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

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

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  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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