Abstract:
A track-and-hold circuit capable of tracking an analog input signal and holding a sampled voltage of the analog input signal at a sampling instant for processing by other circuitry, in response to a track signal that alternates with a hold signal. A first capacitor is provided, having a first terminal connected to a power supply terminal. Tracking circuitry operates when in an on state to apply through a resistor a tracking voltage to a second terminal of the first capacitor that corresponds to the voltage of the analog input signal, by applying the tracking voltage to a first terminal of the resistor, the second terminal of the resistor being connected to the second terminal of the first capacitor. A switch, responsive to the track signal and the hold signal, operates to switch the tracking circuitry to an on state in response to the track signal and to an off state in response to the hold signal, the time of change from the track signal to the hold signal comprising the sampling instant. A second capacitor is provided, having a first terminal connected to the first terminal of the resistor and having a second terminal connected to a power supply terminal. The second capacitor substantially reduces frequency-dependent harmonic distortion.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to track and hold circuits, and more particularly relates to a method for improving the performance of such circuits. 
       BACKGROUND OF THE INVENTION 
       [0002]    High-speed, low-power analog-to-digital converters (ADCs) are widely used in numerous applications, for example in portable wireless communications devices. The general architecture of a typical ADC is shown in  FIG. 3 . An analog input signal I A  is provided to a track-and-hold (or, sample-and-hold) circuit that receives I A  and samples the voltage of that signal at regular intervals. These sample voltages are each held for a period, and provided serially to form an output signal I S  that is provided as an output of the track-and-hold circuit and as an input to the following stage, a quantizer. The quantizer converts the values of the voltage levels of I S  to a digital signal, that it outputs as O D . 
         [0003]    The requirement for ever improved performance in the commercial world of technology results in a corresponding demand for ever improved performance of ADCs. Generally, the performance of high-speed ADC is limited by the performance of its track-and-hold circuit. U.S. Pat. No. 6,489,814, which issued on Dec. 3, 2002, to Gian Hoogzaad et al., discloses an exemplary prior art track-and-hold circuit that attempts to reduce excess currents flowing onto the hold capacitor using a feedback connection. However, prior art track-and-hold circuits still suffer from problems such as harmonic distortion, that adversely affects performance of the ADC. 
         [0004]    There is thus a need for a way of improving the performance of ADC track-and-hold circuits. 
       SUMMARY OF THE INVENTION 
       [0005]    The following summary presents a simplified description of the invention, and is intended to give a basic understanding of one or more aspects of the invention. It does not provide an extensive overview of the invention, nor, on the other hand, is it intended to identify or highlight key or essential elements of the invention, nor to define the scope of the invention. Rather, it is presented as a prelude to the Detailed Description, which is set forth below, wherein a more extensive overview of the invention is presented. The scope of the invention is defined in the Claims, which follow the Detailed Description, and this section in no way alters or affects that scope. 
         [0006]    The present invention is a track-and-hold circuit capable of tracking an analog input signal and holding a sampled voltage of the analog input signal at a sampling instant for processing by other circuitry, in response to a track signal that alternates with a hold signal. A first capacitor is provided, having a first terminal connected to a power supply terminal. Tracking circuitry operates when in an on state to apply through a resistor a tracking voltage to a second terminal of the first capacitor that corresponds to the voltage of the analog input signal, by applying the tracking voltage to a first terminal of the resistor, the second terminal of the resistor being connected to the second terminal of the first capacitor. A switch, responsive to the track signal and the hold signal, operates to switch the tracking circuitry to an on state in response to the track signal and to an off state in response to the hold signal, the time of change from the track signal to the hold signal comprising the sampling instant. A second capacitor is provided, having a first terminal connected to the first terminal of the resistor and having a second terminal connected to a power supply terminal. The second capacitor substantially reduces frequency-dependent harmonic distortion. 
         [0007]    These and other aspects and features of the invention Will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  is a circuit diagram of a first portion of an exemplary prior art sample-and hold circuit. 
           [0009]      FIG. 1B  is a circuit diagram of a second portion of the exemplary prior art sample-and hold circuit of  FIG. 1A . 
           [0010]      FIG. 2  is a circuit diagram of a preferred embodiment of the present invention. 
