You need to put a low pass filter in front of any A/D converter to prevent aliasing. (calles an anti aliasing filter). The time constant of the filter should be selected so that the low pass filter knee is well below the A/D converter sample rate.

If you don't do this in a sampled data environment and then you try to analyze the sampled data to find frequency content you can very easily "see" frequencies in the data that did not actually exist in the original signal.

If the offset across the resistor in the filter is troubling to you then you should implement the filter with an Op-Amp where you can arrange a sufficiently high input impedance and then drive the microcontroller A/D input with the low output impedance of the Op-Amp.


Michael Karas