Purpose of the sample-and-hold (S/H) in ADC systems: During analog-to-digital conversion, what is the function of the sample-and-hold circuit with respect to the input signal?

Difficulty: Easy

Correct Answer: stabilize the input analog signal during the conversion process

Explanation:


Introduction / Context:
Many ADCs require a stable input during conversion to guarantee accuracy, especially successive approximation (SAR) and integrating types. A sample-and-hold (S/H) or track-and-hold (T/H) circuit ensures the converter sees a constant voltage while it determines the digital code.



Given Data / Assumptions:

  • Input can vary with time (audio, sensor outputs, ripple).
  • Converter needs microseconds (or longer) to complete conversion.
  • Goal: prevent input slewing during this interval.



Concept / Approach:
The S/H momentarily tracks the input (sample phase), then opens the switch and holds the captured voltage on a capacitor (hold phase). The ADC measures this held voltage. This freezes the input to avoid conversion errors due to input movement.



Step-by-Step Solution:
Track phase: switch closed; capacitor follows the input.Sample instant: command to hold; switch opens.Hold phase: ADC converts the fixed capacitor voltage.After conversion: switch closes again to resume tracking.



Verification / Alternative check:
Timing diagrams in SAR ADC datasheets show a required acquisition time and a hold period coinciding with the conversion window.



Why Other Options Are Wrong:
Option A confuses SAR logic with input conditioning.

Option B misattributes comparator reference stability to the S/H.

Option D refers to staircase waveforms used in some ADC tests, not the S/H function.

Option E is unrelated; S/H does not remove quantization noise.



Common Pitfalls:
Ignoring droop and aperture uncertainty; inadequate hold capacitor or switch leakage degrades performance at high resolution.



Final Answer:
stabilize the input analog signal during the conversion process

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