Purpose of sample-and-hold (S/H) circuits in ADC systems In an A/D conversion system, sample-and-hold circuits are primarily designed to do what during the conversion interval?

Introduction / Context:
Real-world analog signals can vary while an ADC is converting. Without a stable input, the ADC may digitize a moving target, causing conversion errors. Sample-and-hold (S/H) circuits mitigate this by freezing the input signal level during the critical conversion window.

Given Data / Assumptions:

• An ADC requires a stable input during quantization.
• S/H provides two phases: sample (track) and hold.
• Comparator thresholds and counters are internal to specific architectures and are not the object held by the S/H block.

Concept / Approach:

The S/H captures the instantaneous value of the analog input at the sampling instant and holds it constant while the ADC performs its conversions (especially important in SAR and multiplexed systems). This ensures conversion accuracy matching the rated resolution and timing.

Step-by-Step Solution:

Verification / Alternative check:

Observe that counter-ramp, SAR, and pipeline ADC datasheets commonly include a front-end S/H or track-and-hold, confirming the role described.

Why Other Options Are Wrong:

• Holding a binary counter output is architecture-specific and not the function of S/H.
• Comparator threshold stabilization is not the role of S/H.
• Holding a DAC staircase is not typical; the S/H is applied at the input of the ADC.

Common Pitfalls:

• Confusing S/H with zero-order hold used in DAC reconstruction; the purpose here is input stability for ADCs.

Final Answer:

stabilize the input analog signal during the conversion process