analog-to-digital converters (ADCs), digital-to-analog converters (DACs) delta-sigma modulators, filters, power supplies, and voltage regulators. Generally, switched capacitor techniques allow for the construction of accurate, compact, frequency tunable circuits without the use of resistors. This is particularly advantageous in integrated circuit designs, where the construction of precise resistor-based circuits is difficult and unnecessarily consumes chip area. 
     In the basic switched-capacitor integrator, an input voltage is sampled onto a sampling capacitor during a first clock phase. During a second (non-overlapping) clock phase, the charge on the sampling capacitor is transferred to the integrator capacitors. The output of the integrator is fedback to the summing node with a integrator capacitor. The impedance of the circuit depends on the size of the sampling and integrator capacitors and the frequency of the clock. 
     One of the key performance parameters in switched-capacitor circuits is the settling time. In particular, it is important that the circuit output ramp to its final value quickly and smoothly. This is particularly true in high performance applications, such as audio analog-to-digital and digital-to-analog conversion circuits where discontinuities in the circuit response can affect the ultimate quality of the audio signal presented to the listener. 
     SUMMARY OF THE INVENTION 
     According to one such embodiment, a switched capacitor circuit is disclosed which includes an operational amplifier having an input and an output, a sampling capacitor and a set of switches. During a first phase, a set of switches samples an input voltage by charging sampling capacitor. During a first portion of a second phase, the operational amplifier input is electrically coupled to sampling capacitor with a first path having a first time constant. During a second portion of the second phase, the operational amplifier input is electrically coupled with the sampling capacitor through a second path having a second time constant, the second time constant being smaller than the first time constant. 
     The present inventive principles substantially reduce the settling time when a switched capacitor techniques are being practiced. Specifically, circuits and methods are disclosed which substantially reduce the negative output spikes in switched capacitor circuits by controlling the rate of voltage change at the switch capacitor circuit summing nodes. In particular, multiple sets of switches are provided for transferring charge to the summing node. One set of switches has a large time constant such that the voltage change is relatively slow and a second set of switches has a smaller time constant such that the change in voltage at the summing node is relatively rapid. 
    
    
     BRIEF DESCRIPTION OF THE 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. 1A is a functional block diagram of a single-chip analog to digital converter suitable for illustrating the present inventive principles; invention 
     FIG. 1B is functional block diagram of a 5th Order delta-sigma modulator suitable for use in the analog to digital converter of FIG.  1 A: 
     FIG. 1C is a functional block diagram of a digital radio employing  1 C and analog to digital converter such as shown in FIG. 1A; 
     FIG. 2A illustrates an exemplary fully-differential conventional delayed switched-capacitor integrator; 
     FIG. 2B illustrates the ideal response; 
     FIG. 2C illustrates the embodiment in which this creates a negative spike in the output 
     FIG. 3 illustrates a first technique for substantially reducing or eliminating the negative voltage spike shown in FIG. 2 according to the present inventive concepts; and 
     FIG. 4 illustrates an alternate switched-capacitor network  400  according to the inventive principles. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-4 of the drawings, in which like numbers designate like parts. 
     FIG. 1A is a high level functional block diagram of a single-chip audio analog-to-digital (A/D)  100  suitable for practicing the principles of the present invention. A/D converter  100  is only one of a number of possible applications requiring switched-capacitor integrator stages. Other examples include digital to analog converters (DACs) and Codecs. 
     A/D converter  100  includes two conversion paths for converting left and right channel analog audio data respectively received at the left and right analog differential inputs AINL+/− and AINR+/−. The analog inputs are passed through an input gain stage  101  and then to a 5th order delta-sigma modulator. 
     Each delta-sigma modulator is represented in FIG. 1 by a summer  102 , low-pass filter  104  (quantizer), comparator  105  and DAC  106  in the feedback loop. The outputs from the delta-sigma modulators are passed through a decimation filter  107  and a high pass filter  108 . 
     The resulting left and right channel digital audio data are output through a single serial port SDOUT, timed with a serial clock SCLK and a left-right clock LRCLK in accordance with the Digital Interface Format (DIF). The SCLK and LRCLK clocks can be generated externally and input to converter  100  or can be generated on-chip, along with the associated data, in response to a received master clock MCLK. 
     If 5th order delta-sigma modulators are used in system  100 , each low pass filter  104  comprises 5 integrator stages  109  as shown in FIG.  1 B. Integrator stages  109  are one of a number of possible applications of switched capacitor integrators  300  and  400  according to the inventive concepts discussed below. 
     One possible application of A/D converter is in a digital radio  110 , such as that shown in FIG.  1 C. 
     Digital radio  110  includes an analog section or front-end  111  which receives radio frequency (RF) signals from an associated antenna  112 . Analog front-end  111  is preferably a conventional RF down-converter including a low noise amplifier (LNA)  113  for setting the system noise figure, a bandpass filter  114  and mixer  115  driven by an analog local oscillator  116 . The mixed-down analog signal is then converted into digital form by analog to digital converter  117 . 
     The digitized data output from A/D converter  117  is passed to digital processing section  118 . A pair of mixers  119   a,b  generate in-phase (I) and quadrature (Q) signals from a corresponding pair of clock phases from crystal oscillator  120 . The I and Q signals are next passed through bandpass filters  121   a  and  121   b  and on to digital baseband processor  122 . The processed digital signal is then re-converted to analog (audio) form by D/A converter  123 . 
     A switched mode (Class D) audio power amplifier (APA)  124  is used to drive an external set of speakers or a headset. Preferably, at least some of the components of digital radio  110  are powered by a switched mode power supply (SMPS)  124 . 
     An exemplary fully-differential conventional delayed switched-capacitor integrator  200  is shown in FIG.  2 A. Integrator  200  generally works as follows. During clock Phase 1 (φ 1 ) switches  201   a,b  and  204   a,b  close and the input voltage V In  is sampled onto sampling capacitors Cs  203   a,b.  During clock Phase 2 (φ 2 ), switches  201   a,b  and  204   a,b  open and switches  202   a,b  and  205   a,b  close. Consequently, the charge on sampling capacitors Cs to the summing nodes of op amp  206  and onto capacitors  207   a,b.    
     The size of sampling capacitor Cs, and accordingly the amount of charge transferred, will be very large in high performance A/D and D/A converters, and similar applications. Specifically, the charge being transferred is in accordance with the relation: 
     