           [0011]      FIG. 3  is a block diagram of the architecture of an ADC. 
           [0012]      FIG. 4  is a graph of two input signals, showing a sampling instant. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    The making and use of the various embodiments are discussed below in detail. However, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
         [0014]      FIGS. 1A and 1B  show a circuit diagram of an exemplary prior art sample- and hold circuit. The circuit is powered by a power supply providing a source voltage VDD at a VDD node, and a current return ground at a GROUND node. Referring to  FIG. 1A , a PNP bipolar transistor Q 6  has its emitter connected to VDD through a resistor R 21 , receiving a positive bias voltage from node PBIAS at its base. The collector of transistor Q 6  is connected to the emitter of a PNP bipolar transistor Q 1  and to the emitter of a PNP bipolar transistor Q 0 . A diode-connected PNP bipolartransistor Q 13  has its emitter connected to VDD, and its base and collector connected to one terminal of a resistor R 46  and to one terminal of a resistor R 48 . The other end of resistor R 48  is connected to the base of a PNP bipolar transistor Q 11  and to the collector of an NPN bipolar transistor Q 2 . The other end of resistor R 46  is connected to the base of a PNP bipolar transistor Q 10  and to the collector of an NPN bipolar transistor Q 19 . The emitters of transistors Q 10  and Q 11  are connected to VDD through a resistor R 47 . The collector of transistor Q 11  is connected to the collector of an NPN bipolar transistor Q 9 , and to a connection identified as “C” to the circuitry shown in  FIG. 1B . The collector of transistor Q 10  is connected to the collector of an NPN bipolar transistor Q 8 , and to a connection identified as “B” to the circuitry shown in  FIG. 1B . 
         [0015]    An NPN bipolar transistor Q 7  has its emitter connected to GROUND through a resistor R 29 , receiving a negative bias voltage from node NBIAS at its base. The collector of transistor Q 7  is connected to the emitter of transistor Q 2  and to the emitter of transistor Q 19 . A diode-connected NPN bipolar transistor Q 12  has its emitter connected to GROUND, and its base and collector connected to one terminal of a resistor R 43  and to one terminal of a resistor R 45 . The other end of resistor R 43  is connected to the base of transistor Q 9  and to the collector of transistor Q 1 . The other end of resistor R 45  is connected to the base of transistor Q 8  and to the collector of transistor Q 0 . The emitters of transistors Q 8  and Q 9  are connected to GROUND through a resistor R 44 . The base of transistor Q 1  and the base of transistor Q 2  are connected together and to a HOLD node receiving a HOLD signal. The base of transistor Q 0  and the base of transistor Q 19  are connected together and to a TRACK node receiving a TRACK signal. The power supply node VDD connects to the circuitry of  FIG. 1B  by way of a connection identified as “A”, while the GROUND node connects to the circuitry of  FIG. 1B  by way of a connection identified as “D”. 
         [0016]    Referring to  FIG. 1B , the collector of an NPN bipolar transistor Q 20  is connected to VDD, as is the collector of an NPN bipolar transistor Q 16 . The emitter of transistor Q 20  is connected to the emitter of a PNP bipolar transistor Q 15 , which has its collector connected to GROUND. The collector of a PNP bipolar transistor Q 19  is connected to GROUND, as is the collector of a PNP bipolar transistor Q 17 . The emitter of transistor Q 19  is connected to the emitter of an NPN bipolar transistor Q 14 , which has its collector connected to VDD. The base of transistor Q 14  and the base of transistor Q 15  are both connected together and to an INPUT node to which an INPUT signal is applied, which is an analog input signal to be digitized. 
