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INA271: Not working correctly

Marek Bedlek
Prodigy 20 points
Part Number: INA271

Hello,

I used five INA271 circuits with 20 mOhm shunts on my board to sense current on five different voltage rails.

Two of the INA271 circuits are working fine. Three of them however, are not. Their are experiencing variable gain. Of the three, works with gain cca 35, second one with cca 40 and the third one with cca 18.

I suspect the issue is in improper layout, schematic-wise, the circuits are same for all the voltage rails.However, the two that are working fine and the one that has gain 18, have traces from the shunt in the top pcb layer. The ones with gain 35 and 40, needed to have the traces coming from the shunt crossed, so they are using vias. I can also measure relatively significant voltage on one of the traces from shunt to INA271, on the amplifier that arent working with proper gain, cca 10-15 mV, which throws the INA271 off.

What is interesting is, that even the amplifiers with improper gain, are still working linearly, in other words their gain is constant.

All five circuits have the same +5 VDC power supply. Kelvin connections are used to connect to shunt resistor pads.

Could anyone help me with find the cause of this behavior?

Thank you

  • 0
    TI__Genius 15760 points
    Hi Marek,

    When you say you are experiencing variable gain... what is your voltage reference?

    Are you measuring the Vshunt in the inputs of the INA to confirm you are getting the differential voltage you are expecting? The voltage you have across the shunt can be different to the voltage across the INA inputs if there is improper layout. If there are layout problems it is most likely that the INA is not seeing the voltage you are expecting on the inputs and that's why you see a "variable gain".

    Perhaps, you can share schematic and layout? This will help me understand your application better.

    Also, we have a video series that gives advice on layout and how to troubleshoot our current shunt monitors to find out sources of errors. Refer to Section 13 and Section 14.
    e2e.ti.com/.../467332
  • 0 in reply to Mayrim Verdejo
    Prodigy 20 points
    Hello,

    thank you for your response.

    I can actually confirm that differential voltage across INA input pins is different than on the resistor. Do you know what could be the cause of that? Both circuits that have gain larger than 20 have vias on the traces that go from shunt to inputs of INA.

    I will upload schematic and layout tomorrow.
  • 0 in reply to Marek Bedlek
    TI__Genius 15760 points
    Hi Marek,

    The cause of different values in the inputs of the INA and the shunt resistor is due to layout problems, extra parasitric impedance. Please review the videos that I recommend in previous post.