Analog-to-digital converters (ADCs): What is the primary purpose of the sample-and-hold (S/H) circuit inside or ahead of an ADC during the conversion interval?

Introduction / Context:
Many ADC architectures require a constant input during conversion to avoid conversion errors. Real-world signals may change rapidly or contain noise. The sample-and-hold (S/H) circuit captures (samples) the instantaneous analog value and maintains (holds) it stable while the converter performs quantization. This is crucial for achieving accurate, glitch-free results, especially at high resolution or when input bandwidth is significant.

Given Data / Assumptions:

• An ADC needs a quasi-static input during its internal decision process.
• The S/H circuit consists of a switch and a hold capacitor with low droop and fast acquisition.
• We focus on the role with respect to the analog input, not internal digital nodes.

Concept / Approach:
During the brief sample phase, the hold capacitor charges to the input level. The switch then opens for the hold phase, isolating the capacitor so the input presented to the ADC core remains constant. By eliminating input movement while comparisons are made, the S/H prevents code ambiguity, reduces distortion, and improves effective number of bits (ENOB) for dynamic signals.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets specify acquisition time, aperture time, aperture jitter, hold step, and droop rate. Meeting these ensures the input is effectively stable, which directly correlates with dynamic performance metrics such as SINAD and THD for fast-changing signals.

Why Other Options Are Wrong:

• Binary counter output: Relevant to successive-approximation timing in some ADCs, not what S/H stores.
• ADC threshold voltage: Stabilized by internal references and regulators, not the S/H.
• ADC staircase waveform: A DAC inside an SAR can generate a staircase, but the S/H does not hold that waveform.

Common Pitfalls:
Confusing S/H storage target (the analog input) with internal DAC or comparator nodes; overlooking the impact of aperture jitter on high-frequency inputs.

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