Value of AC coupling capacitor?

Discussion in 'The Projects Forum' started by Shocker33, Jun 11, 2015.

  1. Shocker33

    Thread Starter New Member

    Oct 28, 2013
    I want to use an AC coupling capacitor with a resistor to drive MOSFET at a different voltage to the drive signal. The capacitor and resistor arrangement forms an RC high pass filter.

    The equation for an RC high pass filter is Fc = 1/(2*Pi*R*C).

    If i want an ideal square wave to pass through the capacitor/filter, what do i set the -3dB point to?

    By ideal, i mean an infinitely small rise and fall times, where each level is flat.

    I basically want a square wave at a couple of hundred hertz to pass through this as ideally as possible.

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Pulse fidelity after several seconds:

    Getting there:
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    There will always be some signal loss and distortion, regardless of the cap value. The higher the cap value, the closer the passed signal will be to the ideal square wave. As a very rough rule-of-thumb, the capacitor reactance at the fundamental frequency should be < R/10 for distortion < ~10%.
  4. #12


    Nov 30, 2010
    "Ideal" = "instant headache"
    The large end of the capacitor calculation is about how flat the flat portions are. MikeML covered this beautifully in his first simulation. The high frequency end calls to mind the limitations of large capacitors at megahertz frequencies. You might need to add a parallel capacitor of the ceramic type to get an "ideal" square wave to rise and fall at megahertz frequencies.

    The second sim by MikeMl demonstrates the start-up time to steady state. You might design to compensate for this initial drift...or not...depending on your needs.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    For a single-pole R-C hipass filter, frequency response is down 3 dB at the corner freq, down 1 dB one octave above the corner freq, etc. So figure out what you really mean by "ideal" in engineering terms (frequency response, waveform distortion, etc.) and what your lowest frequency of interest is, and use this to adjust the filter parameters. For example, if -1 dB amplitude flatness for a 250 Hz signal is acceptable, then set the corner frequency at 125 Hz.

  6. crutschow


    Mar 14, 2008
    As has been noted, the output won't be perfect for any real devices.
    You need to quantify "as ideal as possible".
    For example, how much "droop" can you tolerate on the flat portions of the square-wave?