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INA219: Common mode question

Vincent Chen64988
Expert 4480 points
Part Number: INA219
Other Parts Discussed in Thread: INA193, INA198, INA196

Hi Team,

My customer is designing a circuit the block diagram as below.

He chose INA219 as a solution to monitor current and voltage. I am wondering the circuit is workable or not, because the supply voltage is negative and it will conflict the common mode range(-0.3V ~26V), am I correct?

If INA219 is not a candidate, what solutions we can provide? The package as small as possible, it is better to have SOT-23 package like INA219.

Thanks very much.

Vincent Chen

  • 0
    TI__Expert 4480 points

    Hi Team,

    Please find circuit below.

    Thanks a lot.

    Vincent

  • 0 in reply to Vincent Chen64988
    TI__Mastermind 25155 points
    Hello Vincent,

    Thank you for reaching out on the forum. You are correct, your customer will run into issues if their common mode is below -0.3. Unfortunately we do not have any device with I2C capability with a common mode lower limit below -0.2V. However, if your customer has an available ADC on their microcontroller they may consider the INA193, which has a lower common mode limit of -16V and is available in a SOT-23 package.

    Alternatively, if they must have built in I2C functionality, they will need to find a way to bring the device ground below or at the common mode voltage.
  • 0 in reply to Patrick Simmons
    TI__Expert 4480 points
    Hi Patrick,

    Thanks a lot for the advice.
    Customer mentioned it's a reference design from vendor who build 100G optical transceiver, it is quite weird. I am asking for any test data from his vendor, then hopefully we can find any clue.

    Best regards,
    Vincent
  • 0 in reply to Vincent Chen64988
    TI__Expert 4480 points

    Hi Patrick,

    Customer changed solution to INA198 and below is his new circuit. I thought the circuit is OK, could you please provide me your advice?

    Thanks very much.

    Vincent

  • 0 in reply to Vincent Chen64988
    TI__Mastermind 25155 points
    Hello Vincent,

    This looks fine, but your customer should be aware that there is an offset voltage at the input that will make the output greater than 0V, even if there is no load current. This offset is 2mV max. Which means that for any current at or below 0.002V/0.8ohm=2.5mA will results in a large measurement error.
    One other thing to note is the input filter will introduce a gain and offset error. So if the input filter is not absolutely needed, your customer might consider removing it.
  • 0 in reply to Patrick Simmons
    TI__Expert 4480 points
    Hi Patrick,

    If so, can we suggest customer to use INA196(20V/V) then the shunt resistor can be 4ohm then the max. offset error will be reduced to 0.002V/4ohm=0.5mA. Is that workable?

    Thanks very much.
    Vincent
  • 0 in reply to Vincent Chen64988
    TI__Mastermind 25155 points
    Hey Vincent,

    Yes increasing your shunt will allow you measure lower currents with greater accuracy. The value you calculated is reasonable approximation of your lower bound.
  • 0 in reply to Patrick Simmons
    TI__Expert 4480 points
    Hi Patrick,

    Got it, thanks very much for your advice.

    Best regards,
    Vincent Chen