Circuit for processing a time-varying signal

Abstract: A sample-and-hold circuit for sampling a time-­ varying signal comprises a main sample-and-hold subcircuit (44) extending between input and output terminals (42, 46) and an auxiliary sample-and-hold subcircuit (48) extending between the input terminal (42) and a portion of the main subcircuit. Each sample-and-hold subcircuit (44, 48) comprises an input buffer amplifier (52, 82), a diode-bridge switch (56, 84) responsive to a control signal and coupled to the input buffer amplifier, a charge-holding capacitor (76, 100) coupled to the switch, and an output buffer amplifier (78, 98) coupled to the capacitor. The output of the auxiliary output amplifier (98) is further coupled to the main switch and the output of the main output amplifier (78) is coupled to the output terminal (46). A generator (62) generates the control signal for controlling the switches (56, 84) such that the same part of the time-varying signal passing through the main and auxiliary subcircuits (44, 48) is sampled. The time-­ varying signal passing through the main subcircuit (44) is delayed such that the time-varying signals traveling through the main and auxiliary subcircuits reach the main switch at substantially the same time.

Description: Field of the Invention This invention relates generally to circuits for processing a time-varying signal. Background of the Invention The present invention relates generally to high speed circuits where the timing of signals occurring in a main path and an auxiliary path, such as in a feedback path, is critical. Although not limited to such an application, one circuit type where this is particularly true is a sample-and-hold circuit. Sample-and-hold circuits are often used on the input of an analog-to-digital converter for providing a stable input signal for the converter. High speed converters are becoming very common as digital signal processing of high frequency signals is undertaken. Sample-and-hold circuits typically employ an analog switching device, generally referred to as a sampling gate, for periodically connecting a capacitor to an input signal to be sampled. During the tracking state, the capacitor is connected to the signal so that it charges and discharges to track the voltage level of the signal. In the hold state, the switch is turned off, disconnecting the capacitor from the input signal. The capacitor holds that voltage level existing on the input signal at the time the capacitor is disconnected. The charge on the capacitor should stay constant, providing a constant output voltage. The capacitor is also typically connected to a high impedance input of a buffer amplifier which provides the hold output signal of the sample-and-hold circuit, the high impedance input ensuring minimal discharge of the capacitor during the hold phase of the circuit operation. The switches used in sample and hold circuits are typically either Schottky diode bridges or field-effect transistors. The diode bridge, which is perhaps the simplest of the switches, is switched by a pair of controlled current sources. These basic circuits however have significant blow-by distortion caused by the input signal coupling to the output during the hold state due to diode capacitances. This has been improved upon by the addition of a diode at each corner of the bridge associated with the current sources. A fixed bias is applied to the diodes to provide a low impedance path to ground for the blow-by current in the hold state. Since the bias is fixed, though, the charge injected into the hold capacitor at the switching off of the diode bridge varies with the magnitude of the input signal disproportionately, thereby producing nonlinearity in the relationship of the held charge on the input signal represented by it. This in turn has been overcome in conventional circuits by bootstrapping the bias voltage from the sample-and-hold output. Such a circuit is known from EP-A-200 290 and is shown generally at 10 in Fig. 1. Circuit 10 includes an input terminal 12 for receiving the input signal to be sampled. This terminal is connected to an input buffer amplifier 14. The output of amplifier 14 is input into a switch in the form of a Schottky diode bridge 18 which is controlled by a pair of current sources 20 and 22. These current sources receive a control signal from a control signal generator 23. The diode bridge adjacent each current source has blow-by current shunt diodes 24 and 26 which are biased by fixed voltage sources 28 and 30. The output of the switch is coupled to a hold capacitor 32, one side of which is grounded. This capacitor tracks the input signal during the track state and holds a charge proportional to the input signal when in the hold state. The capacitor 32 is coupled to an output amplifier 34 for outputting the output signal on an output terminal 36. As has been mentioned, the bias voltages for diodes 24 and 26 are bootstrapped from the circuit output through a bias circuit 38. This circuit configuration is more effective than prior versions of the circuit. However, in high speed applications, it tends to have limited use because of the longer settling time required by the bootstrapping arrangement. This is due primarily to the time delay in the signal propagating to the output and then feeding back to the operative circuit component, such as the switch or amplifier. Conventional sample-and-hold circuits also typically have a hold capacitor which is grounded so that it must take the complete charge of the input signal. Thus, changes in charge take more time than if less stored charge was required. Summary of the Invention It is an object of the invention to shorten processing delays of a circuit for processing a time-varying signal. The invention is characterized by the features of claim 1. The present invention overcomes the disadvantage of prior art circuits generally, and