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Original question:

INA199: AC Current sensing with Current transformer

INA199: INA199 showing non linear output

Alihussain Vohra
Prodigy 80 points
Part Number: INA199


Tool/software:

Hi All,

I am using INA199A1 to sense current from current transformer. I am attaching the schematic snapshot. I am getting non-linear output . Please suggest what am I doing wrong.

  • 0
    TI__Mastermind 23385 points

    Alihussain, 

    Thanks for using E2E.

    There are a few issues here, but the largest one is an issue is with your front end. Our current shunt devices source a part of their power, as well as their ability to stand up to VCM > Vs from the front end of the device via the bias currents of the device, and as such they have to have a defined common mode against the ground of the device. What this means is that the load they measure must be referenced to the same ground as the pin of the device, and they are not recommended in floating applications, as ground deviation between the two grounds will manifest as unwanted error on the output.

    If you can reference the front end to the device ground (I'm assuming the CT here is stepping the signal down so this is not possible), you also have extremely large sense line resistors, which will inject large amounts of error into the signal. We discuss this in section 7.4.1 of the datasheet, but the 1k you have populated here is creating an additional 40+% error on the measurement. We recommend to keep these resistors to less than 10Ωs for best gain performance. 

  • 0 in reply to Carolus Andrews
    Prodigy 80 points

    Yes I have removed 1k it reduces the error but still there is too much deviation. 

    -How can we make CT ground and circuit ground same? Do you have any reference for this? How to solve this?

  • 0 in reply to Alihussain Vohra
    TI__Mastermind 23385 points

    Alihussain,

    you likely cannot do this, as it would eliminate the isolation the CT is providing. How much current are you looking to measure at the primary? You could potentially consider one of our Hall effect products, which would provide the isolation in place of the CT, but we would need to ensure that the device can handle the current. 

  • 0 in reply to Carolus Andrews
    Prodigy 80 points

    Then what is this solution provided by TI? I have followed the same reference?

    Apart from that I am using 600/5A CT, but in the same design I used 2000/5A CT too.

  • 0 in reply to Alihussain Vohra
    TI__Mastermind 23385 points

    Alihussain, 

    My comment above is that the GND of the device and the ground shown terminating the common mode capacitors in this loop should be the same. It does not appear you have these capacitors in your circuit from the schematic shown above, and as a result, the the secondary voltage is floating against the device pin GND. The common mode capacitors will set the voltage on the secondary side so that it is common GND to the part. Can you add these and see if this rectifies the issue? If it does not (or you have already tried this and it did not help), can I see an oscilloscope shot of the sense voltage the device is measuring, and the output the device is delivering here? How much error are you seeing across the device?