I am working on a board that has no antialisaing filter at the input of the ADC. I have option to I implement my own filter using RC + Opamp circuit. But is it also possible to implement Anti Aliasing filter after sampling by ADC and processing in Digital domain: a digital Anti aliasing filter?
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Just to support Matt's answer and provide a few more details:
Most modern ADCs do most of the hard antialiasing job in the digital domain. Reason is that digital filters tend to produce less by-products for a much lower cost. The actual chain is:
The further illustrate, consider the following:
But with 8x oversampling:
The following book is an excellent, clear resource for these sort of things.
I agree with pichenettes's answer but I would like to add that it is pretty common practice to use a simple inexpensive low-order analog anti-aliasing filter, and do the rest of the anti-aliasing filtering in the digital domain. This implies of course that you do not process at maximum sampling rate but that you downsample after the digital anti-aliasing filter. Summarizing:
No, this does not make sense. Say your ADC sample rate is 1kHz. A 100 Hz sine wave and a 900 Hz sine wave will yield exactly the same sequence of digital samples once fed into your ADC - but you want to pass the former and attenuate the later. How do you expect your digital filter to produce different outputs when fed the same input?
The only thing that could work would be to sample the input signal as fast as your ADC allows, and then downsample it in the digital domain to your target sample rate - but unless you have CPU cycles to waste, you'd better use an analog filter upstream.