Re: Audio Sampling Question



Tom Bruhns wrote:

Eeyore wrote:
Guy Macon wrote:
Henry VIII wrote:

Are the analog filters identical? A slight difference in phase
shift will introduce an error. Even if you use the same analog
input and compare measurements at different times there could be
variations due to temperature.

Most modern audio ADCs don't require a front end filter.


Sorry, Graham, that's not quite true. Because the sampling is at a
high frequency in a delta-sigma converter, typically a couple MHz, the
alias filtering can be very "gentle", but it IS necessary if there's
any chance of high frequencies getting through otherwise.

By which you mean ultrasonic frequencies (close to the Nyquist limit imposed by the
over-sampling) though. NOT audio frequencies.

If used strictly for audio frequencies, a front end filter truly isn't required. The
rather gentle 6dB/octave LPF most application circuits suggest is simply to filter
any out of band signals. The recommended values for the ADC I've been using give you
a 1st order LPF @ 75kHz. That assuredly isn't going to give filter match issues in
the audio band. And, as I said, it could be left out if you're confident of the
bandwidth of the incoming signal ( i.e it doesn't contain anything in the megahertz
region.

Graham

.

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