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INA181: Low Side negative Current sense issue

Kumar Balaji
Intellectual 410 points
Part Number: INA181

I'm using INA181- A1 device for my Low side Leg current sensing.

My Circuit looks like this,

I'm using 2m ohm Sense resistor and my maximum current is 16 A. The Op-amp gain is 20.

The op-amp output is good at positive cycle, But in negative cycle the sine gets distorted and missing negative cycle.

The top waveform is amplifier output. the bottom one is the Actual current output.

What can go wrong in this current measurement?

Please help me solve this issue.

Thanks,

Kumar V

  • 0
    Guru 140200 points

    Hi Kumar,

    not knowing whether this is the cause, but you should drive the REF pin low ohmically. Use a buffer as shown in figure 53 of datasheet.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 410 points

    Dear Kai,

    I have done that too. Now the offset is 1.66 V and i have lowered the resistance value as you said.

    But still i am getting the same waveform. The frequency of the current is 8 khz-20 khz.

    Thanks,

    Kumar V

  • 0 in reply to Kumar Balaji
    Guru 140200 points

    Hi Kumar,

    is the measuring current a pure AC current?

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 26335 points

    Hey Kumar,

    Thanks for considering to use Texas Instruments.  Can you tell me what your common mode is?  If you are using a function generator to generate the differential and have not provided an offset above zero, I could imagine you might see a waveform like this.  I suspect the ESD structure could be distorting the signal.  If this is not your issue, I think you may need to look at your supply line and see if it is stable.  Then if that is stable, we might need to do some continuity tests between your probe points and the device pins to check for soldering issues.

  • 0 in reply to Patrick Simmons
    Intellectual 410 points

    Hi Patrick Simmons,

    Thanks for your reply.My common mode is between the sense resistor of 2m ohm. The maximum current through the sense resistor is 16A. The supply is stable as you could see the bottom waveform is the actual current which is pure sine wave. I'm using 10E resistor in series with 10nf cap filter as mentioned in application note for filtering. What it would cause the negative cycle alone to distort?

    Please help me in this regard..

    Thanks,

    Kumar V

  • 0 in reply to kai klaas69
    Intellectual 410 points

    Dear Kai,

             Yes,it is a purely sinusoidal AC wave.Simply, it is a leg current of an inverter which runs an induction motor in V/F method in which the output to the motor is purely sinusoidal and we are measuring it in the bottom (leg)of the inverter.

    Please help me to sort out this issue.

    Thanks,

    Kumar V

  • 0 in reply to Kumar Balaji
    TI__Mastermind 26335 points

    Hello Kumar,

    You need to show me the IN+ voltage relative to ground and the IN- voltage relative to ground.  Your common mode voltage will be ((IN+)-(IN-))/2.  Once I see that, we can rule that out.

  • 0 in reply to Kumar Balaji
    Guru 140200 points

    Hi Kumar,

    can negative spike voltages be superimposed to the signal from time to time, something like inductive kick backs? Or by other words, could the INA181 be damaged? Can you take a fresh INA181 and check again?

    Kai