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INA181: DC current sensing for AC current

Kotaro Yamashita
Intellectual 2750 points

Part Number: INA181

Hi Team,

My customer would like to sense DC current at AC 50Hz waveform. In this case, they need to add huge R-C filter, however they don't like to use large shunt resistor.

Is there any other way to achive R-C filter for 50Hz? Is it possible at output filter?

Regards,

Kotaro Yamashita

  • 0
    TI__Mastermind 32395 points
    Hello Kotaro,

    What is the value of the current being measured? Is the 50Hz waveform a sine or square wave?

    If you tried filtering on the input, you could inlcude two 10 Ohm input resistor along with a differential capacitor greater than 1/(2*pi*10Ohm*50Hz) = 318uF, but I would not reccommend this because your source voltage would be significantly loaded down and could cause oscillations seen at INA181 input.

    If you tried filtering on the output with a passive RC low-pass filter, you will most likely over load the output stage of the INA181 casuing oscillation and loss of output drive, especially if you are trying to already directly drive an ADC with the INA181. Even though the it is listed in the INA181 datasheet that the max capacitive load is 1nF, you can usually drive a 100 Ohm/1uF RC filter on the output without device output oscillation, but this does not even come close to the RC values you would need for a 50Hz filter.

    Is the INA181 driving an ADC? Many ADCs have internal 50/60Hz filtering or you could average out the 50Hz noise in the processing part of the signal chain.

    Another option is to add a low-pass filter op amp circuit to the output of the INA181. This op amp would also serve the purpose of buffering the INA181 output if you are driving an ADC.

    Peter Iliya
    Current Sense Applications
  • 0 in reply to Peter Iliya
    TI__Intellectual 2750 points
    Hello Peter,

    Output is sine wave and inverter circuit. I'm not sure the actual current of this circuit but they don't add large R-C filter at input for efficiency.

    I think adding filter op amp option is good for them. is there any reference design?

    Regards,
    Kotaro
  • 0 in reply to Kotaro Yamashita
    TI__Mastermind 32395 points
    Hey Kotaro,

    I do not believe we have any current-sensing specific designs involving low-pass filtering the output. You can certainly try using TI's WEBENCH Filter Designer tool though.
    www.ti.com/.../webench-filters.html pass filter design&tisearch=Search-EN-Everything

    Sincerely,
    Peter Iliya
    Current Sense Applications
  • 0 in reply to Peter Iliya
    TI__Genius 14825 points

    Kotaro, Peter,

    We recently released an app note about output low-pass filter design to maintain stability, if it helps.  It applies primarily to current sensors.  You can find it here: Closed-Loop Analysis of Load-Induced Amplifier Stability Issues Using ZOUT.