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INA146: Non linear read in AC current reading (simulation)

Andrew Charnley
Intellectual 640 points
Part Number: INA146

I have a simple AC circuit with a 100 ohm load. I've two INA146 chips (a differential op-amp with offset and second op-amp for gain).  The output will go into a MCU at 3v. I use two voltage offsets to keep the common mode above the minimum for the voltage I'm going to measure. I then (wrongly?) set the resistors and offset to take up as much resolution as possible. The INA146 seems to have a problem going below 0.3v so I choose values to keep within about 2.8v-0.4v at peak AC.

The R4 and R1 resistors are a voltage divider as there's an internal 10k. I choose the values to get 1.5v at 0v AC (This should probably be 1.2v actually).

As the load is constant I then simulate with different voltages. What I've found is the voltage read is consistent but the current read becomes more skewed as voltage increases. Not sure is skewed is the correct word but you can see what I mean by the attached images.

The question of course, is whether it's accurate and if so any means to resolve it? I need both to be accurate with minimal phase as using them in real/apparent power factor measuring.





  • 0
    TI__Genius 10850 points

    Hi Andrew,

    Thanks for your post, I can help you. 

    I’m not real clear on your requirements, so I have a few questions for you so we can figure this out together. What is the input current range through that 50m-ohm resistor and at what common mode voltage? We want to make sure the voltage seen right at the non-inverting node of A2 is within linear range (ground to 23V in your case since you are operating at a 24V single supply). And what is the expected output voltage? 

  • 0 in reply to Tamara M Alani
    Intellectual 640 points

    Common mode is about 50v + 10v peak
    Current through the sense is about 500mA, +100mA peak

    I believe using a pull down on pin 8 (I was using a 7.5M) for the current measurement reduced the resolution so much that when it's amplified the result wasn't accurate. So rather than try to equalise the offset around 1.5v I'm now living with a higher offset. Ultimately less range at the ADC but more accurate amplification.