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?
Answer
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:
- Analog Input.
- Analog Anti-aliasing filter.
- Oversampling (eg, at 8x).
- Digital Anti-Aliasing Filter.
- Decimating (reduction to 1x).
- Digital Output.
The further illustrate, consider the following:
- The audio is sampled at 44100Hz.
- This provides a Nyquist frequency of 22050 Hz.
- Any frequencies above 24100 Hz will alias back to the audible range (below 20kHz).
- 20000Hz to 24100 is about quarter of an octave.
- Even with a steep 80dB/8ve filter you will only be reducing the aliasing frequencies by 20dB.
But with 8x oversampling:
- The audio is sampled at 352.8kHz (44.1kHz x 8).
- Nyquist is 176.4 kHz.
- Only frequencies above 332.8kHz will mirror to the audible range.
- That's about 4 octaves.
- So you can apply a 24dB/8ve analog filter to reduce aliasing frequencies by 96dB.
- Then oversample.
- Then apply linear phase digital filter between 20kHz and 24.1kHz
The following book is an excellent, clear resource for these sort of things.
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