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INA220: Shunt Voltage Consistently Reading High

Chuchen Wang
Expert 3430 points
Part Number: INA220

Hi,

My customer is getting current and power readings that are off.  Below are details, can you provide some insight?

We are using TI’s INA220 I2C voltage/current/power monitor on a design.  We are using multiple INA220s on the board: two for 12V buses, and one for a 24V bus.  I am getting correct bus voltage readings from all the INA220s.  However, the current and power readings are off.  Initially, I assumed I must have the math wrong for setting up the calibration register.  However, when I read the shunt voltage (which is independent of the calibration register), it is also off by the same proportion as the  current/power.

 

For the all of our boards that I’ve tested, the 12 buses are all reading approximately 18% high for the shunt voltage.  For all boards tested, the 24V buses are reading approximately 11% high for the shunt voltage.  All the buses (12V and 24V) are using a .002 Ohm +/- 1% shunt resistor.  The only differences are the bus voltage and the corresponding programming of the BRNG bit in the Configuration Register.

 

Is there an errata for the INA220 related to this?  Are there any other variables we can control (or mis-control as the case may be) that can affect the accuracy of the shunt voltage reading?

Thanks,

Chuchen

  • 0
    TI__Genius 15760 points

    Hi Chuchen,

    Hope all is well!

    As the bus voltages readings are correct and shunt voltages are wrong, this can be a layout issue with the input traces of the INA. Peter will follow up with you on this tomorrow, in the meantime please request the schematic and layout from the customer.

  • 0 in reply to Mayrim Verdejo
    TI__Mastermind 26660 points

    Hello Chuchen,

    How and where are you measuring the actual shunt voltages? Also what are these values?

    How many boards/units are you seeing this problem on?

    If you would like to not share schematic information online you can email me. p-iliya@ti.com

    Peter Iliya

    Current Sense Applications

  • 0 in reply to Peter Iliya
    TI__Expert 3430 points

    Hi Peter,

    Here's feedback from the customer.  I will email the schematic to your email.

    We’ve seen this problem on two boards out of two that we’ve looked at.  These are first articles, so there aren’t many of them yet.  But, each board has two 12V monitors and one 24V monitor.  All four 12V monitors were reading ~18% high.  Both 24V monitors were reading ~11% high.

     

    We determined the voltage drop across the .002 Ohm shunt in two different ways:

     

    1. We measured across the shunt using a 4 wire digitial multimeter. 

    2. We calculated the actual current and bus voltage. 

     

    To measure bus voltage, we’ve used two digital multimeters, a digital scope, and the digital voltage reading on an electronic load.  They all read the same voltage.

     

    To measure actual current, we set the electronic load to a desired current, and confirmed it’s calibration with inductive current meter clamped around the cable carrying the current between our board and the electronic load.

     

    The voltage drop across the shunt varied with current, and the measured and calculated values matched.  However, the value read from the shunt voltage register was either 18% or 11% high depending whether we monitoring a 12V or 24V bus.

    Thanks,

    Chuchen

  • 0 in reply to Chuchen Wang
    TI__Mastermind 26660 points
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