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can a comparator pull down to a negative rail

Discussion in 'Electronic Design' started by panfilero, May 6, 2013.

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  1. panfilero

    panfilero Guest

    I would like to use a comparator so that it's output is either open drain (or collector) or it's pulled down to its negative rail (Let's say -5V for my application) I've looked at a couple of comparators, and they have +Vin, -Vin, and GND pins... but from looking at the datasheets it doesn't look like they can output their negative rail... am I right about this? if i am, then what's the point of the -Vin input?

    I was looking at the LT1011
    http://cds.linear.com/docs/en/datasheet/1011afe.pdf

    and the LM319
    http://www.ti.com/lit/ds/snosbj2a/snosbj2a.pdf

    much thanks!
     
  2. panfilero

    panfilero Guest

    ahh ok, so I could set up my rails at +/-10V and tie my ground to -5V and should be good to go then... great.

    - thanks
     
  3. Tim Williams

    Tim Williams Guest

    Comparators like the LM319 (LM311, ...) would be better called
    "uncommitted transistor output". Like the TL494 outputs, if you're
    familiar with it. You can use it for pull-up (OUT to +V, output from
    "GND") or pull-down (normal), or whatever your imagination can come up
    with.

    Other ones, made for specific logic levels (LM360, TTL level, comes to
    mind), may be more particular about voltages. In the latter case, for
    example, obviously you need the analog supplies to be more than the analog
    input voltages (so you could do +/-5V for a [-3, 3V] range, say), and the
    output has to be referenced to whatever it connects to (i.e., +5V and GND,
    or insert-logic-level-here). Consider: TTL, CMOS, LVDS, ECL...

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://seventransistorlabs.com

    I would like to use a comparator so that it's output is either open drain
    (or collector) or it's pulled down to its negative rail (Let's say -5V for
    my application) I've looked at a couple of comparators, and they have
    +Vin, -Vin, and GND pins... but from looking at the datasheets it doesn't
    look like they can output their negative rail... am I right about this? if
    i am, then what's the point of the -Vin input?

    I was looking at the LT1011
    http://cds.linear.com/docs/en/datasheet/1011afe.pdf

    and the LM319
    http://www.ti.com/lit/ds/snosbj2a/snosbj2a.pdf

    much thanks!
     
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