PWM-DC Low Pass Filter

Discussion in 'The Projects Forum' started by ampotvin, Nov 20, 2012.

  1. ampotvin

    Thread Starter New Member

    Nov 20, 2012
    Hello Everyone, first time user here.
    I have some questions about a Butterworth sallen-key topology low pass filter for PWM TO Analog conversion. The PWM signal is the input to the low pass filter, and has a changing frequency around 45Hz (from roughly 42-47Hz)(this is for a DC motor control feedback circuit). I got this filter from a friend and I calculated the Q factor and cut-off frequency. (0-5V is PWM signal)

    The cutoff frequency fc =2.8Hz and the Quality factor Q=4.2

    I understand why such a low cut off frequency but my question is why does the Q factor have to be so large? From my understanding a Butterworth works best with a Q=1/2. Does a higher Q factor allow for a quicker response to changes in frequency?

    When graphed, the high Q value shows a large amount of oscillation, and when i tried ti simulate a lower Q value, there is less oscillation. Can anyone suggest why a high Q value is good in this situation.
  2. crutschow


    Mar 14, 2008
    Don't know why you would want such a high Q. You might ask your friend.

    Post the circuit diagram.