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op amp input ranges

Discussion in 'Electronic Basics' started by kell, Feb 1, 2006.

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  1. kell

    kell Guest

    I'd like to know how close to the positive rail you can go with the
    inputs.
    I was looking at the LT1013 in particular, but op amp datasheets in
    general don't seem to adress the question explicitly.
    Using an op amp as a comparator in a circuit with a single supply, I
    need to use the inputs to within a tenth of a volt of the positive
    rail.
    I had a 741 in the junk box, so I checked and it worked as a comparator
    with the inputs 0.1 volt from the positive rail. Is this true of the
    LT1013 or other op amps, and how can l tell before ordering?
     
  2. Lord Garth

    Lord Garth Guest

    Why use an op amp that requires output offset adjustment? If you want
    an old op amp, use the LM1458. It too is a jelly bean (cheap) component.
     
  3. Phil Allison

    Phil Allison Guest

    "kell"

    ** See page 3, http://www.farnell.com/datasheets/41645.pdf

    The "input voltage range" is given as (up to) + 3.8 volts with a +5 volt
    supply.





    ......... Phil
     
  4. Usually the datasheet gives an "input voltage range" specified in terms of
    Vcc and Vee; sometimes both a safe range (guaranteed not to blow up the
    chip) and a specified range (guaranteed to work as specified).

    For instance, looking at the TI LT1013 datasheet (it just happened to be the
    first one on Google), I see absolute max input range of Vee-5V to Vcc. The
    specified common-mode input range may be less than the safe range; for
    instance, on the same datasheet I see common-mode input voltage range
    specified from -15V to +13.5V, with +/-15V supplies, at 25C. So that's from
    Vee, to 1.5V below Vcc. What they're saying is that if you get closer than
    two diode drops to the positive rail, you won't hurt anything but they won't
    guarantee proper operation - for instance, it might work okay with less than
    a certain amount of source impedance.

    Some opamps can get close to both rails, some to the positive rail, some to
    the negative. It depends on the circuit topology, and there are tradeoffs.
    So-called "single-supply" opamps, IME, generally let you get close to the
    negative rail but not to the positive rail.
     
  5. Pooh Bear

    Pooh Bear Guest

    You need to check the *input common mode voltage*on the data sheet.

    Graham
     
  6. Pooh Bear

    Pooh Bear Guest

    A 741 does not *require* input offset adjustment. Leave those pins open if
    you don't need to use them.

    Graham
     
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