sample-and-hold circuits in particular, such as the one shown in Fig. 1, by providing a referencing voltage source on a main signal path which has fast settling times. This aspect of the invention is provided in a circuit for processing a time-varying signal comprising a main subcircuit coupled between an input and an output terminal for conducting the time-varying signal along a first path. An auxiliary subcircuit substantially identical to at least a portion of the main subcircuit and coupled between the input terminal and the main subcircuit conducts the time-varying signal along a second path. A delay subcircuit delays the time of propagation of the time-varying signal along one of the first and second paths so that the time-varying signal at a selected point along the first path has a predetermined time relationship with respect to the time-varying signal traveling along the second path at the point it is fed to the main subcircuit. In a preferred embodiment of the invention, a sample-and-hold circuit for sampling a time-varying signal is provided. Included are a main sample-and-hold subcircuit and an auxiliary sample-and-hold subcircuit. Each sample-and-hold subcircuit comprises an input buffer amplifier coupled to an input terminal, a diode-bridge switch responsive to a control signal and coupled to the input buffer amplifier, a charge-holding capacitor coupled to the switch, and an output buffer amplifier coupled to the capacitor. The output of the auxiliary output amplifier further is coupled to the main switch and the output of the main output amplifier is coupled to the output terminal. A control-signal generator generates the signal for controlling the switches such that the same part of the time-varying signal passing through the main and auxiliary subcircuits is sampled. Finally, a delay subcircuit delays the time-varying signal passing through the main subcircuit such that the time-varying signals traveling through the main and auxiliary subcircuits reach the main switch at substantially the same time. One aspect of the present invention also further enhances high speed operation by providing a hold capacitor which only tracks and holds a small charge, thereby charging more quickly. In the preferred embodiment, this is provided by an auxiliary sample-and-hold subcircuit wherein an auxiliary charge-holding capacitor is coupled to a switch for holding substantially the voltage of the time-varying signal received through the auxiliary switch. A main charge-holding capacitor coupled between the outputs of main and auxiliary switches holds the difference in voltage between the voltage of the time-varying signal received through the main switch and the voltage on the auxiliary capacitor. It will be seen that such circuits provide for fast operation in that settling times are avoided by use of an auxiliary subcircuit from which the signal is obtained. This and other features and advantages of the present invention will become apparent from a review of the drawings in conjunction with the following detailed description of the preferred embodiments. Brief Description of the Drawings Fig. 1 is a block diagram of a conventional sample-and-hold circuit. Fig. 2 is a block diagram of a sample-and-hold circuit made according to the present invention. Figs. 3 and 4 are sample waveforms of a signal in the circuit of Fig. 2 at two different times. Detailed Description of the Preferred Embodiments Fig. 1 is described in the section entitled Background of the Invention. A sample-and-hold circuit 40 made according to the invention is shown in Fig. 2. The circuit of Fig. 2, similar to the circuit of Fig. 1, includes an input terminal 42, a main sample-and-hold subcircuit 44, and an output terminal 46, all defining what may be referred to as a first signal path. A second signal path is defined by an auxiliary sample-and-hold subcircuit 48 substantially identical in its main elements, as will be seen, as main subcircuit 44. Further, a delay element 50, such as is provided conventionally by a transmission line, is posed between the input terminal 42 and the input to main subcircuit 44. Main subcircuit 44 includes an input buffer amplifier 52 of conventional and well known design connected to delay unit 50. The output of the amplifier 52 is also input into a conventional Schottky diode bridge forming a sampling gate switch 56, similar to bridge 18 of Fig. 1. It is driven by two controllable current sources 58 and 60. Sources 58 and 60 are controlled by a control signal received from a control signal generator 62 of conventional design. The generator 62 generates the signals which determine when the circuit is in a track state and when it is in a hold state. When current is flowing through the switch 56, it is in a track state, and correspondingly, when current is not flowing through the switch 56, it is in a hold state. The control signal passes through another delay element 64 for delaying the time that the signal reaches the current sources 58, 60 of the main subcircuit 44. The diode bridge also has a pair of blow-by current shunting diodes 66 and 68 and associated voltage sources 70 and 72. These are fed from a bias subcircuit 74. The output of the switch 56 is connected to one terminal of a hold or memory capacitor 76 which is connected between the main and auxiliary subcircuits as will be further described below. The switch output is also input to an output buffer amplifier 78 of conventional design, the output of which is connected to output terminal 46. Auxiliary subcircuit 48 is structured substantially identically to main subcircuit 44 and includes an input amplifier 82, diode bridge switch 84, current sources 86 and 88, blow-by diodes 90 and 92, associated voltage sources 94 and 96, and output amplifier 98. The output of amplifier 98 is used as the voltage source for bias circuit 74. The current sources 86; 88 are