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Problem with INA122

Bryden Smith
Prodigy 50 points

Other Parts Discussed in Thread: INA122, XTR106

Hi,

I am in the process of designing a scale with max weight set at 10KG (22.046LBS)  I am using the INA122 as my instrumentation amplifier.  I am using a Wheatstone bridge circuit to detect the difference in my voltages with the weight differences.  I am using two strain gauges both 350 ohm and two 350 ohm precision resistors.  My voltage source is 5V.  The problem I am having is I am not getting any output on the amplifier.  When the bridge is balanced 0v = Vout = 0KG.  Max weight should equal about 10 mV(ish) difference VA and VB and then I set the gain equal to 500 on the amplifier to set it to 5v = my max weight of the scale.  Still no output on the amplifier.   If I have it balanced I get the 0KG.  If I don't have one leg of the bridge hooked up to the amplifier I get 5v out on Vout = 10KG.  I am having problems with the in between portion of the amplifier.  I am attached a picture of my schematic.  Vout of the INA122 going is to the ADC of a P18F4550 microcontroller.

Thanks for your help

Bryden Smith

  • 0
    TI__Mastermind 25875 points

    Hello Bryden,

    Can you please provide us with the part number and/or datasheet of the strain gauges you're using?

  • 0 in reply to Pete%20Semig
    Prodigy 50 points

    The strain gauge part number: KFG-5-350-C1-11L1M2R from OMEGA

    Website for them is: http://www.omega.ca/shop/pptsc.asp?ref=Prewired_GP_Strain_KFG&Nav=pree02

    Datasheet on them is: http://www.omega.com/Pressure/pdf/Prewired_GP_Strain_KFG.pdf

    Thanks

  • 0 in reply to Bryden Smith
    Prodigy 50 points

    After some more testing today I get these values:

    No Weight:

    Compression strain gauge = 349.5 ohm

    Tension strain gauge = 349.5 ohm

    In a voltage divider I will get 2.5v with the strain gauges on one side of a bridge and my precision resistors (2.5v) on the other side I get Vout = 0v

    Max Weight:

    Compression strain gauge = 348.6 ohm

    Tension strain gauge = 350.4 ohm

    If I put the compression strain gauge as the bottom resistor on the one leg of the bridge and the tension on the top.  I get from the voltage divider 2.49320555v.

    Should I put this in to Vin- because then it would be (Vin+ - Vin-) = (2.5v - 2.49320555v) = 6.79445mv at max weight. and then I just have to adjust my gain to equal 5v out?

    I hope this extra information helps

    Thanks

    Bryden Smith

  • 0 in reply to Bryden Smith
    TI__Mastermind 25875 points

    Hello Bryden,

    Thanks for the additional information. In order to ensure that the voltage at node VA is greater than that of VB, R15 should be greater than R14. Since R14 (compression) decreases in resistance with increasing load and R15 (tension) increases in resistance with increasing load, I would leave them as shown for R15 will always be greater than R14.

    Therefore I'm not sure why you'd want to change the circuit at all. If you're interested in balancing the bridge, there is a discussion in the XTR106 data sheet that may be of interest.