Abstract:
A method and apparatus for an improved track-and-hold circuit is disclosed. By utilizing an amplifier connected to the input signal in combination with, in essence, a replica of the track-and-hold sampling transistor, a track-and-hold technique that reduces distortion and nonlinearities in the sampling process is achieved.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to the field of electronic signal conversion. More particularly, the present invention relates to the ability to more accurately track, sample, and hold a signal for analog-to-digital conversion. 
     BACKGROUND OF THE INVENTION 
     A/Ds are ubiquitous and used in a variety of applications, such as, medical equipment, audio equipment, test and measurement equipment, telecommunications, military applications. imaging and video applications, etc. Many of these applications may benefit from an improved A/D converter. 
     In many applications a need exists for a high-speed, high-resolution front end for analog-to-digital (A/D) converters. An A/D converter takes a finite amount of time to generate a digital output representing the analog input signal strength. Generally, the higher the resolution of the A/D, the longer the conversion time. If during this conversion time the input signal changes, then the digital output may not accurately represent the input signal. Thus, for high-speed signals that change rapidly, or for high-resolution conversion, and for the combination where the signal is high-speed and high-resolution is needed, what is desirable is a way to rapidly and accurately “sample” or “track” the analog input and “hold” it steady while the A/D conversion takes place. In this way, further changes in the high-speed signal do not affect the A/D, because the input signal has been “captured.” Additionally, if the input signal that was “captured” is very close in amplitude to the actual input signal and does not degrade during the A/D conversion time, that is, it is held steady, then the A/D may perform a high-resolution measurement. The circuit that performs this function is often referred to as a “sample-and-hold” circuit or a “track-and-hold” circuit. A track-and-hold circuit is generally placed between the input signal source and the digital portion of the A/D converter, and is often considered the “front-end” of an A/D converter, because it performs the analog function of tracking and holding the analog input for digital conversion. 
     Because the track-and-hold analog “front-end” is, in many applications, the limiting factor for speed and/or resolution, much engineering attention has been directed to how to improve and/or correct for track-and-hold inaccuracies. Approaches have concentrated on virtually every component in the track-and-hold circuit. Parameters that have been focused on include such things as the offset voltage of the input circuitry, gain errors of the input circuitry, gain linearity of the input circuitry, large and small signal bandwidth of the input circuitry, as well as the slew rate of tracking, aperture delays, aperture jitter or uncertainty, and charge transfer or charge injection. The “holding” element, conventionally a capacitor, has also been the subject of much investigation with examination of such things as leakage current, droop rate, etc. 
     In spite of the immense engineering efforts on all facets of the A/D speed and/or resolution issue, the current approaches still suffer limitations. These limitations as noted above are primarily in analog front-end track-and-hold circuit. One of the limitations that has persisted is the inherent nonlinearity of the sampling device that is used in a track-and-hold circuit. Analysis of this nonlinearity indicates that one of the factors is related to the range of the amplitude of the input signal. For lowered track-and-hold nonlinearities, generally, the input signal amplitude should be minimized. On the other hand, for higher resolution and/or dynamic range, generally, the desire is to handle a wide range of input voltage amplitudes from small to large. Thus, there are conflicting requirements. 
     A conventional track-and-hold approach is shown in the simplified circuit diagram of FIG.  1 . In FIG. 1, the metal-oxide-semiconductor (MOS) transistor M1  106  is turned on and off by the voltage applied to its gate Vg  114 . The transistor M1  106  is often referred to as the input switch or the sampling switch. The size of the sampling capacitor Cs  112  is dictated by the resolution of the analog-to-digital (A/D) converter (number of bits=N) and the fundamental thermal noise given by equation (1).                  (     V   S     )     n   2     =     kT     C   S               (   1   )                                
     In equation (1), Vs denotes the noise voltage of the source, Cs is the sampling capacitor capacitance, k is Boltzman&#39;s constant, and T represents the temperature in degrees Kelvin. 
     In practical applications, the size of the sampling capacitor Cs  112  needs to be even larger, because of other noise sources (from the active devices) contributing to the total noise. 
