A sample-and-hold circuit

Abstract: A MOS track-and-hold circuit incorporating cancellation of error due to switch feedthrough is described. To eliminate the channel charge feedthrough due to oxide capacitance, a switched capacitor source (22) is connected to be charged to a voltage V1 during the "hold" phase and between the input node (12) and the switch gate (17) to provide a voltage V1 - Vin during the "track" phase. A dummy transistor (26) biased in an "off" condition has its drain connected to the holding capacitor (15) and its gate switched between ground and the output terminal (Vout which tracks Vin) to also cancel the feedthrough from the gate-drain overlap capacitance and any gate-drain parasitic capacitance.

Description: This invention relates generally to sample-and-hold and track-and-hold circuits (hereinafter collectively referred to as "sample-and-hold" circuits) and, in particular, to sample-and-hold circuits utilizing MOS transistor switches. Background of the Invention Sample-and-hold circuits are used for sampling values of continuous time-varying amplitude signals at discrete time intervals. The sampling may, for example, be done as part of an analog-to-digital signal conversion process, wherein amplitude values of an analog input signal are sampled at regular clocked time intervals for conversion into binary digital values, to give a digital output representation of the time-varying input. Digital signal processing may then be readily applied to the converted signal before converting the processed signal back into analog form for control or other purposes. In a typical application, the rate of sampling must at least be equal to twice the highest relevant frequency component (viz. bandwidth) of the original signal (the Nyquist frequency) in order to accurately capture the meaningful information. A conventional sample-and-hold circuit 10 is shown schematically in FIG.1. A continuous, time-varying voltage signal Vin applied at input terminal 12 is connected through a switch 14 to charge a hold capacitor 15 (Ch), one terminal of which is connected to an output buffer 16 and the other terminal of which is connected to ground. Output buffer 16 may, for example, be a unity gain amplifier. Switch 14 is controlled by a sample-and-hold timing (or clocking) signal applied at node 17, so that when switch 14 is switched "ON," the voltage at capacitor node 19 (and hence the output voltage Vout at output node 21) tracks the voltage applied at input node 12, and when switch 14 is switched "OFF," the last voltage applied at input node 12 is "held" at node 19 (and thus at node 21) by the charge on capacitor 15. Switch 14 is often implemented as a MOS (metal-oxide semiconductor) transistor. When switch 14 is implemented as an NMOS transistor, for example, the input voltage Vin is passed through the N channel when a high gate voltage (e.g., Vdd) is applied at node 17 ("phase A") and is blocked when a low gate voltage (e.g., ground) is applied at node 17 ("phase B"). This causes Vout to track the varying magnitude of Vin during phase A (i.e., Vout = Vin), but "hold" the last phase A value of Vin during phase B (i.e., Vout = Vin (t i )). There are, however, two types of capacitance inherent in the MOS transistor that contribute along with the pass-through input voltage Vin to the captured charge at node 19 when the transistor is turned "OFF." One is the gate-oxide capacitance (Cox). The other is the gate-drain parasitic capacitance (Cgd). Capacitance Cox is the capacitance caused by locating the metal and dielectric oxide directly over the channel. When the MOS transistor is "ON," in order for current to flow, charge is present at the surface of the channel, under the oxide. When the same transistor is then turned "OFF," that charge must be dissipated, some going to the drain and some going to the source. This affects the charge stored on capacitor 15 (voltage "held" at node 19) when switch 14 is turned "off." The charge contribution of Cox is proportional to Vin x Cox, where Cox is the gate oxide capacitance of transistor 14. Capacitance Cgd is a parasitic capacitance due to physical overlap between the gate and drain terminals, as well as from any coupling between the gate and drain connections, in the MOS transistor layered structure. (Where source rather than drain is connected to node 19, the terms "drain" and "Cgd" should be understood to include "source" and "Cgs" in the context of this discussion and the described invention.) Capacitance Cgd exists whether the transistor is turned "ON" or "OFF" and, thus, also contributes to the charge stored on capacitor 15 (voltage "held" at node 19) when switch 14 is turned "OFF." The charge contribution of Cgd is proportional to Vin x Cgd, where Cgd is the gate-to-drain parasitic capacitance of transistor 14. Because the input signal is voltage magnitude time-variant, the influence on the "held" voltage of sample-and-hold circuit 10 due to Cox and Cgd is a function of the input voltage. Capacitance Cox depends on the difference between the applied gate voltage and the voltage at node 19. For example, 3.0 volts may be applied to the gate to turn transistor 14 "ON"; however, when transistor 14 is "ON," the voltage at node 19 will track the (varying) voltage at input node 12. Thus, the amount of charge distributed due to Cox and Cgd at node 19 when the transistor is turned "OFF" will vary with the level of the input voltage. If the effect of Cox and Cgd on the captured value were constant each time the transistor were turned "OFF" to "hold" the sample, it could be readily taken into account. It would simply be a constant voltage "pedestal" that would raise or lower the signal by the same amount. But, because the amount of influence on the captured signal varies non-linearly as a function of the magnitude of the input signal, it causes a problem. With decreasing channel dimensions, the influence of Cox on the "held" value is lessening; however, the influence of Cgd is becoming more significant. There exists, therefore, a