# 3rd order low pass Butterworth active filter

#### tinhnho

Hi everyone

Does anyone know how to design a 3rd order low pass Butterworth active filter with a cutoff frequency of 8khz ? thanks

p/s: Attachment below is what i have

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#### chroot

Staff Emeritus
Gold Member
When you say "3rd order" do you mean 3-pole? I have never seen a 3-pole Butterworth filter, and doubt that such a thing exists.

- Warren

#### tinhnho

3rd other butterworth polynomial:
(s+1)(s^2+s+1)

4th order:
(s^2+.765s+1)(s^2+1.848s+1)

etc...

S

#### SGT

Your first stage is a second order filter and the second stage is a first order filter. In each stage you should choose R and C such as 1/RC = 2&pi;x8x103.
Only, in order to have a Butterworth filter you should not have a unit gain amplifier in the first stage. According to your table, the gain of that stage should be 3 - 1 = 2.

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