Sample and Hold Circuits

Definition

A sample and hold circuit is an electronic circuit that samples an analog input signal at a specific instant of time and then stores, or holds, that sampled voltage constant for a defined period until the next sample is taken.

It typically consists of:

a switch,
a capacitor,
and a buffer amplifier.

During the sample mode, the switch closes and the capacitor charges to the input voltage. During the hold mode, the switch opens and the capacitor retains the voltage, allowing the output to remain approximately constant.

Main Content

1. Basic Structure and Components

Switching element

The switch is the key control device. It may be implemented using a transistor, MOSFET, analog switch, or transmission gate. When enabled, it connects the input signal to the storage capacitor. When disabled, it isolates the capacitor from the input.

Storage capacitor

This capacitor stores the sampled voltage. Its value is chosen carefully: large enough to hold the charge with minimal droop, but not so large that charging becomes too slow. It is the core memory element of the circuit.

A typical arrangement looks like this:

Input signal --> [Switch] --> [Capacitor] --> [Buffer] --> Output
                    |
                 Control

The buffer amplifier is usually a unity-gain voltage follower. It prevents the stored voltage on the capacitor from being discharged by the load connected to the output. Without a buffer, the output load would alter the held voltage and reduce accuracy.

2. Sampling and Holding Operation

Sampling phase

In this phase, the control signal turns the switch ON. The capacitor charges quickly to the instantaneous value of the input signal. The output follows the input, assuming the circuit is designed for low error and fast charging.

Holding phase

In this phase, the switch turns OFF. The capacitor is disconnected from the input, and the stored charge keeps the voltage nearly constant. The output remains fixed until the next sampling event.

This process is crucial when the input signal changes rapidly. For example, if a sensor output is varying continuously and an ADC needs time to convert, the sample and hold circuit “freezes” the value long enough for accurate digitization.

3. Important Characteristics and Performance Parameters

Acquisition time

The time needed for the capacitor to charge to the correct input voltage during sampling. Faster acquisition time means the circuit can handle rapidly varying signals more effectively.

Droop rate

During the hold phase, the held voltage may slowly decrease due to leakage currents and capacitor discharge. This gradual decline is called droop. A lower droop rate means better holding accuracy.

Aperture time and aperture jitter

Aperture time is the interval during which the circuit transitions from sampling to holding. Aperture jitter refers to timing uncertainty in that transition. In precision and high-speed systems, this can introduce significant conversion errors.

Additional practical issues include:

Switch resistance

Affects charging speed and sampling accuracy.

Charge injection

When the switch opens, a small amount of charge may be transferred to the capacitor, causing an output error.

Capacitor leakage

Reduces the accuracy of the stored voltage over time.

These parameters determine how well the sample and hold circuit can preserve the input value and how suitable it is for high-resolution A/D conversion.

Working / Process

1. Apply the analog input and close the switch

The control pulse places the circuit in sample mode.
The storage capacitor charges toward the input voltage.
The buffer output follows the input signal closely.

2. Reach the desired sample instant and open the switch

At the exact moment a sample is needed, the control signal turns the switch OFF.
The input is disconnected from the capacitor.
The capacitor now retains the sampled charge.

3. Hold the voltage constant until the next sample

The buffer amplifier delivers the held voltage to the next stage, such as an ADC.
The output remains nearly constant for the hold interval.
After conversion or processing, the switch closes again to capture the next sample.

Example: If a temperature sensor output is 2.73 V at the sampling instant, the sample and hold circuit captures that value and keeps the output near 2.73 V long enough for the ADC to convert it into a digital number.

Advantages / Applications

Improves ADC accuracy

It allows the input voltage to remain stable during conversion, reducing conversion error in successive approximation, flash, and other ADC types.

Useful in high-speed signal processing

It enables precise capture of rapidly changing signals, which is essential in oscilloscopes, data loggers, and communication receivers.

Supports multiplexed systems

In systems where multiple channels are switched into one ADC, a sample and hold circuit ensures each channel is measured reliably.

Other important applications include:

Digital voltmeters

Data acquisition systems

Audio signal processing

Telemetry systems

Instrumentation and biomedical electronics

Peak detection and tracking circuits

The circuit is especially valuable wherever a continuously changing analog quantity must be temporarily frozen for analysis, storage, or conversion.

Summary

A sample and hold circuit captures an analog voltage at a specific moment and keeps it steady for a short time.
It is mainly used before A/D conversion to improve accuracy.
The circuit usually uses a switch, capacitor, and buffer amplifier.

Sample mode

charges the capacitor; hold mode preserves the voltage.
Important terms to remember: sample mode, hold mode, acquisition time, droop, aperture time, charge injection