Trouble understanding current sense active filter

Discussion in 'General Electronics Chat' started by Obanion, Nov 26, 2011.

  1. Obanion

    Thread Starter New Member

    Nov 26, 2009
    Hi guys,

    I'm in the process of designing a current sensing amplifier/anti-aliasing filter for some digital power control. I found an interesting circuit that's in use for sensing the switch current in a PFC boost converter but I don't understand a certain aspect of it. What is the purpose of the two 220pF capacitors on the differential input? I can't seem to wrap my head around the reason for their placement.

    The active filter in question is:


    The frequency response of this low-pass filter is:


  2. crutschow


    Mar 14, 2008
    The capacitors appear to be to compensate the amp for stray circuit capacitances to improve the frequency response.

    You simulation has no stray capacitances and you are using an ideal op amp so your simulated response may not be what you will see in an actual circuit with the OPA354.
  3. Yako

    New Member

    Nov 24, 2011
    Do you mean to do with attenuation. Like as in XC (capacitive reactance)

    XC = 1 / 2∏ FC + the resistor in parrallel?