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The Nyquist criterion dictates that all signals must be bandlimited to less than half the sampling rate of the sampling system. Many signals already have a limited spectrum, so this is not a problem. However, for broad spectrum signals, an analog lowpass filter must be placed before the data acquisition system. The minimum attenuation of this filter at the aliasing frequency should be at least: tex2html_wrap_inline2056 where B is the number of bits of the ADC. This formula is derived from the fact that there is a minimum noise level inherent in the sampling process and there is no need to attenuate the sensor signal more than to below this noise level.

Tim Stilson
Thu Oct 17 11:25:23 PDT 1996