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are dual comparator really independant?

Discussion in 'Electronic Basics' started by Mark, Apr 10, 2004.

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  1. Mark

    Mark Guest

    Is a dual/quad compartor in the same DIP totally indepentant of each other
    or is there some interaction between the outputs?
     
  2. John Larkin

    John Larkin Guest

    There may be some small crosstalk, in that outputs that cross close in
    time may influence each other a bit.

    Sometimes DC biasing of one stage can affect another. LM339 types can
    interact two ways: if one channel has both inputs very high or open, a
    shared current source can partially collapse and degrade the others;
    and if you pull any input below ground, the entire chip can go nuts.

    Try it!

    John
     
  3. There are no two anything in the universe that are truly independent
    of each other. The two amplifiers in a dual comparator share power
    supply pins and usually some bias circuits, so there are several sneak
    paths between them, besides the field paths that connect them since
    they are so close ot each other. If you are really pushing the
    isolation envelope with your circuit, single units may be helpful.
     
  4. Mark wrote...
    NSC doesn't have a spec for their quad comparator IC, but they
    do for ICs designed at the same time with similar technology.
    The LM392 (dual-function opamp + comparator) has a spec called
    "Amplifier-to-Amplifier Coupling," (f = 1 kHz to 20 kHz), which
    is −100 dB typical (input referred). And their LM324 quad opamp
    data sheet spec is −120dB typ. Both specs have this same note,
    "Due to proximity of external components, insure that coupling
    is not originating via stray capacitance between these external
    parts. This typically can be detected as this type of capacitance
    increases at higher frequencies."

    Assuming a 12V output swing, a -100dB spec would imply a 120uV
    input feedthrough effect. In addition to external-component
    stray capacitance, one should watch out for power-supply-rail
    feedthrough. The LM324 opamps have a 100dB typical power-supply
    rejection ration spec. The opamp's input circuit is similar to
    the LM339 comparator and we can expect it has a similar weakness.
    So care should be taken that current switched by the comparator's
    output doesn't cause too much pin 12 supply-rail noise.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  5. Winfield Hill wrote...
    Sheesh, a 16-bit unicode - symbol? The specs were supposed to be
    -100dB and -120dB.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  6. John Larkin

    John Larkin Guest

    If you drive one LM324 section open-loop rail-to-rail, you'll see
    *big* spikes on the other opamp section outputs. Cheap, and worth it.

    John
     
  7. JeffM

    JeffM Guest

    Is a dual/quad compartor in the same DIP totally indepentant of each other
    Look at the Power Supply Rejection Ratio (PSRR) on your datasheet.
    There's your interaction.
     
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