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Wheatstone bridge/multiSIM help

Discussion in 'General Electronics Discussion' started by Chris13, Sep 16, 2014.

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  1. Chris13

    Chris13

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    Sep 16, 2014
    Hi,

    I am trying to setup a op-amp using multi sim to provide a 0-5V output with a mV input coming from a strain gauge (wheatstone bridge).

    https://www.dropbox.com/s/7i2uborv0dxatex/amp.jpg?dl=0

    The screenshot attached is from my multisim simulation. I don't understand why XMM-3 is showing 500MΩ. It should just be 500Ω.

    Can anyone help?

    Thanks,
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,087
    Dec 18, 2013
    Your DMM will not show the correct reading with external power connected to it. Resistance measurements should be done without power connected.

    I can't see anything wrong with your circuit as far as your results go. You have a gain of about 146 so this would give you the results you have. You need to change your gain if you want a higher output for a given input. It's not clear to me as you say 0-5 Volts output for 1mV input. You can only get this if you change your gain of the amp.

    Adam
     
  3. Chris13

    Chris13

    26
    0
    Sep 16, 2014
    Hi,

    Thanks Adam - such an obvious mistake when it is pointed out!

    I have my gain resistor setup now so that at 20mV from my bridge I get 5V output.

    I notice when my bridge output is 0mV (or as close as possible, 4.749μV) my output is 903mV. Can anyone suggest how I get this to zero?

    Thanks,
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi Chris
    Also just noticed you are using a single supply, just check your not violating the input common mode range for the device and that the outputs can drive to 0V which I doubt. You would be better off using a split +/- 5V supply if you can.
    Adam
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Split supply as mentioned above.
    Adam
     
  6. Chris13

    Chris13

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    0
    Sep 16, 2014
    I intend to make the amp battery powered - would this be possible?
     
  7. Arouse1973

    Arouse1973 Adam

    5,164
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    Dec 18, 2013
    Yes, two batteries would be required in series and connect the common point of the circuit to the centre where the two batteries meet. The other connections go to the power supply pins of the INAMP.
    Adam
     
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