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difference amplifier input

Discussion in 'Electronic Basics' started by kell, Dec 17, 2006.

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  1. kell

    kell Guest

    I was reading up on op amp difference amplifiers
    http://www.analog.com/UploadedFiles/Application_Notes/9710169AN692_a.pdf
    one thing I haven't seen addressed explicitly is the impedance of your
    voltage signals.
    Suppose your input resistors are 100 and feedback resistors 1 Meg, for
    a gain of 10,000.
    Now let's say you are using a 100k pot to set one of the voltages going
    into the difference amplifier. Would this high impedance source affect
    the difference amplifier, like adding resistance to the input resistor,
    or does it matter?
     
  2. Jamie

    Jamie Guest

    IF Op-amps were ideal, it would not effect it with the acception of the
    feed back resistor.
    if you look up the spec's of the op-amp, you'll find the gain of the
    op-amp and impedance. These figures can be used to get the exact gain
    over all.
    The gain is so high on most op-amps that these factors are almost
    non negotiable.
     
  3. Phil Allison

    Phil Allison Guest

    "kell"

    ** The voltage you work with is the one that really exists at a particular
    point in a circuit.

    Where a voltage or signal has a significant series resistance that must
    always be taken into account.


    ** A difference amplifier responds to the voltage difference between the two
    inputs.

    In the example you gave, the input imposes a 200 ohm load on that
    difference. If this seriously alters the signals you want to difference,
    then that particular circuit is no use.

    When high impedance sources are involved, the solution is to add two more
    op-amps, wired as simple voltage followers, to function as input buffers
    for the difference amp's inputs.

    Use FET op-amps for this and the effective input impedance can be G ohms.




    ........ Phil
     
  4. Chris

    Chris Guest

    Mr. Allison's advice is great. The setup he's talking about, with the
    voltage followers preceeding the difference amplifier, is generically
    called an instrumentation amplifier. It's good to use an IA when you
    have to worry about higher or uneven input impedances. Here's a
    reference:

    http://www.national.com/ms/LB/LB-1.pdf

    Also, I guess I should mention that your example is a little
    unrealistic. Gains of greater than 100 have to be done very carefully,
    and a gain factor of 1000 is pretty much the realistic limit for even
    the best instrumentation amp setups. You end up amplifying offsets and
    noise more than your signal. Also, your maximum input frequency goes
    way down.

    Good luck with your studies
    Chris
     
  5. Eeyore

    Eeyore Guest

    An op-amp with feedback isn't a classic difference ( differential ) amplifier.

    Graham
     
  6. kell

    kell Guest

    I have to set up the difference amp so that the output swings to the
    limit when the input differential reaches just a few millivolts. Can I
    do this, given the caveat you express about excessive gain? Frequency
    isn't a problem, it's very slow.
     
  7. Chris

    Chris Guest

    OK, Kell. You might want to just try it yourself, and see what
    happens. Nothing will blow up, and the Electronics Gods will still
    smile on the world.

    I basically found out most of what I know about pulling millivolt-level
    signals out of the grass when I tried to do it for real. It's a good
    education.

    Cheers
    Chris
     
  8. kell

    kell Guest

    Maybe I was thinking too hard. A comparator ought to do. I only need
    to keep the behavior of a physical system within certain bounds, not
    get an accurate measurement of it. Instrumentation amplifiers can wait
    for another day.
     
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