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INA231EVM: the value read from GUI is wrong

Cindy Chang
Intellectual 2475 points
Part Number: INA231EVM
Other Parts Discussed in Thread: INA231

Hi team,

Customer is using our INA231EVM and GUI for evaluation.

However, they found that when they giving 7.7V/100mA source from power supply, GUI only read 34mA. 

May I have your help to know what may cause reading value being not correct?

Thank you.

  • 0
    TI__Genius 17990 points

    Hello Cindy,

    Looking at the "Shunt Voltage" value, it looks like the expected current should be 112.5µV/1mΩ = 112.5mA.  Can you confirm that the correct number was written to the Calibration Register? Using the info in your screenshot, it looks like we should see 0.00512/(48µ*1m) ≈ 106667. You can also look at the calculations tab to help verify this as was done correctly (and maybe look for an error there). Also, you could try changing the Current LSB to a different value (closer to Low LSB) to see if that works.

  • 0 in reply to Mitch M
    TI__Intellectual 2475 points

    Hi Mitch,

    Thanks for your reply.

    We've tried changing "current LSB" but the current still could not meet 100mA.

    And we found that the sensing current will become smaller if we set "max current" smaller.

    Here're the screen shot.

    Best Regards,

    Cindy

  • 0 in reply to Cindy Chang
    TI__Genius 17990 points

    Cindy, 

    Thanks for sending those screenshots.  I'll start looking over the data today.  In the meantime, could you also send me a screenshot of the calculations tab? 

  • +1 in reply to Mitch M
    TI__Genius 17990 points

    Cindy,

    Never mind to that last request, I see the problem now.  Sorry I missed the problem before, but your calibration value is too large to fit in the calibration register (that is why you see 0x7FFF in the calibration register).  You'll need to adjust your shunt value (or possibly your Current_LSB) so that your calibration value falls within the limits.

  • 0 in reply to Mitch M
    TI__Intellectual 2475 points

    Hi Mitch,

    Thanks for your kindly reply.

    Do you mean we need to change Rshunt value, like increasing to 3mohm or 5mohm?

    Because no matter how we change the Current_LSB, the current value measured by GUI still very small, the only way now is changing shunt value?

    And what value of the calibration register would be so that we could let INA231 work in normal?

    Thank you.

    Best Regards,

    Cindy

  • 0 in reply to Cindy Chang
    TI__Genius 17990 points

    Hello Cindy,

    The device will show the correct value for current when the calibration register (CAL) is calculated from the shunt and Current_LSB values.  The equation to find the CAL value is: CAL = 0.00512/(Current_LSB * Rshunt).  The maximum value that can fit in the CAL register is 0x7FFF (decimal 32767).  So, the minimum shunt size is dependent on the Current_LSB.  From you charts above, it looks like you were using a Current_LSB of 48µA, so if we use that those numbers we get:

    maxCalValue >= 0.00512/(Current_LSB * Rshunt)

    32767 >= 0.00512/(48µ*Rshunt)

    Rshunt >= 3.25531mΩ

    So, with your desired Current_LSB, any shunt bigger than ~3.3mΩ would work to get the current value to read out properly.