[sdiy] Odp: ADC - anti-alias filter

Roman modular at go2.pl
Sat Mar 25 08:50:50 CET 2017


Add simple passive filter at least, that's 2 components taking no PCB space really. With corner frequency at 2kHz (point of interest as you say) you'll get about 20dB attentuation at the fs/2.  As others said, all that noise from above will bite you hard at the output, so 20dB less trouble is well worth considering at the price of 4 sqare milimeters of space.   Roman  Dnia 24 marca 2017 19:16 Steve <sleepy_dog at gmx.de> napisał(a):  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?     - Steve      ______________________________  Synth-diy mailing list   Synth-diy at synth-diy.org  synth-diy.org synth-diy.org
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