RFI filter

Discussion in 'General Electronics Chat' started by andrew24, Feb 28, 2010.

  1. andrew24

    Thread Starter Active Member

    Aug 20, 2008
    Hello, in the attached image there's a RFI filter to prevent high frequency radio signals to reach InAmp inputs.
    All is clear to me except the purpose capacitor C2, because R1 and C1 alone provide high frequency rejection while letting lower frequency signals through.

    It's mentioned in the book where i found this circuit that it reduces amplifier's common mode rejection errors due to components mismatching(R1a C1a and R1b C1b) , but i dont understand how..could you explain?
  2. darenw5

    Active Member

    Feb 2, 2008
    The amplifier is sensitive to the difference between its inputs. The usual R-C low pass filters do an okay job of reducing the effects of frequencies higher than we care about, but to really snuff them out, no better way than to put a dead short between those two inputs. For frequencies above those set by the time constant of C2 and the resistors, there's no place to go but through C2.