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3rd order low pass Butterworth active filter

  1. Mar 29, 2005 #1
    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

    Attached Files:

  2. jcsd
  3. Mar 29, 2005 #2


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    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
  4. Mar 29, 2005 #3
    3rd other butterworth polynomial:

    4th order:

  5. Mar 31, 2005 #4


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    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 = 2πx8x103.
    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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