[sdiy] ADC - anti-alias filter

[Tom Wiltshire]

On 24 Mar 2017, at 18:13, Steve <sleepy_dog at gmx.de> wrote:

> Howdy @ DSP experts out there,
>  
> I was wondering:
> if done by the book, if you're doing ADC conversions of an audio signal and like it to be clean, you put a nicely steep LPF before it that, immensely reducing anything >= fsample/2.
> 
> So somebody told me recently, he wants to analyse an audio signal which pretty much only has frequencies up to 4kHz, maybe 6 but very quiet, and he's interested only in 1..2 kHz of that.
> Hence, he wants to omit the LPF to save parts & PCB space, sample at a rate like 15..25 kHz and use a digital filter and then decimate to 4 kHz or so.
>  
> Is that really feasible?
> The thing is, the signal is picked up by an analog mic (piezo) with 2 opamps after another, each has an integrated PGA of up to 16x gain, because the signal get get really quiet.
> 
> Now even if the sound source picked up and the frequency response curve of the piezo make sure that the mentioned upper limit holds.
> Could that stuff not pick up noise in some environments which introduce frequencies above 1/2 of his higher samplerate and ​hence pollute the spectrum?
> 
> Is it *ever* a good idea to omit the analog filter before sampling?

If the signal is being picked up by a mic, I don't see how they can be so sure about the frequency content. Unless the mic is utterly dreadful, I suppose, which is possible with some piezo things I've seen. Can you put the mic output on an FFT and see what frequencies it really picks up?

I'd be inclined to put a simple passive filter in anyway (2 pole is only four parts) and make sure that the op-amp stages are limited to only the required frequency range. It may be that you can get enough roll-off without needing any more active stages. You could use a multiple feedback filter with gain for one of the gain stages, for example.

To answer the final question, I'd say no, it's never a *good* idea, but you might get away with it in a particular situation if you're clever about it.

HTH,
Tom


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