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Voltage measurement with differential amplifier

Discussion in 'General Electronics Discussion' started by loamobn1, May 7, 2012.

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  1. loamobn1

    loamobn1

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    Feb 23, 2012
    I'm trying to measure the resistance of a part whose resistance varies by about 1 ohm, plus or minus (the average is about 5 ohms). So I have a voltage divider with the part in it (lets say the part is R1), and a reference voltage divider that has R1=5ohms. I'm trying to find the difference in voltage between the two signals, and in that way measure the resistance of the part. So I feed both voltages to an instrumentation amplifier http://en.wikipedia.org/wiki/Instrumentation_amplifier

    This hasn't worked for me. I set all resistances to 1k, except the gain resistor (set to 500), and it doesnt work at all like it is supposed to. Does anyone have any ideas?
     
  2. shrtrnd

    shrtrnd

    3,717
    469
    Jan 15, 2010
    One Ohm is a very low resistance to read accurately.
    My input would be that the tolerances of your resistors is critical.
    The accuracy of your meter is critical.
    Yageo/Phycomp and Susumu make resistors in the .5% tolerance accuracy range.
    Your meter must be calibrated, and is hopefully in the accuracy category of a
    Hewlett Packard 3458A or Agilent 34401A.
    I don't know if this is a work project or home brew, but you're going to need super
    accuracy in your circuit and your measurement device to BE accurate.
     
  3. loamobn1

    loamobn1

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    Feb 23, 2012
    As I matter of fact, I am using an Agilent 34401A :). And the resistors I'm using are potentiometers that I've set by hand, using said multimeter. The thing is, I'm feeding the voltage into a (n equally accurate) DAQ system, so I need a voltage that varies linearly with resistance. So how do I map a change in resistance of +/- one ohm to a change in voltage of +/- 5V?
     
  4. duke37

    duke37

    5,343
    753
    Jan 9, 2011
    If you set up a vitual earth op-amp, feed it with a constant current and use your variable resistance for the feed back, then the output voltage will be proportional to the resistance.

    With only 1 ohm you will not get much output voltage, so will need to include a post amplifier.

    Your original post seemed as if you are using a Wheatstone bridge. How do you manage 1k resistors? You would be better with a 1 ohm reference resistor.
     
  5. loamobn1

    loamobn1

    10
    0
    Feb 23, 2012
    duke:
    why make it more complex than it needs to be? Thanks for the suggestion, I made a negative feedback amp work just fine. Duh!
    Thanks
     
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