Sample & Hold#

A sample-and-hold captures an analog voltage at a chosen instant and holds it steady long enough to be measured. It exists because measurement takes time: an analog-to-digital converter needs its input to stay constant while it works out the digital value, and a signal that moves during the conversion produces a wrong reading. Sample-and-hold freezes the signal at a defined moment so the converter has something still to look at.

Switch, Capacitor, Buffer#

The circuit is three parts working together. An analog switch connects the input to a hold capacitor; a high-impedance buffer (an op-amp follower) reads the capacitor’s voltage without draining it.

Sample (track): close the switch and the capacitor charges to follow the input.
Hold: open the switch at the sampling instant, and the capacitor keeps that voltage while the buffer presents it to the ADC.

The moment the switch opens is the sampling instant, which is why the sample-and-hold defines when a signal is measured, not just what value comes out.

The Imperfections That Matter#

Every part of that simple circuit contributes an error worth knowing:

Acquisition time — the capacitor needs time to charge to the input during the sample phase; sample too briefly and it never reaches the true value.
Droop — during hold, leakage and the buffer’s bias current slowly discharge the capacitor, so the held value drifts if held too long.
Charge injection — the switch dumps a little charge onto the capacitor as it opens, adding a small offset to the captured value.
Aperture uncertainty — the switch does not open at a perfectly precise instant, and that jitter in the sampling moment limits accuracy on fast-changing signals.

In practice a discrete sample-and-hold is rare today because most converters build it in: a SAR ADC’s front end is a track-and-hold stage doing exactly this. It is the quiet bridge between the continuous comparator-and-conversion world and the discrete numbers the digital side works with — and the reason an ADC needs its input to settle before a reading.