     For high speed A/D converters, the major source of distortion comes from nonlinearities in the front-end track-and-hold. For the circuit in FIG. 1, the ON resistance of the transistor M1  106  is modulated by the input voltage level as given by equation (2).                R   ON     =     1       k   1     ·     W   L     ·     (       V   g     -     V     i                 n       -     V   th       )                 (   2   )                                
     In equation (2), RON denotes the sampling transistor ON resistance, k 1  is technology dependent constant related to charge carrier mobility, W/L is the sampling transistor gate width divided by the sampling transistor gate length, Vg is the gate voltage, Vin is the input signal voltage, and Vth is the threshold voltage of the sampling transistor. 
     In order to keep the distortion of the front-end at the N-bit level, the total variation of the RC time constant should be less than equation (3).                Δ        (       R   ON          C   S       )       =     1       F     i                 n       ·     2   N                 (   3   )                                
     In equation (3), RON denotes the sampling transistor ON resistance, Cs is the sampling capacitor capacitance, F in  is the maximum input signal frequency, and N denotes the A/D converter resolution in number of bits. Since the capacitor size is fixed and determined by noise considerations, the ON resistance of the input switch M1  106  should have a minimal variation over the whole input voltage range in order to minimize distortion. 
     A straightforward implementation of the input switch M1  106 , as shown in FIG. 1, uses a transistor with an extremely large width to length (W/L) ratio. Such a transistor will also have large parasitic capacitors from the source and drain junctions, Csb  108  and Cdb  110 , respectively. These parasitic capacitors (Csb  108  and Cdb  110 ) have a strong voltage dependence, so together with the signal source impedance Rs  104  (on the order of 50 ohms), they create a distorting nonlinear filter. Moreover, the amount of distortion is directly affected by the signal source impedance. 
     Thus, in many applications a need exists for a high-speed, high-resolution front end for analog-to-digital (A/D) converters. In particular, it is advantageous in an A/D system to have the input track-and-hold circuit as accurate as possible over a wide range of input signal amplitudes while at the same time having the track-and-hold circuit introduce as few of its own artifacts as possible. In this way, the A/D can achieve better resolution with lowered nonlinearities. 
     Therefore, it is desirable to provide a track-and-hold circuit having high speed and lowered nonlinearities. 
     SUMMARY OF THE INVENTION 
     The present invention includes a method and apparatus for a track-and-hold circuit having improved input signal tracking and reduced nonlinearities. An input signal is presented to a sampling device for sampling the input signal and at the same time the input signal is presented to a control circuit. The control circuit controls the sampling device to reduce nonlinearities during sampling. 
     Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
     FIG. 1 is a schematic diagram of an implementation of a track-and-hold front end circuit. 
     FIG. 2 is a block diagram for an improved track-and-hold. 
     FIG. 3 is a schematic diagram of an embodiment of a circuit for an improved track-and-hold front end. 
     FIG. 4 is a diagram of an embodiment of a circuit for a unity gain buffer. 
    
    
     DETAILED DESCRIPTION 
     An improved track-and-hold circuit is described. The invention, by utilizing the input signal to affect part of the track-and-hold sampling circuit is capable of reducing nonlinearities in the sampling process. More specifically, a key idea underlying this invention is to bootstrap the gate voltage of a sampling transistor during the track phase to maintain a substantially constant input signal to sampling transistor gate voltage. 
     FIG. 2 is a block diagram for an improved track-and-hold. The Input  202  is coupled to the inputs of the Sampling Device  204  and the Sampling Device Driver  206 . The output  208  of the Sampling Device Driver  206  is coupled to the Sampling Device  204 . The output  210  of the Sampling Device  204  is coupled to the Sample Storage  212 . In operation, the Input  202  is presented to both the Sampling Device  204  and to the Sampling Device Driver  206  at the same time. Based upon the Input  202 , the Sampling Device Driver  206 , determines how to drive the Sampling Device  204  with a signal  208  such that the output  210  of the Sampling Device  204  has lowered nonlinearities due to the track-and-hold process, such that the sample  210  stored in the Sample Storage  212  also has lowered nonlinearities. 
     FIG. 3 is a schematic diagram of an embodiment of a circuit for an improved track-and-hold front end. The input signal Vin  302  exhibits a source resistance Rs  304  which is coupled to the positive input of amplifier A1  318  and the source of an n-type metal-oxide-semiconductor (NMOS) sampling transistor M1  306 . The drain of M1  306  is coupled to the track-and-hold sampling capacitor Cs  312 . Amplifier A1  318  is configured as a unity gain voltage follower, where the output of A1  318  is fed back to the negative input of A1  318 . The output of A1  318  is also coupled to the source of NMOS transistor M2  316 . The gate of sampling transistor M1  306  is coupled to the gate and drain of transistor M2  316  and to the current source I  314 . 
