Anti-aliasing filter

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zionico90

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Hy guys,

I have to design an anti-aliasing filter for a 16-bit ADC with a sampling frequency of 200 kHz with the following specification:

- Pass Band Gain H=10dB with +/- 1 dB ripple
- Off Band Gain Hoff=-40dB

My understanding is that the staff frequency (the one at -40dB) should be 200kHz/2 for Nyquist. However, it seems that there are no information about the cut-off frequency of the filter which is fundamental parameter for the design. Is it possible to derive it considering that we have 16-bit ADC or it is a missing input?

Thanks!
 

I think you're right. If you have a two-pole filter the cutoff (3db) point will be much lower than for, say, an 8-pole filter.
 

It is crazy! So I have the sampling frequency and then dividing by the number of channels I can set the stop frequency. For what concerns the cut-off...it is simply free! Crazy...provocatively I would select a 2nd or 3rd order filter
 

It could depend on bandwidth of interest, in other words if your signal is nominally 100hz, a two pole filter would be fine
 

To avoid aliasing, the stop band attenuation of e.g. 40 dB must be already achieved at the Nyquist frequency of 100 kHz, respectively the upper pass band corner must be lower than 100 kHz, e.g. 90 kHz. In a multiplexed ADC application, the calculation applies for the effective sampling frequency per channel.

See below the characteristic of an almost ideal digital antialiasing filter of an oversampling audio ADC.



A filter with the said characteristic needs a much higher order than 2 or 3. The anti-aliasing filter specification depends however on the out-of-band noise spectrum. If you know there are no or only weak aliasing components, the filter specification can be relaxed.
 

Hi,

Just to clarify. I see you have multiple channels:

Then your hardware setup should be:
(Signal_input --> AAF) × channels --> MUX --> ADC.
And not:
(Signal_input) × channels --> MUX --> AAF --> ADC.

Every time you select another channel you need to wait for the ADC input voltage to settle (ideally to +/- 0.5LSB deviation).

Klaus
 

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