[sdiy] Sallen-Key Bessel Low Pass - behaviour and RC values

[Justin Owen]

Hello,

I've been asked to help set up an anti-alias (input) and reconstruction (output) filter for a micro-based project a friend is working on. I'll be using a Bessel type Sallen-Key Low Pass config for both - but I had some initial questions.

In a general sense, my understanding is that the real world differences in behaviour/performance between Bessel and Butterworth in a SallenKey config are mainly down to Q and cutoff?

...and the differences in component selection is that for a Butterworth topology there is a 2:1 ratio between the values of C1 and C2 (or C2 and C ATCMB...) and for a Bessel C1 = C2.

Is that right?

Secondly, there seems to be two schools of thought on whether to set the filter cutoff frequency at Nyquist (which I believe is currently 96KHz) or around the highest frequency the DAC will output (in our case I believe this is about 16KHz).

Any thoughts on this from those who've been round this block before?

Lastly - I hope you guys don't me asking this on list but I'd be curious to hear from Tom W. and Olivier G. about their RC selections of 12K/10nF (plus some gain?) and 39K/100pF - and anyone else on there fave or suggested RC values.

Thanks!

- J