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Sample and Hold Settling Time

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A track and hold does not see jumps in input as often as a sample and hold, so if there are long pauses between taking samples, a track and hold reaches high precision more quickly than a sample and hold. Because it is easy to signal average a long list of numbers, it has become common to take data very fast, then average or take subsets of the data for emulating infrequent measurements. Thus, sample and hold is more common than track and hold.

Unfortunately, there are non-idealities, so just dealing with RC time constants is over-optimistic. The switch does not turn on or turn off instantaneously. There is some resistance associated with the switch when it is on, and some stray capacitance when it is off. During switching, there are noise spikes or glitches. Thus, after the switch opens, one must wait some period (usually less than 1 μs) before starting digitization. Given the numbers in the last problem above, it is little wonder that typical 16 bit analog to digital conversion for many years was limited to 50 to 100 kHz. In recent years, conversion rates over 1 MHz have become common. What led to the change? Low resistance switches with few glitches, and the smallest possible capacitors.



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