need to compensate for the adverse effects of Cox and Cgd in sample-and-hold circuits. SUMMARY OF THE INVENTION The invention provides a method and apparatus for reducing the time-varying deleterious effects of gate-to-channel (Cox) and overlap (Cgd) capacitance in MOS transistor sample-and-hold (i.e. sample-and-hold and track-and-hold) circuits. The invention further provides sample-and-hold circuits incorporating such features. According to a first aspect of the invention there is provided a method for reducing the time-varying effects of overlap capacitance (Cgd) in the operation of a sample-and-hold circuit having a time-varying input voltage (Vin) applied to an input terminal through a transistor switch to charge a hold capacitor, the hold capacitor connected to supply an output voltage (Vout); the sample-and-hold operation comprising connecting a gate of the transistor switch to a first voltage (Vdd) to turn the switch "ON" during a first "track" phase of operation (A), and to a second voltage (ground) to turn the switch "OFF" during a second "hold" phase of operation (B); the method characterized by: connecting a dummy transistor to the hold capacitor in parallel with the transistor switch; and connecting a gate of the dummy transistor to the second voltage (ground) during the first phase of operation (A) and to the output voltage (Vout) during the second phase of operation (B). According to a second aspect of the invention there is provided a sample-and-hold circuit having an input terminal for receiving a time-varying input voltage (Vin), a hold capacitor connected to supply an output voltage (Vout), and a transistor switch connected to selectively apply the input voltage (Vin) to charge the hold capacitor responsive to voltage at a gate of the transistor switch, and control circuitry for connecting the gate of the transistor switch to a first voltage to turn the switch "ON" during a first "track" phase of operation (A) and to a second voltage to turn the switch "OFF". during a second "hold" phase of operation (B); characterized in that: a dummy transistor is connected to the hold capacitor in parallel with the transistor switch; and other control circuitry is provided for connecting a gate of the dummy transistor to the second voltage during the first phase of operation (A) and to the output voltage during the second phase of operation (B); the other control circuitry and dummy transistor being connected and configured to reduce the time-varying effects of overlap (Cgd) capacitance in the sample-and-hold circuit. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be further described by way of example with reference to the accompanying drawings in which: FIG. 1 (prior art) is a schematic view of a conventional sample-and-hold circuit; FIG. 2 is a schematic view of a sample-and-hold circuit in accordance with the principles of the invention. Throughout the drawings, like reference numerals are used to refer to like elements. DETAILED DESCRIPTION FIG. 2 shows a sample-and-hold (viz. track-and-hold) circuit 20 having a continuous, time-varying voltage signal Vin applied to an input terminal 12 connected, as before, through a MOS transistor switch 14 to charge a capacitor 15, one terminal (node 19) of which is connected to an output buffer 16 in the form of a unity gain amplifier, and the other terminal of which is connected to ground. However, in departure from the arrangement in circuit 10 of FIG. 1, instead of connecting the gate node 17 of transistor 14 alternately between a supply voltage Vdd (e.g., 3.0 volts) at phase A and ground at phase B, node 17 is alternately connected between a voltage V1 + Vin at phase A and ground at phase B. This is accomplished in the illustrated embodiment by providing a capacitor 22 having terminals 23 and 25 that are switched across voltage source terminals V1 and ground during phase B and across gate node 17 and input node 12 during phase A. Capacitor 22 is, thus, charged to voltage V1 during phase B, when switch 14 is turned off. Then, during phase A, when capacitor 22 is switched over to gate node 17 it will apply a gate-to-source overdrive voltage differential ((V1 + Vin) - Vin = V1) which is independent of the applied input voltage V1. Thus, the channel charge injected into hold capacitor 15 when "hold" mode phase B is entered will also be independent of variations in Vin. This removes non-linearities due to Cox. However, the charge injection due to the gate-to-drain parasitic capacitance Cgd is still dependent on Vin. And, with the trend toward shorter and shorter channel lengths, Cgd becomes comparable to Cox. Thus, there remains a need to cancel the non-linear charge injection component due to Cgd. Circuit 20 does this by the addition of a "dummy" MOS transistor 26, connected as shown in FIG. 2 to negate the effect of charge injection due to the parasitic capacitance of transistor 14. Transistor 26 is formed with structural parameters (viz. channel length, channel width, oxide thickness, layer overlap, etc.) that match (at least from a parasitic capacitance contribution standpoint) those of transistor 14. Transistor 26 has its source connected, to a voltage Vdd, its drain connected to node 19, and its gate connected to node 27 which is switched between ground at phase A and the output voltage Vout at phase B. Because output buffer 16 is a unity gain amplifier, Vout will be the same as the voltage at node 19 (approximately equal to Vin during and at the end of phase A). Voltage Vdd is chosen relative to the anticipated values of Vin so that the gate voltage of transistor 26 will never be higher than its source or drain voltage, either in phase A or in phase B. Consequently, transistor 26 will always be "off." However, when the circuit switches from "track" phase A to "store" phase B, the gate of transistor 26 will see a transition that is equal and opposite to that seen by transistor 14. This cancels the effect on the stored value of non-linear charge feedthrough due to the drain-gate parasitic capacitance.

