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Instrument Amp Issue

Brad Foster
Prodigy 30 points
Other Parts Discussed in Thread: INA122

I have two INA122 instrument amp circuits in series in a current loop with 4-20 mA.  The first circuit in the loop has a 50 ohm resistor across pins 3 and 2 of the IA.  The second circuit has a 40 ohm resistor across pins 3 and 2.  Pin 5 Vref is tied to ground (pin 4).  Vcc on the first IA is 24 VDC and 5 VDC on the second.  No gain resistors are installed so the gain is 5.  The second circuit in the loop works fine but the first circuit has a 0.1 volt offset on the output.  What should be a 1-5 Volt output is 1.1 to 5.1.  Any suggestions would be greatly appreciated.

  • 0
    TI__Mastermind 21831 points

    Hi Brad,

    I am going to move this thread to the precision amplifiers forum who own the INA122 device so that it can get the proper visibility and support.

    Thanks,

    John

  • 0
    TI__Genius 12335 points

    Hi Brad,

    The common-mode voltage of instrumentation amplifiers is specified at the mid-supply point (+12V for the first amplifier, +2.5V for the second). The further the input voltage deviates from mid-supply, the greater the error at the output. With an input voltage 12V away from mid-supply this can produce an offset voltage of 12* 10^(-90/20) * 5 = 1.9mV assuming a CMRR value of 90dB.

    One suggestion would be to reduce the power supply voltage of the first amplifier such that the input voltage is no longer so far from the mid-supply point. 

  • 0 in reply to John Caldwell
    Prodigy 30 points

    Thank you for your response.  From your calculation, you came up with an offset voltage of 1.9 mV while I am seeing upwards of 80-100 mV offset with a supply voltage of 24 VDC.  

    Per your suggestion,  I lowered the supply voltage to 11.8 VDC and looped four of the upper inputs together in series so now there are four inputs with 50 Ohm resistors and one with 40 Ohms (plus two diodes).  The upper most load has a high side voltage of 2.2 volts and low side voltage of 2.0 volts.   The output of the first INA122 is 1.08 V.  The second INA output is 1.067, third is 1.078 and fourth is 1.087.  I swapped the first two INAs and the readings didn't change.

  • 0 in reply to Brad Foster
    TI__Genius 12335 points

    Hi Brad,

    My mistake, I read your first post incorrectly and thought you had a 1mV offset. Are all of these amplifiers located physically close to each other on the PCB? I'm wondering if a ground potential is increasing the voltage at the reference pin which is causing a shift in the output readings. Try measuring the output directly at the amplifier with respect to the reference pin. 

  • 0 in reply to John Caldwell
    Prodigy 30 points

    I found the problem.  I had a 0.1 uF disk capacitor on the output to get rid of any noise.  As soon as I removed it, the circuit began behaving as expected.  This is kind of puzzling to me as the IA with the 40 ohm resistor on the input has a 1.0 uF Tantalum Capacitor on its output and it works fine.

    Thanks for your time and efforts on my behalf.

  • 0 in reply to Brad Foster
    TI__Genius 12335 points

    Brad,

    If you view the output of the IA with an oscilloscope you will most likely see oscillations due to the large capacitive load. The ESR of the 1.0uF tantalum was most likely sufficient to prevent oscillation in one of the IAs. However, ceramic capacitors typically have extremely low ESR values, explaining the oscillation in the other IA circuits.