Resistor between op amps and Comparators

Discussion in 'General Electronics Chat' started by Andrew1234, Apr 20, 2015.

  1. Andrew1234

    Thread Starter New Member

    Feb 16, 2015
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    Hi Forum
    My application uses an op amp and a comparator. I want to take the output of the op amp, and connect it to a comparator where one input of the comparator is a reference voltage and one input is from the op amp. Since the op amp should be low impedance at the output, and the input of the comparator should be high impedance, I think there should be no issue with signal loss.
    My question: Does it make sense to include any resistor value between the op amp and the comparator? If yes, what purpose does the resistor serve, and what value do you recommend?

    Thanks
    Andy
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I wouldn't bother but there are other opinions. I believe there was a thread here not long ago that gave reasons for adding the resistor to the input of the comparator.
     
  3. #12

    Expert

    Nov 30, 2010
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    I can remember some variations caused by the input impedances not matching. Input bias current changes with time and temperature, going through the input resistor and causing a difference in switching point, but that's usually millivolts in the worst case. Most of the applications we see here have far worse sources of error than input bias current.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Thanks #12, that does jog with my recollection. The issue was input bias current.
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    I would put 1kΩ to 10kΩ on the input of the comparator to limit the input current. I have seen LM311 comparators get blown with no input resistors.
    The only negative effect would be slightly slower rise/fall time on the input.
     
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  6. #12

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    Nov 30, 2010
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    I accede to reality.
    "One experimental result is worth a dozen theories."
     
  7. crutschow

    Expert

    Mar 14, 2008
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    I would think that would occur only if the input voltage is above or below the LM311 supply rails (as when its power is off but a signal is applied).
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Agreed. I was thinking if the opamp and comparator have the same power supply then you don't need the resistor.
     
  9. Andrew1234

    Thread Starter New Member

    Feb 16, 2015
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    My input signal is at 12MHz, so maybe I should worry about the rise and fall times for my application. I think what I'll do is put some 0 ohm resistors there, and if there are problems I can always put more resistance.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    What comparator are you using?
    Is the op amp carrying 12MHz? If so what part is it?
     
  11. Andrew1234

    Thread Starter New Member

    Feb 16, 2015
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    the op amp is ADA4940 and it carries a 12MHz signal, which then goes into the AD8611 comparator. do you think these parts are suitable? They seemed fast enough to me..
     
    Last edited: Apr 20, 2015
  12. crutschow

    Expert

    Mar 14, 2008
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    The ADA4940 is designed for differential output.
    You might be better is using a conventional high-speed op amp with a single-ended output.
    The comparator looks sufficiently fast.
    Note that those frequencies require careful layout and decoupling, preferably on a board with a ground plane, to get stable operation and good signal fidelity.
     
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