Wheatstone Bridge Question

Discussion in 'The Projects Forum' started by nevjc, Jan 7, 2014.

  1. nevjc

    Thread Starter New Member

    Jan 7, 2014
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    0
    Hi
    Could anyone tell me how I can add the two output voltages from two seperate wheatstone bridges without adding any further loading to each individual output.

    Regards
    Nevjc
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    You use a differential instrumentation amp on each bridge output, then add the two outputs together with a summing op amp circuit.
     
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  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,790
    Add a high impendence op amp follower to each side of the bridge (connect the bridge nodes to the op amps with 3 to 10 M ohm resistors.

    Hopefully your bridge are higher impedence than 100 k ohm.

    A TLV2242 dual op amp would be a reasonable low power high impedence op amp for DC work. Hundreds of others will also work.
     
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  4. nevjc

    Thread Starter New Member

    Jan 7, 2014
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    Thank you crutschow and gophert fo the advise. I will build the opamp circuit ad give it a try. I am trying to build a system where I can measure two individual weights using two seperate low cost scales and then also display the total weight of both scales using a third LCD display from a third scale.
    Hope that makes sense. Thank you for your assistance

    Nevjc
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    An instrumentation amplifier may very easy be built with discrete opamps. It may be tempting to do so in order to save some money. But an instrumentation amplifier need to be constructed with closely matched resistors for the best accuracy. Instead of using a high impendence op amp follower to each side of the bridge. I think I intuitive would have chosen to use a instrumentation amplifier with a high impedance FET input stage instead.
     
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  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Do they share the same excitation voltage, or do you have two isolated supplies?
     
  7. nevjc

    Thread Starter New Member

    Jan 7, 2014
    3
    0
    Hi Mike,
    They do have seperate 3v excitation batteries but I can combine if necessary from a single battery source.
     
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