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INA191: Negative terminal measurement

Nicolás González
Prodigy 110 points
Part Number: INA191
Other Parts Discussed in Thread: INA2191, INA2181

Hi, 

I am planning to use this device with a shunt resistor for measuring a 54 V battery current. Would it be any problem if I do this measurement in the negative terminal of the battery? I'm a bit messed up with the ground references and all that stuff.

Best regards,

Nicolás

  • 0
    Guru 140200 points

    Hi Nicolas,

    of course, you can. But at zero battery current the output of INA191 cannot go all the way down to signal ground. See "Vsn" and "Vzl" specifications in the table of section 6.5 of datasheet and see section 7.3.7.

    In this case connecting Vref (of INA2191) to a bias potential >0V will shift the output signal by the amount of this bias potential and zero battery currents can be measured again.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 110 points

    Thank you for your kind answer. Let me check if I have understood correctly:

    In the case that I am supplying the device from +3.3V LDO, now I only have to connect GND pin to other reference (for instance +1.2V) and then consider it for the amplifier gain.

    Am I right?

  • +1 in reply to Nicolás González
    Guru 140200 points

    Hi Nicolás,

    not to the GND but to the REF pin. You would need to take the INA2191 in this case, because the INA191 does not have the REF pin.

    Imagine you get a voltage drop across shunt of 0V...10mV at the inputs of INA2191A3 (gain = 100) and you give a bias voltage of 1.5V to the REF pin, then the output voltage of INA2191A3 would be 1.5V...2.5V.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 110 points

    Nice! I understand now. Thank you so much Kai.

    Best regards!!

  • 0 in reply to Nicolás González
    TI__Mastermind 19445 points

    Nicolas,

    By placing the shunt at the negative terminal of the battery, you are performing what we call a "low side" measurement, as the common mode voltage across the shunt and therefore the input pins of the device will be approximately GND. Ensure that if you decide to move to the high side of the battery, i.e., the positive terminal, that you choose a different device, as the common mode of the INA191 tops out at 40V, and the 54V common mode would damage the device. 

    Kai is spot on here regarding the device's ability to swing to ground. Placing a reference voltage that sits above this limitation is ideal, as it will ensure the device remains in linear operation all the way down to 0A.

  • 0 in reply to Carolus Andrews
    Prodigy 110 points

    Hello Carolus,

    Thanks so much for you kind advice. Not planning to move to the high-side. But anyway, is there any other device that you would recommend for low-side current sensing?

  • +1 in reply to Nicolás González
    TI__Mastermind 19445 points

    Nicolas,

    Any of our devices that can withstand a 0V common mode should be capable of a low side implementation. It really comes down to what you need in the system, i.e., cost, performance, features (comparator integration, digital power monitoring, enable pin, etc.) Is there any way you could share a bit more of your spec for what you're looking for here?

    From the above, the INA2191 is a dual channel version of our popular INA19x family, which is made for low current precision measurements (due to its 12uV offset, 120dB CMRR, and picoamp iB). Do you need a dual channel solution here? If you don't necessarily need this precision performance and are more cost sensitive, you may have a look at the INA2181, which is a cost optimized dual channel amplifier also capable of low side bidirectional measurement.

  • 0 in reply to Carolus Andrews
    Prodigy 110 points

    Thank you so much for your detailed answer!