AD1835: Output filters

Document created by analog-archivist Employee on Feb 23, 2016
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I have a little question about your audio codec AD1835. There are schematics of
a typical ADC input antialiasing filter and a DAC output reconstruction filter
in the datasheet for AD1835. But there are no comments about parameters of
these filters. So I had been simulated them and I found out that critical
frequency (fc at -3dB) of ADC filter is about 200kHz and critical frequency of
DAC filter is 100kHz. I don't understand why these filters have so high
critical frequencies. I think that if samplling freq. is for example 96kHz than
maximum frequency component in input signal must be 96kHz / 2 = 48kHz in order
that no aliasing arise. The same situation is about output reconstruction

    Can you explain me why these filters have so high critical frequencies?
Wouldn't it be better if these frequencies will be about 40kHz for both ADC and
DAC filter?


The filters uses for sigma-delta codecs don't need tight cutoff frequencies.
There main aim is to have good attenuation at the frequency the modulators run
at, 3MHz in the case of the AD1835. We selected values which give the cutoff at
100KHz so we could operate the parts at sample rates up to 192khz if required.
We have also tested these parts with simple RC filters (51ohm and 2.2nF) with
no degradation in performance.