Microvolt strain gauge amplifier

Discussion in 'The Projects Forum' started by guitarhero, Oct 9, 2012.

  1. guitarhero

    Thread Starter New Member

    Oct 2, 2012
    3
    0
    Dear all,

    I'm trying to build a strain gauge bridge signal conditioning circuit. I'm interested in measuring microvolt differences in signal. This variation will sit on an offset of up to 20mV. I've so far designed the attached circuit.

    I would be very grateful for any advice on the attached circuit and particularly how to add an offset trim to remove the comparatively large offset.

    Many thanks for your time.
     
    Last edited: Oct 9, 2012
  2. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Typically you put a trimpot in series with one of the fixed bridge resistors (and reduce that fixed resistor value by about 1/2 of the trimpot value). Use a film type multi-turn (10-15) trimpot and make its value no more than 1/4 of the fixed resistor value.

    Why do you have a pot connected from the output to ground? All that does is change the load impedance. :confused:
     
  3. guitarhero

    Thread Starter New Member

    Oct 2, 2012
    3
    0
    Thanks for your reply. The bridge is a 4 active gauge bridge so I don't think that's an option for me.

    You're absolutely right about the pot, I miss-copied my hand drawn schematic. It should be connected to the non-inverting input so the op amp functions as a non-inverting amplifier with adjustable gain.

    For the offset, I was thinking of using a buffered resistor divider between the power rails and wiring that to the input of the instrument amplifier but I think with the constraints on noise that isn't possible. I think I should be able to wire it to the non-inverting input of the final op-amp though?
     
  4. guitarhero

    Thread Starter New Member

    Oct 2, 2012
    3
    0
    Thanks for your reply. The bridge is a 4 active gauge bridge so I don't think that's an option for me.

    You're absolutely right about the pot, I miss-copied my hand drawn schematic. It should be connected to the non-inverting input so the op amp functions as a non-inverting amplifier with adjustable gain.

    For the offset, I was thinking of using a buffered resistor divider between the power rails and wiring that to the input of the instrument amplifier but I think with the constraints on noise that isn't possible. I think I should be able to wire it to the non-inverting input of the final op-amp though?
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Yes, you could apply offset to the output op amp. You can also adjust the output offset by a voltage at the REF input of the 1NA128 amp (the REF pin should be tied to ground otherwise).

    But, depending upon the gain of the 1NA128, you could have problems with the offset saturating that amp.
     
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