PH4705/ET4305: Sample & Hold

1. PH4705/ET4305: Sample & Hold Measurement system often can’t keep pace with changing input signal particularly where a conversion into a digital signal is being made. Instead of continuously tracking the analogue signal it is sampled at a specified time interval, the sampled value being held until the next sample is taken. In this way the following circuit has time to complete its processing. A circuit that performs this task is called a Sample and Hold.

2. PH4705/ET4305: Sample & Hold The simple capacitor and switch illustrates the principle; close the switch to sample, open it to hold. Real sample & hold uses an electronic switch (FET) and the capacitor must be buffered or the following circuit will discharge it. To respond fast enough to store the input the capacitor must be small, so its storage time will be limited. Again Op Amps can be used to achieve adequate performance.

3. PH4705/ET4305: Sample & Hold The switch (an FET) and capacitor are surrounded by OP Amp circuitry to buffer the input and output and to enable the switching to be done by an ordinary logic signal.

4. PH4705/ET4305: Sample & Hold Semiconductor manufacturers produce sample and hold ICs with superior specification. A typical device is the National Semiconductor LM398, an 8 pin IC that uses a single external capacitor, 1-100nF, to hold for 1-1000 µs, sampling being controlled by a standard logic signal. It operates from a wide range of power supplies, is low noise, fast capture, and has a unity gain accurate to 0.002%

5. PH4705/ET4305: Sample & Hold Typical integrated Sample & Hold IC