Filter Design low pass passive filter.

Discussion in 'General Electronics Chat' started by saint_jay77, Aug 3, 2007.

  1. saint_jay77

    Thread Starter Member

    Oct 19, 2006
    Hi All,
    I have a question regarding filter design. I have a momentary push button switch (E stop) which needs a filter designed for it; to prevent noise from getting to the FPGA which this signal line is connected to and causing false triggers. Transients or ESD test pulses could cause it to false trigger so the filter is needed. The only signal i would like to see that the FPGA gets is a clean DC signal. I need a good common mode inductive filter (choke) to choke the high frequency transients and further a capacitive filter to short high frequency to ground. Furthermore this signals should pass through another LC filter to ensure any other noise that is not filtered by the common mode filter to be attenuated.
    The only frequency that should pass through this is the signal of a human pressing the Momentary push button switch, assuming it would be a square pulse.
    The unknowns that i need help with.
    1) The typical pulse width of the signal when the button is pressed. I am speculating it wont be in millisecond range but rather in microsecond range. Does anyone have any ideas?
    2) Once this time period is known i need a formula for the fourier transform for the square wave calculation for the pulse width from part 1 of the question. I think this would give me a cut off frequency that i should design the filter for. -3bd margin as it would be indicated on the BODE plot. Any frequnecy above this cut off frequency will be attenuated so it wont reach the control circuit. A schmitt trigger will ensure proper switching. The feature of the (2 thresholds) schimitt trigger inverter that decides if it is a ON or OFF.
    3) The calculation of LC values are required also.
    IF someone could help me with this analog filter i would very much appreciate it.
  2. nanovate

    Distinguished Member

    May 7, 2007
    You need a switch debounce circuit which can be implemented in both harware and software. A simple ckt would be an RC filter which would also reduce stray transients. You can also throw a TVS from the input line to ground near the pin.

    If a human pushes the button the pulse width will be in the hundreds of milliseconds range. It will also NOT be a perfect square pulse since the switch contacts will bounce before settiling down.