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INA128 Output

Mandar Sabhapatikar1
Prodigy 30 points
Other Parts Discussed in Thread: INA128, INA326, ADS1247, ADS1120, ADS1220, ADS1147

Hello,

I am using INA128P precision amplifier for my Temperature Logger application. I am using PT100 RTD as my temperature sensor. I have designed bridge circuit whose output is fed to V+ & V- Pins of INA128P. I am selecting the Gain of 1000 (Rg = 50.05 ohm, 51 Ohm used by me). For more details i am attaching my schematic.

R4 is my RTD. R5, R6 & R7 are negligible lead resistances of RTD. At room temperature as the bridge is balanced, the differential voltage between Pt A & B is 0mV. As the temperature is increased the bridge will be unbalanced & the voltage between A & B is increasing at rate of 0.1mV/*C. As this is much lower voltage, it is fed to INA128P with the Gain of 1000. The output voltage should change as per the input differential voltage, But the voltage is kept constant at 92.8 mV irrespective of the input voltage. What must be the cause of such behaviour?? Is my circuit wrong?? Do i need to use the dual supply?? or should i use the bias of 2V to the reference pin??? Please help me. Resolution required by me is just +-2*C.

Thanks in advance

Mandar

  • 0
    TI__Genius 15410 points

    Hi Mandar,


    The results you are getting is because the common mode voltage of your circuit is not within the range stated in the datasheet. The common mode voltage of your circuit is about 54mV. Since you are supplying the INA128 with 5V and ground, your common mode voltage needs to be between 1.7V and 3.6V. Also the output of the INA128 can swing from 1.4V to 3.6V. There are a couple ways to correct this issue. The first would be to do as you stated and supply the INA128 with +/-2.5V. This will lower the common mode voltage range to -800mV to 1.1V and also lower the output swing from -1.1V to 1.1V. Another way would be to add a 10k resistor in series with R2 and R4. This will increase the voltage at A and B and put the common mode voltage at about mid supply. Or you could decrease the 10k resistors (R1 and R2) to 110ohms. The down side of that is that there will be an increased current. If you are looking to use an instrumentation amplifier you could use the INA326, this device allows you to have a common mode voltage below your negative supply and above your positive supply and also has an output swing 75mV from the rail.

    I would also consider looking at our ADS1220, ADS1247, ADS1120, ADS1147. These parts are much more common and accurate for this application because they use a current excitation instead of a voltage excitation.

    Hope this helps!

    Tim