In data converters, what is the primary purpose of a sample-and-hold (S/H) circuit? (Consider typical ADC front-end usage in embedded systems and instrumentation.)

Introduction / Context:
A sample-and-hold (S/H) circuit is a standard building block placed ahead of many analog-to-digital converters (ADCs). It samples a time-varying analog signal and then holds that value steady while the ADC performs the conversion. This question tests your understanding of why S/H stages are paired with certain ADC architectures (successive-approximation, dual-slope, pipeline, etc.).

Given Data / Assumptions:

• The input is an analog voltage that may change over time.
• The ADC requires a finite conversion time during which its input should be constant.
• The S/H has two phases: sample (track) and hold.

Concept / Approach:
During the sample (or track) phase, a small capacitor is connected to the input and charges to the instantaneous input voltage. During the hold phase, the switch opens, isolating the capacitor so its voltage remains nearly constant. The ADC then reads this stable level, avoiding conversion errors caused by input movement during the conversion window.

Step-by-Step Solution:
Define the problem: ADCs need a stable input while converting.Identify function: S/H freezes the analog value at a specific instant.Relate to accuracy: A stable held voltage prevents code ambiguity and aperture error.Select the option that states “hold analog voltage constant for ADC conversion.”

Verification / Alternative check:
Compare ADC performance with and without S/H for a fast-changing input. Without S/H, measured codes jitter. With S/H, codes stabilize for the same signal at the same sampling instants.

Why Other Options Are Wrong:
Option A: “Temporary memory” refers to digital storage, not analog holding.Option C: DACs generate analog outputs from digital codes; they do not need an S/H to “wait.”Option D: Multiplexer selection is orthogonal; S/H purpose is analog stability, not datapath selection.Option E: Isolation from clocks is not the fundamental role.

Common Pitfalls:
Confusing an S/H (analog) with a digital latch or register. Also, assuming all ADCs inherently track fast signals without needing a stable input during conversion.

Final Answer:
To hold an analog voltage constant so an ADC has time to produce a stable digital output