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Op Amp Comparator hysteresis

Discussion in 'Electronic Basics' started by panfilero, Aug 31, 2012.

  1. panfilero

    panfilero Guest

    I see circuits that add hysteresis to op-amp comparators by putting a positive feedback resistor network that acts as a voltage divider to the output.

    Am I right in thinking that this will only work if you have a negative rail? If your negative rail is grounded this aint gonna give you hysteresis... is there a way this would work with a single supply opamp?

    thanks!
     
  2. panfilero

    panfilero Guest

    I think I have an idea.... I would have to figure out my thresholds and float my opamp so that the positive feedback voltage divider gives me the thresholds I need...

    So, if I want my high threshold at 2V and low at 1V then (assuming my divider splits my voltage in half) I'd need my high opamp rail at 4V and my low rail at 2V... so I'd have to make sure my opamp works off of 2V and my input signal never goes beyond my rails...

    ???????
     
  3. Jamie

    Jamie Guest

    Yes, it works the same way. You need to put a virtual common node at the
    (+) input when operating single rail. This feed back R is calculated
    working with this virtual network so that you can select the hysteresis
    window.

    Jamie
     
  4. panfilero

    panfilero Guest

    I went with an OpAmp out of convenience, and because what I'm looking at isa really slow signal which, basically DC, it would rise and fall real slowly... I'm not sure if using a comparator buys anything when your not concerned about speed... maybe it does, I don't know, I'm all ears though, and I'm gonna dig into it some more...
     
  5. Everyone should have a few LM393's in their parts box. I sometimes
    find a comparator to be too fast, but you can hang a capacitor on the
    output to slow it down.... nice linear ramps from one rail to the
    other.

    George H.

    (for a fast 'twichy' comparator I like the LT1016)
     
  6. Guest

    Comparitors have outputs that are designed more like gates than linear
    devices. Opamps may not be well behaved when their outputs are driven hard
    into the rails. They're also not designed to have a differential voltage on
    the input. Using opamps as comparators can be done but it's not recommended.
    If you drive a capacitor, watch the output current.
    Your boss must be filthy rich. ..or perhaps dirt poor, after buying LT
    stuff. ;-)
     
  7. Nah, not much quantity so saving a few bucks is not worth my time.
    (the $3 comparator with a $3 opamp replaced a ~$100 (?) Amp-tek part.
    so my boss is happy.)


    George H.
     
  8. Guest

    If I specified a $3 comparator, it would be my last. In fact, anything LT
    will draw lightning in a design review (even a no-pop, BTDT).
     
  9. I know it's like I'm 'living in the past' selling circuits with two
    connectors, a rotary switch (or two) a bunch of 0.1% resistors, and an
    opamp (or two)... sprinkle in some C's.

    The opamp is often the least expensive part of the equation,
    (we match the C's when it's important).

    George H.
    (Hey, there must be someone besides me buying these IC's!)
     
  10. Guest

    Hardly. A couple of high-end DSPs, a uC, and a couple of thousand other parts
    (BOM cost in the hundred$ - anticipated AOQ perhaps half million). Your
    contempt and cluelessness is noted, though.
    When you're making a million of something, a $3 opamp doesn't make much sense
    when a $.10 opamp will do, now does it? Yes, I even get beat up for 1%
    resistors. Money matters.
    Little guys with rich (and clueless) bosses, evidently. ;-)
     
  11. Huh? I was describing what I do. No contempt was intended.
    Sure that's fine. I'm making tens of something. Next to the $3 opamp
    is a $10 switch, surrounded by $2 worth of R's. Different world.
    You seem angry for some reason?

    George (just trying to get along) H.



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