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BQ34Z100EVM: Grounding I2C on BQ34Z100EVM causes measurement errors

richard mcdougall
Prodigy 185 points
Part Number: BQ34Z100EVM


I have a design following the schematic in the BQ34Z100EVM guide which puts the pack on BAT+/BAT- and the load/charger on BAT+/PACK-.

The gauge works well when connected to the EV2300, however when it's placed on the host device and I2C is connected, current measurement goes off by 5x.

I think this is because the host electronics system that connects to I2C is grounded common with the load (it is actually part of the load) - because the I2C in the EVM is grounded to BAT-. This likely creates a ground loop back through the I2C ground since the LOAD has a signal ground on PACK-.

What is the best way to solve this problem? Why is the EVM setup to have I2C on BAT- when the load that would read the EVM would almost always be signal grounded to PACK-?

Thanks,

Richard

  • 0
    TI__Guru* 76230 points

    Hi Richard,
    Have you tried calibrating the current measurement when your load is connected to and drawing current from the battery to see if that helps?

    Also, what is the values of your pull-up resistors from your comm  lines and what voltages are the lines pulled up to?

    thanks
    Onyx

  • 0 in reply to Onyx Ahiakwo
    Prodigy 185 points
    > Have you tried calibrating the current measurement when your load is connected to and drawing current from the battery to see if that helps?

    Yes but I found that it just fixes the zero offset but swings still occur in either direction according to load (e.g. +20 amps when discharging)



    > Also, what is the values of your pull-up resistors from your comm lines and what voltages are the lines pulled up to?

    Resistors are 4.7k.

    I found a temporary solution: put a 10 ohm resistor in series with the I2C ground. That breaks the current flow bypass in the ground loop.

    I notice however that the EVM has two grounds on the circuit diagram - AGND and GND. And on the EVM trace there looks to be a place to break the connection from GND to BAT-. Would it make more sense to connect GND on the EVM to PACK-?

    Is there a best practice here for grounding he host device vs BQ or does this not come up as an issue?