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problem with ina122

Other Parts Discussed in Thread: INA122

 

I have a problem using an instrumentation amplifier(INA122), 
www.ti.com/.../ina122.pdf
I've used the scheme in page 9 figure 5. Is powered by +5volt from arduino diecimila.
The bridge have 2 resistors of 10k ohm ,one of 100 ohm and the last is a 3wires pt100(due to this kind of sensor i conect one the 3rd wire to the pin2). The circuit is powered by +5volt from arduino. The problem is that the output is 0v on Vout, so amplifier does not amplify my input imbalance when i change temperature on the sensor(im sure its working rigth because its resistance value is the same as its datasheet). I've tryed to change Rg but nothing, the output is always 0v(i used a Rg of 1.2k and 10k). All connections are right and without short circuits.
I have on Vin+ and Vin- 3.8 milliVolt and 3 milliVolt on output, and the output don't change.
What i'm doing wrong?

  • hi Gabriel ,

    I read in the first sentence that you have connected the third wire of the sensor to pin 2 , which is Vin -
    Now an increase in resistance of this 100 ohm resistor will result in the input differential signal to be increasing in negative direction due to the configuration you have mentioned above



    hence please verify if the connection is exactly as shown in the schematic above
    That is your variable resistance (sensor ) is connected to pin number 3 ( Vin +)
    and your fixed 100 ohm resistor going to pin number 2 (vin -)

    Also with a multi-meter , please probe pin 3 and pin 2 of the INA
    Positive end of the multimeter to pin 3 and negative probe of the multimeter to pin 2
    And check if the DC voltage that you read when Rx = 100 ohm is 0V in the multimeter
    and Rx = 140 ohm it reads around 19.5mV
    thank you
  • hi Shreenidhi,

    Thanks for your help. I was connecting the wires wrong, but when I connected them like you said I had another problem: the output was really unstable (it was oscillating from 0.2v to 2v). So I measured the output from the bridge with my voltemeter and I read the value that I was expecting with my circuit (around 5mv), but when I tested it with the osciloscope I saw that the value was really oscillating between -5mv to 20mv, which was cause due to interference in my university (which was the place where I was using the circuit)

    Finally to solve the problem, I changed the resistance from 10k ohm to 470 ohm (I wanted to make sure that the output from the bridge was from 100mv to 300mv, so the interference wont affect too much my measure) and the gain resistance from 1.2k ohm to 18k ohm.

    Thanks again for you help

    Sincerely, Gabriel

  • Wonderful Gabriel,

    I am glad it worked

    All the best :)