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INA228_237_238EVM: wrong current measurment from INA228 EVM module.

kobi cohen1
Prodigy 60 points

Part Number: INA228
Other Parts Discussed in Thread: INA228

hi,

I have communication to this module,when I'm measuring on shunt a resistor of 1oHm(very accurate resistor) with a ext volt meter I get 1.5mV

however the data that i get from INA228 is 4mA (suppose to be 1.5mA).

Please advise.
thanks Kobi.

  • 0
    TI__Mastermind 29405 points

    Hello Kobi,

    From the screenshot above, it looks like you are getting an average shunt voltage of about 1.5mV. It just looks like there is a ±1.25mV noise in the system. It's possible that your ext volt meter is doing more averaging than the INA228. If you want, you can increase averaging and conversion time of the INA228. 

    As a side note, if the current result is not showing as expected, make sure that the correct SHUNT_CAL value calculated in the configuration tab was written to the register. This should happen automatically, but it's possible that it didn't. 

    Regards,

    Mitch

  • 0 in reply to Mitch M
    Prodigy 60 points

    Hi Mitch.

    the load that I have used is CR EXT load that has impact on the SHUNT resistor.

    when I have used TH resistor of 1.2Kohm and R- SHUNT OF 1Ohm I got good results:

     

    the only problem is with the equation of the current.

    when I calculate, it gets too high.

    Kobi. 

  • 0 in reply to kobi cohen1
    TI__Mastermind 29405 points

    Hey Kobi,

    It makes sense that you would have a better result with at 10Ω resistor than a 1Ω resistor. From the calculator tool snip you sent above, with a 1Ω resistor you are only using 4mV out of the ±40.96mV full scale range. With a 10Ω resistor you would be utilizing more of the full scale range which would improve overall accuracy.

    as for the calculation, you should take the value from the current register and multiply it by the current LSB. So, for the info you showed me above in the GUI calculator tool, it would be a current result of 0x166F5 which is decimal 91893. This would be a current of: 91893 * 7.62939453125e-9 ≈ 0.701mA.

    The problem mis that in your calculation, you are using 10A as your max current when calculating your LSB, but in the calculator tool you only use 0.004A as your max current. You need to pick a maximum current to be measured and use the same value in the GUI tool as you do in your math. (so use the current LSB from the GUI tool). And make sure that the calculated SHUNT_CAL value is the one written to the register to ensure accurate data.

    Regards,

    Mitch

  • 0 in reply to Mitch M
    Prodigy 60 points

    Hi Mitch,

    i don't understand how to configure MAX curren to be lower than 10A , in the table i have configure it for max 4mv.

    the reading voltage above the SHUNT resistor is correct 2.75mv

    the current that i should have with a load of 1.2kohm is 2.74mA.

    So what is the problem here?

    thanks.

  • 0 in reply to kobi cohen1
    TI__Mastermind 36060 points

    Hi Kobi,

    Were you able to figure out the issue? If not, can you check the following? You can keep the setup identical as before –

    • The value stored in Shunt_Cal register (2h)
    • The values from Vshunt (4h) and Current (7h) registers

    There seems to be a scaling problem and it is most likely that Cal register was not programmed with the correct value.

    Regards, Guang  

  • 0 in reply to Guang Zhou
    TI__Mastermind 36060 points

    Hi Kobi,

    I’m going to close this thread, please post again if further assistance is needed.

    Regards, Guang