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LMC6001 output spikes

Louis S
Intellectual 325 points
Other Parts Discussed in Thread: LMC6001, TINA-TII'm hoping to use an LMC6001 in an electrostatics measuring application, and have been attempting to simulate the design in LTSpice to tune the filtering characteristics.  The LMC6001 is used as a buffer before a filtering stage, and is single-supplied with 12V.  The AC signal is around 100 Hz, A/C coupled, and biased to 6V.  The unity gain functionality seems ok, except for occasional spikes up or down of about 1V.
  • 0
    TI__Genius 12325 points

    Louis,

    Would it be possible for you to post a schematic of your circuit to help us debug this problem?

  • 0 in reply to John Caldwell
    Intellectual 325 points

     

    I've tried to reduce it down to the simplest similar circuit that has the same symptoms.

    The green output trace follows the input except for the occasional spikes.

  • 0 in reply to Louis S
    TI__Genius 12325 points

    Louis,

    I duplicated your circuit in Tina-TI and a colleague built it in TopSPICE (shown below) and neither of us were able to duplicate the spikes shown in your above post. This leads me to believe it is an artifact of the simulation program itself and not the circuit. I'm not very familiar with LTSpice but it may be helpful to change the timestep used during simulation to see if there are any effects on the spike.

     

  • 0 in reply to John Caldwell
    Intellectual 325 points

    I really appreciate you guys investigating this for me.  At this point I'm almost certain it's a simulation artifact since I've found the spikes at the output net go away when I disconnect the input to another LMC6001 elsewhere on the schematic sheet (but unconnected to the spike test circuit).

    Thanks,
    Louis 

  • 0 in reply to Louis S
    Intellectual 325 points

    One last bit of information just in case someone runs into something like this:

    In the full circuit, I had the LMC6001 buffer feeding into an odd (novel?) passive twin-t filter followed by a an active lowpass filter with gain.  The net result was virtually no DC load on the output of the LMC6001 since there was no DC path to ground.  I put a 10K resistor to ground at the output of the LMC6001 and that cleans up the simulation.  It doesn't explain why that more complex circuit was fouling up the simplified test circuit which was unconnected.