         [0017]    The base of transistor Q 16  is connected to receive connection “B” from the circuitry of  FIG. 1A , while the base of transistor Q 17  is connected to receive connection “C” from the circuitry of  FIG. 1A . The emitters of transistors Q 16  and Q 17  are connected together and to a terminal of a resistor R 3 . The other terminal of resistor R 3  is connected to the common connection node of terminals of a capacitor C 1  and a capacitor C 13 . The other terminal of capacitor C 1  is connected to GROUND, while the other terminal of capacitor C 13  is connected to VDD. The common connection node of resistor R 3  and capacitors C 1  and C 13  is connected to the common connection node of the base of a PNP bipolar transistor Q 21  and the base of an NPN bipolar transistor Q 22 . The emitter of transistor Q 21  is connected to the base of an NPN bipolar transistor Q 27  and to the base of an NPN bipolar transistor Q 23 , and to the collector of a PNP transistor Q 3 . The base of transistor Q 3  is connected to PBIAS, while its emitter is connected to VDD through a resistor R 51 . The emitter of transistor Q 27  is connected to the base of transistor Q 20 , to the base of transistor Q 19  and to the emitter of a PNP transistor Q 26 . The collector of transistor Q 27  is connected to VDD and to the collector of transistor Q 23 . The emitter of transistor Q 22  is connected to the base of a PNP bipolar transistor Q 26  and to the base of a PNP bipolar transistor Q 24 , and to the collector of an NPN transistor Q 4 . The base of transistor Q 4  is connected to NBIAS, while its emitter is connected to VDD through a resistor R 50 . The collector of transistor Q 26  is connected to GROUND and to the collector of transistor Q 24 . The emitters of transistors Q 23  and Q 24  are connected together and to the collectors of transistors Q 21  and Q 22 , the common connection of all of which forming the output node VOUT. 
         [0018]    Referring now back to  FIG. 1A , in operation, transistor Q 6 , which is set using the value of PBIAS, functions as a current source, and gets switched to either resistor R 43  or resistor R 45 , depending on the state of signals TRACK and HOLD. Transistor Q 7 , which is set using the value of NBIAS, functions as a current sink, operating in complementary operation to the current sourcing of transistor Q 6 . Transistors Q 8 , Q 9 , Q 10  and Q 11  operate in cooperation with transistors Q 6  and Q 7  to establish a simultaneous current source through connection “B” and current sink through connection “C”, or, alternatively, a simultaneous current source through connection “C” and current sink through connection “B”. 
         [0019]    Thus, during the tracking phase, when the signal TRACK is high, turning transistor Q 0  off and transistor Q 1  on, the current from transistor Q 6  is switched to resistor R 43 , thus establishing a voltage at node T. This voltage sets up a corresponding current sinking through transistor Q 9 , and thus through connection “C”, while at the same time a current sourcing is set up though transistor Q 10 , and thus through connection “B”, since the high TRACK signal turns on transistor Q 19 , and, consequently, also turns on transistor Q 10 . 
         [0020]    During the holding phase, when the signal HOLD is high, the reverse occurs. That is, the high HOLD signal turns transistor Q 1  off and transistor Q 0  on, switching the current from transistor Q 6  to resistor R 45 , thus establishing a voltage at node H. This voltage sets up a corresponding current sinking through transistor Q 8 , and thus through connection “B”, while at the same time a current sourcing is set up though transistor Q 11 , and thus through connection “C”, since the high HOLD signal turns on transistor Q 2 , and, consequently, also turns on transistor Q 11 . In general, the circuitry shown in  FIG. 1A  functions as a current switch, switching current sinking and sourcing between connections “B” and “C” in response to the state of the TRACK and HOLD signals. Also in general, a typical track-and-hold circuit will have some circuitry that has a switching function, in response to the state of the TRACK and HOLD signals, for the signal holding circuitry, which in this embodiment is shown in  FIG. 1B . 
         [0021]    Referring now back to  FIG. 1B , and continuing to describe the operation of the circuit of  FIGS. 1A and 1B , during the tracking phase current is sourced on connection “B” and sunk on connection “C”, thus turning on transistors Q 16  and Q 17 . Now, transistors Q 14 , Q 15 , Q 16  and Q 17 , together form a unity gain amplifier, with the current sourced/sunk via connections “B” and “C”, transistors Q 19  and Q 20  are off. During this tracking phase, the analog INPUT signal is basically buffered in the unity gain amplifier and, since transistors Q 16  and Q 17  are on, the buffered voltage is transferred to the plates of capacitors C 1  and C 13  through resistor R 3 . 
