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

Martin Fay martin at martinfay.com
Thu Feb 6 16:17:28 CET 2014


Hi Justin,

Cutoff is the -3dB point, but at Nyquist the filter needs to be into  
the stop band "totally" blocking frequencies above Nyquist.

Martin

Sent from my iPhone

On 6 Feb 2014, at 14:32, Justin Owen <juzowen at gmail.com> wrote:

> 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
>
>
>
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