Sample-and-hold (S/H) building blocks: does a minimal S/H circuit necessarily require an instrumentation amplifier, or is a buffer amplifier sufficient when paired with an analog switch and a hold capacitor?

Introduction / Context:
Sample-and-hold (S/H) circuits are used to “freeze” an instantaneous analog value for conversion or processing. A common misunderstanding is that an instrumentation amplifier is a required element. This question clarifies the canonical minimal topology.

Given Data / Assumptions:

• Basic S/H function: acquire and hold an analog voltage for a defined interval.
• Idealized components to explain roles.
• No requirement for differential inputs or precision gain beyond buffering.

Concept / Approach:
A minimal S/H uses an analog switch (or transmission gate), a hold capacitor (C_H), and a buffer amplifier with high input impedance and low output impedance. During “sample,” the switch closes and the capacitor charges to the input. During “hold,” the switch opens and the buffer isolates the capacitor from the load, presenting the stored voltage at the output. An instrumentation amplifier is not inherently required; it is used when differential inputs, high common-mode rejection, or programmable gain are needed, which go beyond the basic S/H function.

Step-by-Step Solution:

Verification / Alternative check:
Reference implementations in textbooks and ADC datasheets show MOSFET transmission gates feeding a small capacitor followed by an op-amp buffer or source follower; no instrumentation amplifier is shown in the simplest case.

Why Other Options Are Wrong:
Mandatory instrumentation amplifier: incorrect for basic S/H. Comparator/DAC substitutions do not provide the unity-gain buffering needed. “No amplifier required” fails because the held node would sag under load without a buffer.

Common Pitfalls:
Confusing differential front ends (instrumentation amplifiers) with the buffering role; underestimating droop and output loading without a buffer.

Final Answer:
A buffer amplifier with an analog switch and a capacitor is sufficient