         [0022]    During the holding phase current is sourced on connection “C” and sunk on connection “B”, thus turning off the unity gain amplifier, as well as transistors Q 16  and Q 17 . These currents flow through transistors Q 20  and Q 19 . Q 19  and Q 20  act to limit the voltage on the nodes U and L in such a way that the resulting reverse bias of Q 16 s and Q 17 s base emitter junction is independent of the incoming signal at the “Input”. This helps to preserve the voltage across capacitors C 1  and C 13  at the time of transition from tracking phase to holding phase. This voltage is buffered in a unity gain output amplifier formed by transistors Q 21 , Q 22 , Q 23  Q 24 , Q 26  and Q 27 , and resistors R 50  and R 51 , and provided as an output voltage VOUT to the output node VOUT. 
         [0023]    A problem with the sample-and-hold circuit of  FIGS. 1A and 1B  arises from the use of resistor R 3 . This resistor is provided in the circuit to limit the noise bandwidth of the unity gain amplifier described above. The transistors of that amplifier, transistors Q 14 , Q 15 , Q 16  and Q 17 , have wide bandwidths, and thus are a source of considerable noise. This noise is integrated over the period of the tracking phase by capacitors C 1  and C 13 , and is therefore a significant source of error. Resistor R 3  limits the noise bandwidth and thus reduces this error. 
         [0024]    However, resistor R 3  introduces another problem. It limits how quickly transistors Q 16  and Q 17  turn off. This, in turn, produces a signal-dependent sampling instant, which has the effect of introducing an error, namely, harmonic distortion with increasing input frequency. This can be better understood by referring to  FIG. 4 , which shows two graphs of voltage versus time, time aligned in the figure, one above the other. The top graph shows a plot of one cycle of a low frequency INPUT signal, while the bottom graph shows a plot of one cycle of an INPUT signal having a higher frequency than that of the signal in the top graph. The vertical dashed line  40  represents an instant in time at the transition between a tracking phase and a holding phase. A small vertical line  41  intersects the plot of the low frequency INPUT signal a period of time after the time depicted by the dashed line  40 , the period representing the time it takes the transistors Q 16  and Q 17  of  FIG. 1B  to turn off. During such period the transistors Q 16  and Q 17  will continue to conduct, and thus the voltage on capacitors C 1  and C 13  continue to follow, to some extent, the continued rise of voltage of the INPUT signal. The additional voltage rise during this period is shown in the figure as Δ L . 
         [0025]    A small vertical line  42  intersects the plot of the higher frequency INPUT signal the same period of time after the time depicted by dashed line  40  as in the top graph. Since the frequency is higher than that of the signal in the top graph, the voltage rise during this period is greater, as shown in the graph as Δ H . 
         [0026]    Thus it will be appreciated that this additional voltage component that appears, at least to some extent, on capacitors C 1  and C 13 , has a magnitude that is frequency dependent. This manifests itself in the output signal as harmonic distortion that increases with increasing frequency. 
         [0027]      FIG. 2  is a circuit diagram of a portion of a preferred embodiment of the present invention, corresponding to the unity gain amplifier of  FIG. 1B , resistor R 3  and capacitors C 1  and C 13 . It can be seen that in the preferred embodiment there are two additional capacitors, capacitors C 0  and C 3 , with capacitor C 0  being connected between the connection node of resistor R 3  and transistors Q 16  and Q 17 , and GROUND, and capacitor C 3  being connected between the connection node of resistor R 3  and transistors Q 16  and Q 17 , and VDD. Both capacitor C 0  and C 3  preferably have a capacitance that is in the range of 10% to 20% of the capacitance of capacitors C 1  and C 13 , although the invention contemplates capacitances outside that range for capacitors C 0  and C 3 . 
         [0028]    These additional capacitances provide a relatively low impedance path to help in the discharge of charge in transistors Q 16  and Q 17  as they turn off, thus significantly shortening the time it takes for them to turn off. By careful selection of the magnitude of their capacitance values, these additional capacitors can be made to have a negligible effect on the noise bandwidth of the unity gain amplifier during the tracking phase. 
         [0029]    The invention is applicable to numerous track-and-hold circuit architectures. For example,  FIG. 5  is the same as  FIG. 3  of the aforementioned U.S. Pat. No. 6,489,814, while  FIG. 6  shows the same circuit as that of  FIG. 5 , but having an additional capacitor C I  added in accordance with the present invention. 
         [0030]    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.