connected to receive the control signals directly from generator 62. A hold capacitor 100 is connected conventionally between the output of switch 84 and ground. Capacitor 76 is connected between the two outputs of switches 56 and 84. Auxiliary subcircuit 48 is thus a basic sample-and-hold circuit without bootstrapping of the blow-by diodes associated with the switch 84. It can be seen that what is provided by circuit 40 is a pair of substantially identical sample-and-hold circuits, essentially in parallel, and both receiving the same input signal. However, one of the circuits receives the input signal delayed relative to the other. The circuit which receives the input signal is used to generate what would normally be the feedback or bootstrap voltages for the circuit with the delayed signal. The preferred timing of the two signals in the two circuits is illustrated in Figs. 3 and 4. In these figures an input signal 102 is shown as it appears in the two subcircuits at two points in time. For simplicity, it is assumed that the propagation delay of the signal through the subcircuit components is in equal increments, as represented by the vertical dashed lines. Fig. 3 represents the existence of the signal at a first time, T₁. The signal level is lowest at 104, the input to auxiliary input amplifier 82 and rises through the auxiliary subcircuit to its highest point 106 at the output of bias circuit 74. It will be appreciated that this time varying input signal does not in actuality have the same voltage level as it progresses through the circuit. However, it is shown as having values of the same scale throughout so that its progress through the circuit will be more apparent. The figures are thus primarily illustrative of the timing of the signal. The delay provided by delay element 50 is set to be equal to the time of propagation of the signal through the input amplifier 82, switch 84 and output amplifier 98 of the auxiliary subcircuit 48. If the various delays are not equal, then the difference in propagation time through the bias circuit 74 and the main input amplifier 52 would also need to be included. Thus, the signal at point 108 is at the input of the main input amplifier 52, while the output signal, assuming the circuit is in the track state, is shown by point 110. At instant T₁ control signal generator 62 outputs a signal to switch to the hold state. Thus, auxiliary switch 84 switches so that the signal at the output of the switch at point 112 is held on capacitor 100. Fig. 4 illustrates the location of the input signal throughout circuit 40 at a second instant in time, T₂. At this instant point 112 of the signal is at the output of main switch 56. The input signal voltage has been held on capacitor 100 since T₁, as represented by the horizontal line segment 114. The held value of the signal has thus had time to propagate along path two through auxiliary output amplifier 98, bias circuit 74 and switch 56. Along path one, the signal has propagated through delay element 50, main amplifier 52 and switch 56. The control signal from generator 62 has also been delayed by element 64 so that the change from track state to hold state in the main subcircuit occurs at time T₂. The signals along paths one and two thus arrive at switch 56 in phase. That is to say, the same point on the input signal reaches the switch 56 along the two paths at substantially the same time. Since the auxiliary subcircuit 48 has been in a hold state since time T₁, the voltage levels have had time to settle and the biasing signal fed to the main subcircuit 44 is advanced over what it would be if the output of the main subcircuit 44 was bootstrapped. There is thus a savings in the delay time that it would take the signal to propagate through the main output amplifier 78, bias subcircuit 74 and switch 56 so that it could settle to a relatively steady state output. An important aspect of the present invention is illustrated in the circuit of Fig. 2 wherein the hold capacitor 76 of the main sample-and-hold subcircuit 44 is coupled between the output of the main switch 56 and the auxiliary capacitor 100. Since the auxiliary capacitor 100 is coupled to ground, it holds the voltage of the input signal on it. The main capacitor 76 therefore only holds any voltage differential which exists between the voltage on the auxiliary capacitor 100 and the voltage output from the main switch 56. Thus, the main capacitor 76 has very small charge variations with variations in the input signal. The main diode bridge bias current is therefore much lower than that of the auxiliary diode bridge 84. Further, the main input buffer 52 has less driving to do. This is particularly true in the hold state, wherein the voltage held on the auxiliary hold capacitor 100 is substantially the same as the voltage at the output of the main diode bridge 56. The differential voltage is then quite small in comparison. The charge time of the main hold capacitor 76 is correspondingly quite small. Although the delay times have been treated idealistically and simplistically, it will be appreciated by those skilled in the art that the actual delay times incorporated in practicing the present invention are preferably verified, if not actually determined, on an empirical basis, for each circuit, so that inherent variations in circuit components can be compensated for. It can be seen that the general concept of the present invention, that of using duplicate circuits, with relative delay between signals propagating through the circuits to accomplish desired bootstrapping of signals between the circuits has general applicability. It will therefore be apparent that although the present invention has been described with specific reference to a particular embodiment, other forms and variations of circuits may be envisaged and made without varying from the scope of the invention as defined in the claims.

