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>30V output AND 0V offset from a single op-amp?

Discussion in 'Electronic Components' started by Lostgallifreyan, Apr 26, 2006.

  1. CA3140 data sheets show that the output can approach the positive rail and
    (allegedly) reach the negative rail. This might be false. On a single rail
    supply the offset cannot be trimmed to get a true zero output, you MUST
    have some negative.

    I want to measure up to 30 watts at 1W/V from a thermopile sensor, on a
    multimeter, so my idea is a split rail supply whose total is 34 volts, with
    get through-zero adjustment for offset error.

    Will this work without frying the IC? Can anyone suggest common, low price,
    good performing IC's for this? It doesn't have to be a single rail op-amp,
    it just has to accept a VERY asymmetrical split rail. I'm hoping the LF411
    or LF412 (with external offset trim) can do this.

    I can make the dual supply stage, that's not the tough bit, but I'd like
    some advice about the extreme asymmetry I want for the gain stage. (Both
    will hopefully be workable from one dual op-amp IC).
     
  2. Franc Zabkar

    Franc Zabkar Guest

    Why not just use a resistive potential divider to reduce the input to
    a manageable level? Does the source need to see a high impedance, ie
    will it be unduly loaded by a potential divider?

    - Franc Zabkar
     
  3. Sorry, I nearly missed this one. :)

    It's not the input that's the problem though, that's lower than +3V. What I
    was after was up to +30V output for measuring on a single range of a
    voltmeter. I was concerned that dual rail op-amps might only be designed
    for symmetrical supply rails, and might fail if ground was biased almost to
    negative while output was driven close to positive rail.

    I reposted this in simpler form when this post appeared to get lost, with
    subject "How asymmetrical is an op-amp supply allowed to be?"

    I've been told there by Tim Williams that any op-amp will allow a strongly
    biased ground, just 2 volts or so above negative, and still allow an output
    to get close to the positive rail even with max supply voltage; he says
    that both transistors in the output stage (and presumably the others) must
    be able to handle the full supply voltage.

    If this is all I need to consider when using an LF412 with the small
    negative voltage to help zero the offset to my ground (made by the first
    stage in the op-amp and two resistors as potential divider), then the
    question is answered. :)
     
  4. Franc Zabkar

    Franc Zabkar Guest

    Sorry, brain fart. Sadly they're getting more frequent as I get older.

    - Franc Zabkar
     
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