     Referring now to both FIG.  2  and FIG.  3 . The Input  202  in FIG. 2, in one embodiment, may be Vin  302  and Rs  304  as shown in FIG.  3 . An alternative embodiment might be the output of an amplifier stage. The Sampling Device  204  in FIG. 2, in one embodiment, may be M1  306  and the parasitic capacitances Csb  308  and Cdb  310  as shown in FIG.  3 . An alternative embodiment might be a complementary-metal-oxide-semiconductor (CMOS) switch. The Sample Storage  212  in FIG. 2, in one embodiment might be Cs  312  as shown in FIG.  3 . An alternative embodiment might be any charge storage device. The Sampling Device Driver  206  in FIG. 2, in one embodiment might be A1  318 , M2  316 , and I  314  in FIG.  3 . Signal  208  in FIG. 2, in one possible embodiment may be signal Vg  320  in FIG.  3 . 
     A key idea underlying the circuit in FIG. 3 is to bootstrap the gate of M1  306  during the track phase to maintain a substantially constant gate overdrive voltage gate-to-source voltage (Vgs) minus threshold voltage (Vth) (Vgs−Vth). Thus, referring to equation (2), the term (Vg−Vin−Vth) remains relatively constant and thus RON is relatively constant. A simple level shifter is not adequate because of the back bias effect that changes the transistor M1  306  threshold with its input source level Vin  302 . The circuit in FIG. 3 includes among other things, an input signal source Vin  302  with a source resistance Rs  304 , the sampling transistor M1  306  and parasitic capacitors from the source and drain junctions (Csb  308  and Cdb  310  respectively), and the sampling capacitor Cs  312 . Additionally, the circuit in FIG. 3 uses a bias circuit I  314  in conjunction with M2  316  and A1  318  to generate the M1  306  sampling transistor gate voltage Vg  320 . The transistor M2  316  has a constant drain current, and, since it operates in saturation, has a constant gate overdrive. The transistor M2  316  source is connected to a voltage follower A1  318 . The voltage follower A1  318  is also called a buffer. This configuration has the advantage that it compensates for the back bias threshold variation. Since the sources of transistors M1  306  and M2  316  are at the same voltage potential, these two transistors will have the same threshold voltage. Another advantage of this configuration is that the large gate capacitance of transistor M1  306  is not connected directly to the buffer A1  318  output. This arrangement improves the phase margin and the stability of the buffer  318 . 
     The bandwidth of the buffer  318  needs to be significantly higher than the maximum input signal frequency to avoid phase shifts between Vin and Vg. One embodiment of a buffer  318  is shown in the schematic diagram of FIG. 4 as  402 . The buffer is a two-stage operational amplifier style amplifier circuit, with the input p-type MOS (PMOS) transistors M4  406  and M5  408 , and the second stage with an n-type MOS (NMOS) transistor M9  416 . The input signal In  404  is connected to transistor M4  406 , whose output is coupled to M6  412  and to M9  416  and the first input of the RC circuit R  420  and C  422 . Transistor M9  416  output is coupled to the second input of the RC circuit R  420  and C  422 , to the input of transistor M5  408 , and is the output Out  418  of the operational amplifier. Transistor M7  414  is connected to the output of M5  408 . Current source I  410  is connected to the input transistors M4  406  and M5  408 . Transistor M74  428  in FIG. 4, which is controlled by a bias voltage Bias  430 , can be the current source I  314  in FIG. 3, and transistor M31  424  in FIG.  4  and the gate voltage Vg  426  can play the role of the level shifter M2  316  in FIG.  3 . In the amplifier shown in FIG. 4, the input stage M4  406  and M5  408  is not balanced. This imbalance creates an input offset on the order of 100 mV which does not let the output voltage of the output transistor M9  416  collapse to ground for input voltages close to ground. This technique avoids large output distortion and recovery problems in the buffer and allows input voltages close to the ground rail with minimal output distortion. A tradeoff is that the potential at the sources of M1  306  and M2  316  in FIG. 3 does not track exactly, therefore, the gate overdrive voltage for M1 is not perfectly constant. This small difference, however, determines a second order effect on the M1 ON resistance. 
     Thus, an improved track-and-hold circuit has been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.