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INA281-Q1: Input Pin Bias when Unpowered

Part Number: INA281-Q1
Other Parts Discussed in Thread: INA280, INA281

Hello, I am using the INA281A1-Q1 in a configuration for high-side current sense of a 50V supply. The Vs pin of the current sense amplifier is powered at 3.0 V. When the 50V rail is powered, the current sense resistor is biased as expected and the amplifier correctly amplifies the differential voltage and rejects the common mode offset, producing the expected output on the OUT pin. However, in our particular application there are scenarios where the amplifier is still powered at its Vs pin, but the In- and In+ pins have no voltage bias (they are essentially high impedance other than the current sense resistor) as the 50V rail is disconnected through a power MOSFET before the current sense resistor. In this configuration, we measure between -0.7 and -7V on In- and In+ with respect to ground, depending on the exact loading on this rail. This high negative voltage is causing issues with our power MOSFET's controller/driver IC.

Is is this negative bias voltage normal for the INA281A1-Q1 if the In- and In+ pins are unpowered while Vs is powered? If so, can you provide more information about the input bias circuitry? We would like to keep the input bias voltage between -0.3 and 0V when this 50V rail is off in order to not reverse bias other parts on this rail when it is unpowered.

  • Josh,

    Can you share a schematic of your setup, as well as share the nature of the load sitting below the sense resistor? When the 50V line is cut to the load, are there any additional supplies present?

    Also, I apologize in advance, TI observes Thursday and Friday as holiday for Christmas Eve and Christmas Day, so responses from our team may be delayed. Thank you for your patience. 

  • Hi Carolus,

    Please see the attached schematic above. I have tested the circuit in three conditions:

    Condition 1: NFET gate is low so drain-source is high impedance. No load is connected to the Power-Out net. The voltage I would expect on V+ and V- is 0 V, however it is -0.7V.

    Condition 2: Same as condition 1, but I add a 2.2 kOhm resistor between Power-Out and ground. The voltage on V+ and V- changes to -0.06 V.

    Condition 3: Same as condition 1, but I remove the INA280 amplifier from the board. The voltage on V+ and V- is 0V as expected.

    I am trying to determine why the INA280 is biasing V+ and V- to -0.7 V. It is causing issues with the switch controller as it does not expect to see a negative output voltage when the NFET switch is open.



  • Hi Jish,

    what you observe is quite usual. It's just the result of the input bias currents of INA281.

    Why not mounting a diode or Schottky diode across the load?

    You can also connect a low ohmic resistor in parallel - like your 2k2 resistor - to furtherly bring down the unwanted negative voltage.


  • Hey Josh,

    I would recommend Kai's suggestion with a diode at the V+/V- node to clamp this node to the forward voltage drop of the diode, which should be around 0.3V. This might be the most straight-forward solution to this issue. So the goal is to limit the input voltage of the switch controller to >-0.3V.

    Unfortunately, floating the input pins of the INA281 is not a recommended configuration because there will be no return path for the input bias currents, although it seems the V+/V- nodes are shared with the switch controller. I would also try testing what would happen if you repeat condition 1, but disconnect the switch controller from V+/V- nodes. During condition 1, INA281 input bias currents are going to be pulled out from the switch controller according to the common-mode voltage of device; however, since these pins are floating, the VCM is undefined and the V+/V- voltage is going to float to wherever the input bias currents pull it to.

    Could you explain what the front-end of the switch controller is, specifically its V+/V- nodes? I assume this controller has some internal current sense feature?

    Could you also explain what the output of the INA281 is connected to during the problem?