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INA3221: Bus and shunt voltage on a negative rail

John B
Intellectual 325 points
Part Number: INA3221
Other Parts Discussed in Thread: INA296A, INA226, ISO1643

Tool/software:

I am wondering if possible to use INA3221 for monitoring bus and shunt voltage on a negative rail. The negative rail is -5 V supplied by the linear regulator. The -5 V is connected to the load which require current up to 10 mA. We need to measure this current. Kindly let me know if we can use INA3221 for this application. Thanks in advance. 

  • 0
    Guru 296910 points

    This device cannot measure voltages below GND. If if you connected the GND pin to −5 V, then you would need to shift the voltages at the digital pins.

    You probably want a current-sense amplifier that supports negative voltages like the INA296A/B.

  • 0
    TI__Expert 4580 points

    John,

    Thank you for your patience.

    We are currently out of the office for the U.S. Thanksgiving holiday. We will reply to this thread when we return on Monday, 12/2.

    Louis

  • 0 in reply to Louis L
    Intellectual 325 points

    I found application note TI Designs: TI TIDU361A

    In the attached diagram taken from TI TIDU361A. The Vin- of INA is connected to the negative power supply. In my case the negative power supply is -5 V. The same Vin- is also connected to GND_INA in the diagram, which is the ground pin of the INA device. This look like the GND pin of INA is connected to negative power supply which is -5 V. 

    I have marked two circles in red in the attached diagram. The red circle on the top is Vbus of INA. What will be the voltage of Vbus connected to Zener diode ? The resistors R1 and R2 making voltage divider also connected to Vbus. Do we need to make Vbus positive ? What is Vbus voltage ?  

    The other red circle on the bottom in the diagram. Is that "Vs" of INA ? With Zener diode (+4.7 V) and resistor Rz on the bottom, are we making Vs positive ?  

    According to datasheet, the pin Vs of the INA3221 is Analog Power supply, 2.7 V to 5.5 V. 

     

  • 0 in reply to John B
    Guru 296910 points

    That circuit uses the voltage divider and the 24 V Zener to ensure that the voltage at Vbus is in the valid range. (Note that the load uses a −48 V supply.) With the INA3221, Vbus is allowed to be between 0 V and 26 V (relative to the GND pin).

    Yes, that unlabelled connection is the INA226's supply. When you have proper GND and −5 V supply rails, you do not need a Zener.

    How many of the other digital outputs of the INA3221 do you need? The ISO1643/4 have additional GPIO channels.

  • 0 in reply to Clemens Ladisch
    Intellectual 325 points

    @

    Thank you very much for your comments. I learn a lot from your comments. This help to understand the circuit diagram I posted in the original post taken from an application note in which INA226 was used as bus and shunt monitor for negative power supply -48 V.

    In my application, I have -5 V instead -48 V. The negative power supply -5 V is connected to a load. The load current is not more then 10 mA.

    The power supply pin of INA226 (or INA3221) is Vs and according to datasheet Vs has to be within 2.7 V and 5.5 V with respect to ground.

    If I connect the INA_GND to -5 V and Vs to the 0 V which is PCB_GND (the signal and power ground for positive power supply on the PCB).

    If the Vs pin is connected to 0 V (PCB_GND) and the INA_GND is connected to -5 V, then Vs is INA_GND + 5V already. Do I need to connect a diode, marked as "D2" red in the attached circuit ? Do I need Rz ?

    Regarding Vbus, if I also connect Vbus to 0 V (PCB_GND) then then Vbus is also INA_GND + 5V already. Do I need to connect a diode, marked as "D1" red in the attached circuit ? Do I need R1 and R2 ?

  • 0 in reply to John B
    Guru 296910 points

    You do not need either diode.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 23730 points

    Thanks Clemens for your support on this.

    John,

    Clemens is correct that for your application you will not need either diode. You will also not need the voltage divider (R1 and R2, since your VCM is within limit with respect to device GND.

    Regards,

    Mitch