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Comparator oscillation

  1. Nov 27, 2009 #1
    I have a LT1394 comparator in a circuit that I'm trying to troubleshoot.

    The problem is that once the variable voltage approaches the reference, just before a switch, the output signal oscillates between LO and HI.

    I haven't been able to dive too deeply into it, to figure out if its noise or AC coupling on the variable IN, i'm also not a pro at this in any regard. Currently by raising the cap on the output by a factor of 10 I'm able to lower the oscillation but this is a band aid solution and doesnt fix the fundamental problem.

    I know that the LT1394 has no built in hysteresis and assume that the resistor on the positive feedback path is whats going to control that for me. Should I lower it? Anything else I can do?
     
  2. jcsd
  3. Nov 27, 2009 #2

    vk6kro

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    Science Advisor

    I had a look at the data sheet for this device. The maker claims it is stable even with slowly changing inputs.

    So, it might be input noise.

    If your application is just using variable DC voltages, you could try common mode bypassing. You put a capacitor between the inverting and non inverting inputs. Maybe 47 pF if it won't matter for the application.

    This is a very fast chip, so it could be reacting to pulses on the input that would be hard to see with an oscilloscope.
    You may be able to get a clue from the frequency of the output.
    If the output is 60 Hz it may mean you need better shielding of the input wires. 120 Hz may mean there is a problem with hum from the power supply.
     
    Last edited: Nov 27, 2009
  4. Nov 27, 2009 #3
    Why don't you try about 10 mV of hysteresis by lowering the resistor from the output back to the positive input.
    Bob S
     
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