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Need an op-amp expert !

Discussion in 'Electronic Design' started by Jim, May 16, 2008.

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  1. Jim

    Jim Guest

    All,

    I think I've messed up an op-amp (actually a comparator) design at
    work.

    I'm using an LMV321 (dual low voltage op-amp) operating at 4V.

    Using one half as an amp, it seems to amplify a 50KHz square wave to
    about 1V peak to peak quite happily.

    Then comes the trouble !

    The 1V square wave is connected to a clamp (DC restorer) with the
    output square wave bottom side approx 1.6VDC (so top side should be
    2.6V). That connects directly to the - input of the other half of the
    LMV321 used as a comparator.

    The reference voltage (2.1V) connects to the + input of the comparator
    via 22K. There is a 330K feedback resistor from output to + input for
    Schmitt trigger action.

    The output of the comparator seems to change the mark/space duty cycle
    (weird). The problem gets much worse at cold. Adjusting the voltages
    slightly on the comparator inputs seems to vary the output duty cycle
    (weird).

    My best theory is that an LMV321 is not fast enough as a comparator
    (even though it amplifies the 50KHz to 1V peak to peak quite happily).

    I have cunning plan. An LMH6658 is an RF op-amp that is pin compatible
    and shouuld drop in for the LMV321.

    Has anybody any experience with using the LMH6658 as a comparator ?
    Will it make an OK fast(ish) comparator ? Are there any circuit tweaks
    to speed up the LMV321 ? Do op-amps get slower at cold (-30C) ?

    Please help !

    Thanks,

    Jim
     
  2. Guest

    Sorry, should have said the problematic op-amp was LMV358.
    [LMV321 is single version, LMV358 is dual.]
     
  3. Jamie

    Jamie Guest

    --
    like most other op-amps, most likely the capacitance in the package
    to prevent oscillation is causing a phase shift, much more so than
    what you may see in a real comparator.

    You may want to look at "Miller effects" It might shed some light
    on it.

    http://en.wikipedia.org/wiki/Miller_effect

    I don't know if you want to believe everything you read from
    the wiki but it's a start.

    Have you tried a fast comparator ?


    http://webpages.charter.net/jamie_5"
     
  4. Jim

    Jim Guest

    Thanks guys.
    Deep down this is what I figured.

    Jim
     
  5. Eeyore

    Eeyore Guest

    Exactly. You really need a proper comparator.

    Graham
     
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