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Question about differential amplifier(isolated input signal)

  1. Dec 19, 2012 #1

    when using op amp as differential amplifier,
    can it used for input signal isolated from power supply of that op amp?
    as this figure:


    so if the input signal is sinewave 5 Vp.p,
    will be the output of this op amp sinewave 5 Vp.p also ??
  2. jcsd
  3. Dec 19, 2012 #2
    Yes.....with condition.

    If you look at the data sheet, given the + and - supply, the input range is usually about 1V below the either supply voltage. Look at the data sheet under "input range". There are so called rail to rail input op amp that can accept input up to the supply voltages. I would put some protection diode at the input of the amp to protect from over voltage.
    Last edited: Dec 19, 2012
  4. Dec 19, 2012 #3
    yes that right,i know that

    i not ask about the range of input signal,
    i ask if op amp(as differential amplifier) can or can't ,work with input signal isolated from power supply(the signal come from other circuit) ??
  5. Dec 19, 2012 #4
    Yes, that's what I said the first time. As long as you conform to the input range.
  6. Dec 19, 2012 #5
    Yes samaaa, there is no need to shout at someone trying to help.

    We can see that you have put significant effort into your work.

    And yes the differential configuration will work with a non ground referenced signal ( ie floating) as you have drawn.

    However your circuit does not isolate the signal from the op amp circuitry and therefore there is the opportunity for an electrical short to the power supply or other parts of the circuit.

    To properly isolate the signal you would need to interpose an optoisolator or other device between the signal source and the differential amp input.

    Please note this is unlikely to be necessary at the voltages you have here, I am just being pedantic for you to see correct use of terminology.

    Please also note that your implementation of a differential amp using a single op amp is a poor one. A better circuit would use three (from the same package)
    Last edited: Dec 19, 2012
  7. Dec 20, 2012 #6
    thank you (Studiot & yungman) for help
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