       
           C=Q ( V   LEFT   −V   RIGHT )  (1) 
       
     
     where: 
     C is the capacitance, Q is the charge stored on the capacitor, V LEFT  is the voltage on the left capacitor plate, and V RIGHT  that on the right capacitor plate. In the present example, the charge Q and capacitance C are fixed. 
     During the initial period of Phase 2, (after switches  202   a,b  and  205   a,b  have closed), the voltage on the left plate of sampling capacitors C S  changes from V IN  (i.e., the final voltage at the end of Phase 1) to V cm . At the same time, the charge on sampling capacitors C S  remains the same, just before it begins to be transferred out. According to Equation 1, the right plate voltage on sampling C S  (i.e. the summing nodes) also must change by the same amount as the left plate voltage, which normally is a larger voltage swing. Consequently, op amp  206  must pull the summing nodes back to common mode voltage (V CM ) before it can start to ramp-up the output voltage V Out  to its final value. This creates a negative spike in the output as shown in FIG. 2C (the ideal response is shown in FIG. 2B for reference). Because the length of Phase 2 is fixed, this phenomena seriously affects the settling of the circuit output 
     Moreover, in order to improve the settling behavior of the conventional integrator shown in FIG. 2, switches  202   a,b  and/or  205   a,b  are usually selected to be large so as to decrease the time constant of the switched-capacitor network (i.e. speed up the response). However, as the speed of the network increases, the negative spike shown in FIG. 2C also grows larger. 
     FIG. 3 illustrates a first technique for substantially reducing or eliminating the negative voltage spike shown in FIG. 2 according to the present inventive concepts. In this embodiment, the time constant of the switched-capacitor network  300  is varied using sets of parallel switches  301   a,b  and  302   a,b  in place of switches  202   a,b  in the embodiment of FIG.  2 A. 
     In network  300 , switches  301   a,b  are selected to be small such that they have a large resistance while switches  302   a,b  are selected to be large such that they have a small resistance. Small switches  301   a,b  therefore increase the time constant and make network  300  slower. On the other hand, large switches  302   a,b  decrease the time constant and make the network faster. Network  300  preferably operates as follows. 
     During Phase 1, switches  204   a,b  close and the input voltage V In  is sampled onto sampling capacitors C S . At the same time, switches  205   a,b  are open such that the summing nodes of op amp  206  are at the voltage on integrator capacitors C I . 
     Small switches  301   a,b  close first during Phase 2 in response to the clock signal φ 2  and switches  204   a,b  open. When only small switches  301   a,b  are closed, and the network  300  is slower (i.e. the RC time constant is increased) which forces the voltages the summing inputs to integrator  206  to change more slowly. Consequently, the magnitude of “negative” spike at the integrator output becomes much smaller. 
     Large switches  302   a,b  are subsequently closed, after the introduction of a delay, by the clock signal φ 2D . In particular, after op amp  206  has pulled the summing nodes almost back to common mode voltage VCM, large switches  302   a,b  are closed such that network  300  becomes significantly faster. The electric charge on sampling capacitors C S  is transferred to the integrator capacitors C I  at full speed. 
     FIG. 4 illustrates an alternate switched-capacitor network  400  according to the inventive principles. In this case, switches  205   a,b  in the conventional embodiment are replaced by sets of parallel switches  401   a,b  and  402   a,b.  Small switches  401   a,b  are closed during the start of Phase 2 in response to clock signal φ 2  to slowly bring the voltages at the summing nodes the common node voltage and minimize the negative voltage spike. Large switches  402   a,b  are subsequently closed after a delay by delayed clock signal φ 2D  to transfer the charge on sampling capacitors C S  on to the integrator capacitors C I  rapidly. In other words, as was done in network  300 , a large time constant is used during the initial phase to minimize spikes in the output while a smaller time constant is used to fully charge the integrator capacitors and ramp the output voltage. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.