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INA228: combining multiple shunts

Barry Fields
Prodigy 20 points
Part Number: INA228
Other Parts Discussed in Thread: INA4230, INA4235

Tool/software:

Dear TI,
Below is a representation of my solar system. There are 4 separate battery banks. The OP amps are intended to generate an individual charge current value to be sent to a separate analog peak charge current detector. The absolute accuracy of those measurements is not a priority as those values will only be used for comparison between battery banks.
I feel comfortable with what I have here and submit it as an overall view for my real questions below.


 
I need to somehow combine the individual shunt signals to obtain a total charge current for all banks and communicate that to an MCU. Again, absolute accuracy is not required because the result would be compared to the same result 15minutes ago. This would yield a indication of stabilized current.
1) Is this a valid configuration?

 
2) My simulations show that the resistor divider network acts like a 400a 75mv shunt. Are these calculations correct?
    From data sheet
            Rshunt = 75mv  ÷  400amps =  1875 uΩ
    equation #3    current LSB = 400 ÷ 219  = 76.3 x 10-6 
    equation# 2    shuntcal = ( 13107.2 x 10 6 ) ( 76.3 x 10-6 ) ( 1875 x 10-6  = 3000d  = BB8h

Thank you in advance for your response.
Barry Fields

  • 0
    TI__Mastermind 26525 points

    Hello Barry,

    Here are my responses to your questions:

    1. Looking over this configuration, it does seem like it will work. My main concern would be with mismatch from the 1kΩ resistors on the input of the INA228. The datasheet says not to use higher than 100Ω, as a higher value can cause measurement error based on the mismatch of the resistors. However, if you need the higher resistors then it can be fine if you are ok with the error. 

    2. Your equations look correct, but the value you calculated for equation #2 is wrong. When I calculate that equation, I get 1875d (0x753).

    You may be interested in the INA4235 or INA4230, which are 4 channel devices. If you used one of these, then you could just read the current register for each shunt and then add them together in the MCU.

    Regards,

    Mitch

  • 0 in reply to Mitch M
    Prodigy 20 points

    Thank you very much. I will hand select 1% 1k resistors for best match.

    You are correct, my math was wrong. I sent my ABACUS of for adjustment.

    I appreciate the 4channel suggestion but I have no need for individual currents.

    Best regards,

    Barry