Question

Sample and hold circuits in A/D converters are designed to

• A. \( \text{Stabilize the comparator's threshold voltage during the conversion process} \)
• B. \( \text{Sample and hold the output of the binary counter during the conversion process} \)
• C. \( \text{Sample and hold the D/A converter staircase waveform during the conversion process} \)
• D. \( \text{Stabilize the input analog signal during the conversion process} \)

Hint:

The main purpose of a Sample and Hold circuit in an ADC is to mitigate the effects of changing input signals during the finite conversion time. It effectively takes a "snapshot" of the analog voltage at a specific instant and holds that value constant, ensuring accurate analog-to-digital conversion even for rapidly varying input signals. This is critical for preventing "aperture error" or "aperture uncertainty."

Answer 1

The Correct Option is D

An Analog-to-Digital Converter (ADC) takes an analog input signal and converts it into a digital representation. This conversion process takes a finite amount of time, known as the conversion time, \( t_{\text{conv}} \). If the analog input signal changes significantly during this conversion time, the resulting digital output will be inaccurate because the ADC is trying to convert a fluctuating voltage. A Sample and Hold (S/H) circuit is placed at the input of most ADCs to address this problem. Its function is as follows: \textbullet \ Sample Mode: When a sampling command is received, the S/H circuit quickly measures (samples) the instantaneous value of the analog input signal.
\textbullet \ Hold Mode: Immediately after sampling, the S/H circuit enters the hold mode. During this mode, it maintains (holds) the sampled analog voltage constant at its output, even if the actual input signal changes. This held voltage is then fed to the ADC. By stabilizing the analog input voltage throughout the ADC's conversion time, the Sample and Hold circuit ensures that the ADC converts a fixed, unchanging voltage, leading to accurate digital representation. Let's evaluate the options: (A) \( \text{Stabilize the comparator's threshold voltage during the conversion process} \): The S/H circuit acts on the *input analog signal*, not on internal reference voltages or comparator thresholds, which are typically stable by design.
(B) \( \text{Sample and hold the output of the binary counter during the conversion process} \): The binary counter is part of the ADC's digital logic (e.g., in a successive approximation ADC) and its output is already digital. A S/H circuit works with analog signals.
(C) \( \text{Sample and hold the D/A converter staircase waveform during the conversion process} \): In many ADCs (like successive approximation or counter-type), an internal DAC generates a staircase or varying voltage for comparison. The S/H circuit is at the *input* to the ADC, not monitoring its internal DAC output.
(D) \( \text{Stabilize the input analog signal during the conversion process} \): This accurately describes the primary function of a Sample and Hold circuit in an A/D converter, which is to provide a constant analog voltage to the ADC while it performs the conversion.