A simplified model of a sample-and-hold circuit is a switch connected between the input and one terminal of a holding capacitor. The other terminal of the capacitor is tied to a ref­erence voltage (ground), and the output of the sample-and — hold is the voltage across the capacitor. The switch is con­trolled by a digital signal that determines the sampling time and the hold duration. Ideally, the output voltage tracks the input exactly when the switch is closed (track or sample mode) and stores a sample of the input voltage when the

Sampling moment


Figure 3. Settling time and pedestal voltage during the sample-to — hold transition.


switch opens (hold mode). In reality the behavior of the circuit deviates from this ideal model affecting the performance. The following subsections describe the specifications used to char­acterize sample-and-hold circuits.

Sample-Mode Specifications

The operation during the sample mode is similar to a voltage or current amplifier. Thus any specifications used to charac­terize an amplifier can be used to characterize the sample — and-hold circuit in sample mode. Some of the key specifica­tions in this mode of operation are offset, settling time, gain error, and nonlinearity. The offset is defined as the difference between the input and the output with no signal applied at the input. This is shown in Fig. 1. The time it takes for the output to settle within a certain error band around its final value with a step applied at the input is called the settling time. The size of the step is usually full scale as shown in Fig. 1 unless specified otherwise. Gain error and nonlinearity are both steady-state errors that describe the deviation of the magnitude transfer characteristic from a straight line with a slope of 1. As represented in Fig. 2, the gain error appears as the deviation of the slope of line P1P2 from 45° and can be expressed as Gerror = 1 — tan в. One definition of nonlinearity is the maximum deviation of the transfer characteristic from line P1P2. This is also shown in Fig. 2. Other sample-mode specifications include bandwidth, slew rate, distortion, and noise, which also characterize a general amplifier, are defined in a similar fashion.

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