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Oscillations on current measurement

Beat Arnet
Intellectual 810 points
Part Number: INA240

Hello TI Experts,

I am using an INA240 (20x) in conjunction with a 8 mOhm resistor for measuring the fairly non-linear inductor current of a buck converter (half-bridge).

At 20 kHz switching, the output of the INA240 looks pretty decent, but small oscillations can be observed during the linear portion of the current decay.

1) Green trace: inductor current measurement with Tektronix current probe
2) Red trace: inductor current measurement with INA240
3) Blue trace: half-bridge output
4) Orange trace: buck converter output

If I now increase the switching frequency to 30 kHz, these oscillations become substantially more pronounced:

Would you be so kind to help me understand what the root cause of these oscillations is and let me know if/how they can be reduced?

Thank you in advance,

Beat

  • 0
    TI__Expert 4290 points

    Beat,

    Thanks for the question and using the E2E forum.

    We're looking into this and will give you an update when we return from the holiday week.

    Louis

  • 0 in reply to Louis L
    TI__Mastermind 30910 points

    Hi Beat,

    These may not be oscillations after all.

    The device uses a switched capacitor architecture for offset reduction. The ripples might be due to this operation, especially if the period is about a couple of uS. This ripple can grow bigger if the input voltage change is large and rapid, as the circuit scrambles to catch up.

    Regards, Guang  

  • 0 in reply to Guang Zhou
    Intellectual 810 points

    Hello Guang

    I know that these are not actual current oscillations (as the Tektronix probe does not show them), but the INA240 measurement is still oscillating.

    Could you please point me to the right section in the INA240 data sheet that describes this phenomenon/limitation and provide some guidance on how to reduce the oscillations?

    Thank you and kind regards, Beat

  • 0 in reply to Beat Arnet
    TI__Mastermind 30910 points

    Hi Beat,

    The most relevant is probably the settling time spec, most of which is due to the behavior mentioned previously.

    Maybe some filtering on the output can help alleviate the issue to some extent.

    Regards, Guang