RC Time Constant in voltage divider

Discussion in 'General Electronics Chat' started by philm01, Jun 22, 2015.

  1. philm01

    Thread Starter Member

    Mar 22, 2014
    Hello everyone,

    I seem to be having a brain-fart today. So, I have a resistor divider circuit, R1, R2, where R1 is the top resistor, connected to signal in and R2 is the bottom resistor, connected to ground . The output will go to a ADC. The frequency of the waveform going into the ADC (signal out) needs to be 1/5th that of the sampling frequency (specified in the datasheet). I am creating a low pass filter on signal out by connecting a capacitor (C1) to ground.

    My question is this, for the RC time constant, will it be R1 * C1 or R2*C2?
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    Neither. It will be C1 times the equivalent parallel value of R1 and R2: R1*R2/(R1+R2).
    philm01 likes this.
  3. crutschow


    Mar 14, 2008
    To elaborate on OBW0549's comment, note that each resistor goes to a low impedance, one to ground and one to the signal source. Thus looking back at the junction of the two resistors, the impedance would appear as the two resistors in parallel.
    Last edited: Jun 22, 2015
    OBW0549 likes this.
  4. philm01

    Thread Starter Member

    Mar 22, 2014
    Oh, ok, thanks guys!