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Comparator and hysteresis

Discussion in 'Electronic Basics' started by MRW, Jul 9, 2007.

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  1. MRW

    MRW Guest

    Hello! :)

    I am reading about comparators. I happened to be using this one:
    http://www.national.com/pf/LM/LMC6762.html

    According to the application hints in the datasheet, "generally, it is
    a good goal to reduce the delta voltage on the power supply to a value
    equal to or less than the hysteresis of the comparator. For example,
    if the comparator has 50 mV of hysteresis, it would be reasonable to
    increase the value of
    the local bypass capacitor to 0.01 μF to reduce the voltage delta to
    10 mV."

    I understand this portion clearly. But what I don't understand is: how
    do you determine the comparator hysteresis? Does this refer to the
    built-in hysteresis in the comparator? Or is this related to any
    hysteresis due to external networks?

    By the way, I could not find any values associated with the comparator
    in the operating tables.

    The application hints also has a section on hysteresis, and they show
    an example by putting a resistor and capacitor in the feedback network
    of the non-inverting input. However, they do not indicate what kind of
    hysteresis value you can get with this setup.

    Thanks!
     
  2. Andrew Holme

    Andrew Holme Guest

    Hello! :)
    There is no built-in hysteresis. Hysteresis is provided by external
    positive feedback.

    The output state changes when the input level (-ve terminal) crosses the
    threshold (+ve terminal).

    The threshold is set by a resistive divider network, connected to the
    output.

    There are two different thresholds: one for output high, and one for output
    low.

    Hysteresis is the difference between the two thresholds.
     
  3. MRW

    MRW Guest

    Thanks, Andrew! My next question would be. How do I start designing a
    external hysteresis circuit and how would I analyze it?

    I'm skimming thru the Arts of Electronics, and it says comparators
    always use positive feedback and that the voltage levels at the non-
    inverting and inverting terminals are not the same (so opamp golden
    rule 1 does not apply page 230).
     
  4. MRW

    MRW Guest

    By the way, you are referring to the hysteresis of the LMC6762 right
    and not comparators in general?

    I found a Maxim application note about adding extra hysteresis to a
    comparator. In it, they say, "Hysteresis is designed into most
    comparators, usually with a value of 5mV to 10mV. Internal hysteresis
    helps the comparator avoid oscillation due to small amounts of
    parasitic feedback. Though sufficient to prevent the comparator from
    self-oscillating, such internal hysteresis can easily be swamped by
    any external noise of greater amplitude. In that case, you can improve
    performance by adding hysteresis externally."
     
  5. Eeyore

    Eeyore Guest

    Actually a 'pure' comparator uses no feedback at all.

    You get the hysteresis by addding some positive feedback.

    Graham
     
  6. Andrew Holme

    Andrew Holme Guest

    1. Connect the top of a potential divider to the comparator output
    2. Connect the middle of the potential divider to the non-inverting input

    The comparator output is either high or low. You can get the high and low
    output voltages from the datasheet. Once you have these, you can calculate
    the voltages at the non-inverting input.
     
  7. Andrew Holme

    Andrew Holme Guest

    Yes, some comparators have internal hysteresis.
     
  8. JeffM

    JeffM Guest

    You're so close, it's painful to watch.
    Just a couple of more pages...
    http://books.google.com/?q=noting-that-the-hysteresis-equals

    A sawtooth on the input is a good test.
     
  9. neon

    neon

    1,325
    0
    Oct 21, 2006
    hysterisis is the amount of feedback from the output to the input it can be calculated as voltage F/B lets say out is 10v the input has a divider of 2 10k resistor a 100k F/B resitor will give you the ratio of F/B
     
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