sampling rate

Hi
In analogue to digital conversion ,it is often that sampling frequency should be greater than twice the maximum frequency contained in the
original signal.How to choose a an analogue to digital conveter if we have no idea about the frequency content of the signal to be conveterd to digital.
Thanks
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FAISAL wrote:

If you "have no idea" - then you need to get one.
One common approach is to apply a low pass filter to limit the frequency content to the range that you are interested in measuring. The choose an A/D to suit. But that does require "some idea".
Another is to use a converter with a sample rate way, way beyond any signal frequency that you think is possible or likely. But that does require "some idea".
Another is to measure the signal. Always a good idea but sometimes impractical.
Another is to model the system and predict what the range of signal frequencies will be. That can be surprisingly effective.
etc..
-- Sue
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Quite right. A quick & dirty method is to look at the signal on a 'scope and eyball the frequency content (it's a knack!) If you have the equipment, sample it at a "high enough" frequency and do a Fourier transform to see what's in it, i.e. get the power spectrum. Figure out what you want to capture for your application what you have to filter out. Design a steep low-pass cut-off filter to select what you want. Always use an anti-aliasing filter before sampling and always respect Nyquist! Cheers, Roger
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