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Eval op amp achieves +/- power from single power supply?

  1. Mar 14, 2017 #1
    Picture for reference is below.

    I'm confused as to how this is able to receive a differential voltage of +/- from a single power supply. (Yes, I know the picture says dual power supply.)

    Normally I have to short the negative to ground on one side of a dual power supply, and short the positive on the other side of a power supply in order to get a +/- Vpower to an op amp.

    I actually wired up this evaluation module with just using one side of a power supply and it worked. There were no rails on either side.

    Looking at the circuit board: http://www.ti.com/lit/ug/slou148/slou148.pdf

    I'm not entirely sure how it is able to do this, or how the circuit would look.


    Circuit Question.png
     
  2. jcsd
  3. Mar 14, 2017 #2

    Averagesupernova

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    The little photo in your attachment that represents a power supply of some type does not really mean anything. It SAYS "Dual Power Supply". But how would it be drawn on a schematic if it were a battery or several batteries? If you don't know, then you cannot know whether your op-amp can work or not and cannot expect any predictable results.
     
  4. Mar 14, 2017 #3

    NascentOxygen

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    Staff: Mentor

    Are you saying you connected the board to your power supply but omitted the blue connection, and found the differential amplifier on the board still operated as normal under every test you devised?
     
  5. Mar 15, 2017 #4
    All the connections where made, however the op amp could not handle much of an offset.

    Then when I hooked up in dual mode the op amp operated as expected when applying a large offset on one signal.
     
  6. Mar 16, 2017 #5
    I'm not sure that I understood this part.

    Anyway, the '+' and '-' are not fixed for an opamp. All it requires is that the input signals should be higher than '-' and lower than '+' by a specified margin. If this fits, then it'll work and produce an output voltage higher than '-' and lower than '+'.

    There are opamps which will work even from 3.3V (without any negative power supply) perfectly, without any hitch.
     
  7. Mar 22, 2017 #6

    rbelli1

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    There also may be input protection diodes or another path from the input to the supply rails. You will essentially be powering the device from the input signal. This will give very poor results as the load on your input will not be constant and your power supply will also not be constant and generally below the peaks of your signal. Check for major distortion on your output.

    BoB
     
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