Re: REGARDING SAMPLING
An A/D converter requires a certain amount of time to perform a conversion. So, at best, the ADC output can represent what the analog input is as a series of "snapshots" of the input, separated by an amount of time that is equal to, as a minimum, the time required to perform an A to D conversion. Usually a sample and hold device is used in front of the converter proper to prevent the converter input from changing during the conversion process. The sampling interval (frequency) must be at least twice the frequency of the highest frequency component in the input in order to prevent aliasing.
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Your question is rather general. I hope this helps.
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Regards,
Kral