[sdiy] Help with a strange filter

[Richie Burnett]

> 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 
resistors.

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!

-Richie,