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

[Tom Wiltshire]

Hi Justin,

> 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?

Q alone, if the cutoff is the same, surely?

> ...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?

Dunno. There are tables for that sort of thing.

> 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).

Get the cutoff as low as you can within reason to increase the amount of rejection you get for everything else. If you're passing audio, put the cutoff at the highest audio frequency you want.

> 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.

I used the Analog Devices Filter Wizard:

http://www.analog.com/designtools/en/filterwizard

Designing filters is no magic. You just plug values in until you find a set which give you the cutoff you want and the response you want (or reasonably close) with some fairly sensible values. I start by deciding on the caps, since they're much harder to get in arbitrary values than resistors. 1% metal film resistors in E96 are fairly cheap and easy to find, but the same is not true for caps.
Once you've got a basic capacitor value, you can tweak the resistors.

I'd seriously advise you to not get too tied up in it (filter component choice). You can waste hours trying to get it spot on. No-one cares, not even you in three months time. Get something that works and uses easy-to-get values, even if it's out by 5% or 10%. So much "mojo" is just sloppy engineering that not struggling to get technical perfection isn't a crime, and might even be a benefit (if you get lucky).

HTH,
Tom