Why is a sample-and-hold circuit used before an ADC?

To keep the input constant while the conversion takes place

To amplify the signal so the ADC reads larger values

To remove noise by averaging many samples

To convert the analog signal into a digital one

What happens to the switch and the hold capacitor in hold mode?

The switch opens and Ch keeps its charge, so Vout stays at the value at the instant of opening

The switch stays closed and Ch follows Vin

The switch closes and Ch discharges fully

The switch opens and Vout drops to zero immediately

Why is the output buffer A2 given a very high input impedance?

So the capacitor does not discharge through the load

So Ch charges more quickly in sample mode

So feedthrough from the input is increased

What is the trade-off in choosing the hold capacitor Ch?

Larger Ch lowers droop but increases acquisition time

Larger Ch increases droop and shortens acquisition time

Larger Ch lowers both droop and acquisition time

Ch has no effect on droop, only on aperture time

Year not given · 8 marks

Sample and hold circuit

Short answer

A sample-and-hold circuit consists of an input buffer, an analog switch, a hold capacitor and an output buffer. In sample mode the switch closes and the capacitor charges to the input voltage; in hold mode the switch opens and the output stays constant. Key parameters are acquisition time, aperture time and droop rate.

Explain sample and hold circuit with necessary circuit diagrams.

A sample-and-hold (S/H) circuit takes a sample of an analog signal and holds its value constant for the required time. It is used before an ADC so that the input does not change during conversion.

Basic circuit

            +----+      SW (FET)       +----+
 Vin o----->| A1 |------o/  o----+---->| A2 |-----o Vout
            +----+        ^      |     +----+
                          |     === Ch
                    control      |
                    (S/H)       gnd

A1 is an input buffer (low output impedance, so Ch charges quickly).
SW is an analog switch (MOSFET or JFET) controlled by a logic signal.
Ch is the hold capacitor (low leakage, e.g. polystyrene or Teflon).
A2 is an output buffer with a very high input impedance, so the capacitor does not discharge through it.

Operation

Sample mode: the control is high and the switch is closed. Ch charges through the switch to Vin, and the output follows the input.
Hold mode: the control is low and the switch opens. Ch keeps its charge, and the output stays constant at the value it had at the instant the switch opened.

 control:  |‾‾ S ‾‾|__ H __|‾‾ S ‾‾|__ H __|
 Vout:      tracks   held    tracks   held
            Vin      flat    Vin      flat

Important parameters

Acquisition time: the time needed to charge Ch to the input value within a given accuracy.
Aperture time: the delay between the hold command and the switch actually opening.
Droop rate: the fall of the voltage across Ch in hold mode due to leakage (dV/dt = Ileak/Ch).
Feedthrough: the input signal leaking through the open switch.
Hold-mode settling time.

A larger Ch lowers the droop but increases the acquisition time, so a compromise is made. A feedback version (with the switch inside the op-amp's feedback loop) improves accuracy. S/H circuits are used in DAS, ADC front ends and for demultiplexing DAC outputs.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Sample and hold circuit

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