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INA229: Monitor negative power rail.

user1901441
Prodigy 50 points
Part Number: INA229
Other Parts Discussed in Thread: INA226, INA228

Can I configure the chip to measure -5V power supply? Is there a reference circuit that I can follow? Thanks!

  • 0
    TI__Expert 4720 points

    Hello,

    The Vbus can sense voltages anywhere from 0 to 85V relative to INA ground. There would be ways around this by playing with the INA GND being the lowest potential.

    If you can provide a schematic and/or describe your circuit. I can provide more input.

    Regards,

    Cas

  • 0 in reply to Castrense Nigrelli
    Prodigy 50 points

    I saw 226 discussion as below. Positive side is ok. Negative side is the one I have question, especially VBus. I don't think tie to GND is correct but pin10/9. Please help and clarify, thanks!

  • 0 in reply to user1901441
    TI__Expert 4720 points

    Hello,

    Connecting to GND will be the correct way to do this as the Vbus mux will sample from Vbus to GND pin (Voltage = Vbus(pin) - GND(pin)). Therefore if you want to measure the node voltage of Vin+/Vin- they are at the -28 V, so Vbus must be at GND. This will cause Vbus - GND = 0 -(-28) = 28 V. Or you can think of it as GND is 28 volts above the -28V rail therefore to get 28V on the Vbus measurement it must be connected to GND.

    You will have to distinguish in the code that it comes from the negative rail INA229 therefore should be negative.

    Regards,

    Cas

  • 0 in reply to Castrense Nigrelli
    Prodigy 50 points

    In that case, should VBUS of U8 tie to Vi+? Thanks!

  • 0 in reply to user1901441
    TI__Expert 4720 points

    Hello,

    It depends on if you want to measure the rail voltage or load voltage. The only change is the voltage drop of the shunt resistor. You can put it on either side of the shunt resistor. In this specific example for U8 we are measuring the load voltage (+28V - I*Rshunt). You could place Vbus at Vin+ and get 28V.

    For U6 the lowest potential must be gnd in this case it is -28V we can not monitor the load voltage as it would require to place the Vin- in a voltage of violating abs min. If you would like to get the load voltage of the negative rail you would not use the configuration above but connect to the high side of the negative rail load, and place Vbus on the In- pin. But if you want to measure the negative rail voltage than the above is fine. See below figure.

    (This is just for illustration I am just trying to describe the way to connect for negative rail load voltage measurement)

    If you would like to measure the positive rail voltage you would place Vbus at Vin+.

    Regards,

    Cas

  • 0 in reply to Castrense Nigrelli
    Prodigy 50 points

    Thank you for the clarification. One last question. INA229 has this diagram and MUX is hard tied to ground. Is it true or just a typo? If it is hard tied, negative voltage side may not work. Thanks!

     

  • 0 in reply to user1901441
    TI__Expert 4720 points

    Hello,

    This just means the INA mux is tied to the GND pin of the device. 

    Regards,

    Cas

  • 0 in reply to Castrense Nigrelli
    Prodigy 50 points

    The final confirmation. I can use INA229/8 the same way as INA226 to monitor both positive and negative power supply. I understand that I will need to add the isolator for negative SPI/I2C transmission. Thanks!

  • 0 in reply to user1901441
    TI__Expert 4720 points

    Hello,

    Yes, the INA228/9 can be used to monitor negative power supply with the proper circuit configuration.

    When you get the circuit schematic and layout done you can create a new thread for a schematic/layout review with our experts.

    Regards,

    Cas