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Sample and Hold Circuit

"Time is what keeps everything from happening at once." If we want to measure a signal, we can not ignore the fact that it changes with time. There are several approaches to dealing with this. We could measure the signal very fast, do it many times, and after we're done figure out what the right time scale would have been. We could average the signal. Or we can snatch the value of the signal, hold that value steady, and digitize the single, sampled value at our leisure. A sample and hold circuit (or its first cousin, track and hold) can be employed with digitizers to pluck a single value from an analog source, keep that value stable for at least the time required for digitization, and can then be set to grab (sample) a value at a later time.

Consider the following circuit:

Sample and Hold Circuit

If the switch is closed, the capacitor is charged to the potential Vin. When the switch opens, the capacitor holds its charge, so the follower op amp has an output potential of what potential was on the capacitor at the moment the switch opened. If the switch is typically closed, but opens momentarily when we want to make a measurement, this is a track and hold circuit. If we typically leave the switch open, but momentarily close it when we want to grab a measurement, it is a sample and hold circuit.

This sounds too easy. Time for a reality check.



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