Claims: A method for reducing the time-varying effects of overlap capacitance (Cgd) in the operation of a sample-and-hold circuit having a time-varying input voltage (Vin) applied to an input terminal (12) through a transistor switch (14) to charge a hold capacitor (15), the hold capacitor (15) connected to supply an output voltage (Vout); the sample-and-hold operation comprising connecting a gate (17) of the transistor switch (14) to a first voltage (Vdd) to turn the switch "ON" during a first "track" phase of operation (A), and to a second voltage (ground) to turn the switch "OFF" during a second "hold" phase of operation (B); the method characterized by : connecting a dummy transistor (26) to the hold capacitor (15) in parallel with the transistor switch (14); and connecting a gate (27) of the dummy transistor (26) to the second voltage (ground) during the first phase of operation (A) and to the output voltage (Vout) during the second phase of operation (B). The method as claimed in Claim 1, for also reducing the time-varying effects of gate-to-channel capacitance (Cox) in the sample-and-hold circuit, further comprising: connecting the gate (17) of the transistor switch (14) to a voltage substantially equal to the sum of an input voltage-independent voltage (V1) and the input voltage (Vin) during the first phase (A). The method as claimed in Claim 2 further comprising: charging a second capacitor (22) to the input voltage-independent voltage (V1) during the second phase (B); and connecting the charged second capacitor (22) between the input terminal (12) and the gate (17) during the first phase (A). The method as claimed in any of Claims 1 to 3, wherein the wherein the steps of actuating the transistor switch (14) and connecting the dummy transistor (26) comprise actuating an NMOS transistor switch (14) connecting an NMOS dummy transistor (26). A sample-and-hold circuit having an input terminal for receiving a time-varying input voltage (Vin), a hold capacitor (15) connected to supply an output voltage (Vout), and a transistor switch (14) connected to selectively apply the input voltage (Vin) to charge the hold capacitor responsive to voltage at a gate of the transistor switch (14), and control circuitry (17) for connecting the gate of the transistor switch to a first voltage to turn the switch "ON" during a first "track" phase of operation (A) and to a second voltage to turn the switch "OFF" during a second "hold" phase of operation (B) ; characterized in that : a dummy transistor (26) is connected to the hold capacitor (15) in parallel with the transistor switch (14); and other control circuitry (27) is provided for connecting a gate of the dummy transistor (26) to the second voltage during the first phase of operation (A) and to the output voltage during the second phase of operation (B); the other control circuitry (27) and dummy transistor (26) being connected and configured to reduce the time-varying effects of overlap (Cgd) capacitance in the sample-and-hold circuit. The circuit as claimed in Claim 5, wherein the gate (27) of the dummy transistor (26) is arranged to be connected to a voltage substantially equal to the sum of an input voltage-independent voltage (V1) and the input voltage (Vin) during the first phase (A), to also reduce the time-varying effects of gate-to-channel (Cox) capacitance in the sample-and-hold circuit. The circuit as claimed in Claim 6 further comprising: a second capacitor (22) arranged to be charged to the input voltage-independent voltage (V1) during the second phase (B) and for connection of the charged second capacitor (22) between the input terminal (12) and the gate (17) of the transistor switch (14) during the first phase (A). The circuit as claimed in any of Claims 5 through 7, wherein the transistor switch (14) and the dummy transistor (26) each comprise an NMOS transistor. The circuit as claimed in Claim 8, wherein a source of the transistor switch (14) is connected to the input terminal (12), the hold capacitor (15) is connected between a drain of switch (14) and the second voltage terminal (ground), and a drain of the dummy transistor (26) is connected to the drain of the switch (14). The circuit as claimed in Claim 8 or Claim 9 further comprising: a unity gain amplifier having an input connected to a drain of the switch (14), and an output connected to the output terminal (21). The circuit as claimed in any of Claims 8 to 10 further comprising: circuitry for selectively connecting the gate of the dummy transistor (26) alternately to ground and to the output terminal (21).

Inventors: NAGARAJ KRISHNASWAMY, NAGARAJ, KRISHNASWAMY
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1998
Main classification: G11C27
IPC codes: H03K17, G11C27