Comparator input impedance matching..

Discussion in 'General Electronics Chat' started by ChateauduChillon, Nov 7, 2014.

  1. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
    Hi All,

    Using a comparator at the output of a ~30kHz sine wave oscillator to detect a level crossing.

    My question is simple: will I gain or lose anything by including resistor R2?

    I'm thinking it would null out effects from variation in input bias current with temperature, but am I missing something more crucial? I have tried in on my breadboard circuit, not much happened, unsure whether to include it on PCB design or not.

  2. MrChips


    Oct 2, 2009
    I've seen LM311 burnout when input series resistors are missing. I would keep it in.
  3. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
    Thanks, appreciate the advice. On the other hand is my stated motivation just crazy talk? Or could it help?
  4. #12


    Nov 30, 2010
    I can see the burn-out function on page 3. If you're out of common mode range, Q1 and/or Q2 can commit suicide.

    Page 1 tells me 150 na input current. That's 1.5 mv of input offset voltage error with a 10k resistor, and it drifts with temperature if the input impedances are not matched. Page 5 says 7.5 mv to 10 mv inherent input offset voltage. These numbers are usually not a difficulty with a comparator because they are so much less than the voltage level you are comparing. Still, you should get, "in the know" about how input impedance interacts with the input bias current of differential amplifiers because they are used in both op-amps and comparators.
  5. ronv

    AAC Fanatic!

    Nov 12, 2008