[sdiy] Help with a strange filter

Richie Burnett rburnett at richieburnett.co.uk
Thu Jun 14 01:48:26 CEST 2012

> This is an interesting read on the topic too:
> http://www.maxim-ic.com/app-notes/index.mvp/id/738
> The gist of it is that for filters with gain=1, the sensitivity is 
> significantly reduced.

Yep.  If you want a 2 pole lowpass stage with high-Q it is better to achieve 
this by using a large ratio of capacitances and a unity gain op-amp in the 
Sallen-Key filter rather than by increasing the op-amp gain with resistors 
in the negative feedback path to peak the response.  It also saves two 

Peaking the filter response by increasing the op-amp gain makes the filter 
more and more sensitive to component tolerances.  However, if you want 
variable resonance the adjustable feedback method is a convenient way of 
achieving it!


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