Claims: A circuit for processing a time-varying signal comprising: a main circuit (44) coupled in a first path (50, 52, 56) between an input terminal (42) and an output terminal (46) for conducting the time-varying signal along the first path (50, 52, 56) from the input terminal (42) through the main circuit (44) to the output terminal (46); and an auxiliary circuit (48) coupled to a feedback point of the main circuit (44), characterized in that the auxiliary circuit (48) is substantially identical to the main circuit (44) and is coupled between the input terminal (42) and the feedback point of the main circuit (44) for conducting the time-varying signal along a second path (82, 84, 74, 98) from the input terminal (42) to the main circuit (44) at the feedback point, and in that means (50, 62, 64) are provided for delaying the time of propagation of the time-varying signal along the first path (50, 52, 56) to the feedback point so that the time-varying signal at a selected point along the first path (50, 52, 56) has a predetermined time relationship with respect to the time-varying signal along the second path (82, 84, 74, 98) at the feedback point such that the time-varying signal along the first path (50,52,56) reaches the feedback point substantially at the same time as the time-varying signal along the second path (82,84,74,98). The circuit as recited in claim 1 wherein the main (44) and auxiliary (48) circuits each are characterized by: a switch means (56, 84) coupled to receive the time-varying signal; means (58, 60, 86, 88) for controlling the switch means (56, 84) in response to a control signal; and a capacitor (76, 100) coupled to receive the time-varying signal from the switch means (56, 84), the capacitor (100) for the auxiliary circuit (48) having one end coupled to the switch means (84) and to the second path (98, 74) and having the other end coupled to a reference potential, and the capacitor (76) for the main circuit (44) being coupled between the one end of the auxiliary circuit capacitor (100) and the output terminal (46). The circuit as recited in claim 2 wherein the delaying means is characterized by: a first delay means (50) coupled to the input terminal (42) as part of the first path (50, 52, 56) to receive the time-varying signal for the main circuit (44); a control signal generating means (62) coupled for providing the control signal to the main (56) and auxiliary (84) switch means; and a second delay means (64) coupled between the control signal generating means (62) and the main switch means (56), the delays of the first (50) and second (64) delay means being set to produce the predetermined time relationship. The circuit as recited in claim 1 wherein the first path (50, 52, 56) is characterized by: a delay means (50) coupled to the input terminal (42) to receive the time-varying signal; an input buffer amplifier (52) coupled to receive the time-varying signal from the delay means (50); and a switch means (56) coupled between the input buffer amplifier (52) and the output terminal (46) to transfer the time-varying signal from the input buffer amplifier (52) to the output terminal (46) in response to a control signal. The circuit as recited in any one of claims 1 to 4 wherein the second path (74, 98) is characterized by a supplemental circuit (74) coupled between the auxiliary circuit (48) and the main circuit (44) for processing the time-varying signal along the second path (74, 98), the delaying means (50, 62, 64) compensating for any propagation delay of the time-varying signal passing through the supplemental circuit (74). The circuit as recited in any one of claims 1 to 5 wherein the predetermined time relationship is characterized by substantially no delay between the time-varying signals from the first (50, 52, 56) and second (74, 98) paths.

Inventors: SANIELEVICI SERGIO A, SANIELEVICI, SERGIO A.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1989
Main classification: G11C27
IPC codes: G11C27