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differential amp

Discussion in 'Electronic Design' started by Jamie Morken, Jun 29, 2004.

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  1. Jamie Morken

    Jamie Morken Guest


    Is it possible to use a differential amplifer, single opamp single supply
    like this one:
    to ampifly the difference between +12.000V and +11.999V, when the opamp
    supply is equal or less than +12.000V?

    This works in the simulator I am using (electronics workbench multisim) with
    an LM358 opamp even when I drop the opamp supply to 5V! Is this typical or
    is it something that works in the simulator and not on the circuitboard?

  2. Tim Wescott

    Tim Wescott Guest

    That depends, but probably.

    If the circuit is working the opamp will hold it's negative input equal
    to the positive input. So if the positive input (the junction of R3 and
    R4 here) is within the common-mode input range of the amplifier, and if
    the amplifier output isn't saturated, then yes.

    You can build circuits like this that will allow the common-mode voltage
    ("VCM" in the drawing) to be hundreds of volts, even with a low-voltage
    opamp, you just have horrid resistor matching and amplifier offset problems.

    The opamp data sheet will tell you the input common mode range, and the
    output drive range. The rest is up to you.
  3. Ian Buckner

    Ian Buckner Guest

    Good luck getting hold of significantly better than 0.01% R's.
    A giveaway might be "Send your schematics and electrical
    requirements to the Applications Engineering Department"
    on the datasheet ;-)

    Assuming all R's are equal, there is a 2:1 attenuation of the
    input signals. That would put the CM voltage at 6V at the
    amplifier inputs, which would be fine for a LM358 on 12V.
    At a 5V rail, afraid not.

  4. Linearman

    Linearman Guest

    Take a look at the AD629 from Analog Devices. It is an integrated cicuit
    that does just that.
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