[sdiy] Basic 4th order LP butterworth filter papers

[Magnus Danielson]

Aaron Lanterman wrote:
> On Feb 5, 2010, at 1:45 AM, Paul Maddox wrote:
> 
>>> Like this, or something else?
>>> http://focus.ti.com/lit/ds/symlink/tlc14.pdf
>> How do these sound in a synth?
>> has anyone ever tried them?
> 
> I wouldn't expect one to be particularly exciting. The defining aspect of a Butterworth is that it has the steepest cutoff you can get for a given order without any ripple. 
> 
> A two-pole lowpass filter is a Butterworth if you use a Q of 1/sqrt(2) (if I recall correctly). 

Does not help for a four-pole lowpass since the poles should all lie on 
the left hand side of a circle evenly spread out by 180/n degrees 
between them with the left-most pair being at 180 +/- 90/n degrees to be 
a Butterworth response. However, there is handy tables for it and 
picking up most standard op-amp books, AOE or similar would give the 
details for it. Sallen-Key setups is most likely.

Cheers,
Magnus