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INA128: Output offset and unexpected gain

user6239208
Prodigy 40 points
Part Number: INA128

Hello,

I'm using an INA128 to measure the voltage difference across a small resistor (1mOhm), using the layout in the attached image. For technical purposes, a current is fed to the resistor using a voltage source and two 50 nF capacitors. The 47 kOhm resistors are used to provide a return path to the input bias current. The voltmeter VM1 is only used to compare that the output in Vout corresponds to the voltage drop across the resistor. In the second picture the voltage at VM1 and Vout are plotted, and instead of having the same magnitude, the output voltage in the INA has a certain offset and is roughly 1000 times larger. If the resistor to be measured is larger (> 1 Ohm), both measurement voltages agree. I'm not sure whether there's a problem with the layout, the simulation, or if the INA has to be used differently to measure a small voltage drop.

Cheers,

Alberto

  • 0
    Prodigy 40 points

    I realized what was going on. The voltage drop across the resistance is really small (O(100 nV)), which is signifcantly below the offset voltage of the INA (nV-mv, depending on the model and the particularities of each component). Additionaly, I'm using G=1 in this case. Thus, the output I'm seeing is just this offset voltage. Once the voltage drop, amplified by the chosen gain, is above the offset voltage, everything works as expected. 

  • 0 in reply to user6239208
    TI__Genius 10850 points

    Hi Alberto,

    Thank you for your post. I'm happy you were able to resolve this one. I am going to close this thread now. Best of luck on your designs.

    -Tamara