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Sampling frequency Confused :S

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Member level 2
Jun 20, 2011
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hi there ... I am working on EEG signals .. And sampled it using ADC0804...
As I read in the forums here that :
" Theoretically (Nyqist theorem) the cutoff frequency of the anti-aliasing filter should be 1/2 of the sampling frequency. In practice the cutoff frequency should be even smaller; it depends (among other things) on the number of poles in the filter. "
It means if I have an anti-aliasing filter of cutt of 25 hertz then my sampling frequency will be of 50 Hz ... Am I rite .. It means that ADC0804 conversion time or its RC constant i.e (1/1.1RC) doesn't plays any role...
Please Guide ...

Theoretically you are right. Practically you need more like 60Hz for even a very good 25Hz filter. But this is a very odd way to start the problem.

Usually you start with the frequency of the signal you want to digitize, the voltage resolution you need, and the amount of data you can handle.
Your filter must not attenuate the real signal more than the resolution allows. Based on the sharpness of the filter this sets the filter frequency.
Your filter must attenuate any unwanted signal to below the resolution level before you reach 1/2 the sampling frequency. Based on the sharpness of the filter and the noise level this sets the sampling frequency.
If this sampling frequency is too high and generates too much data to store or process, you must spend more money/power/space/weight on a sharper filter or lower noise signal.
Also consider the phase distortion of the signal as it gets near the filter cutoff.

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