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INA241A: INA241A IN+/- Pins Floating to -7.4 V

Christopher Liu
Prodigy 20 points
Part Number: INA241A


Hi,

I'm using the INA241A2 current sense amp with 5mOhm sense resistor to monitor high side currents from an H-bridge driver. I was testing the circuit out with just a resistive load (~7Ohm) across J4 (pins 1+3) and a DC power supply in current control mode to output 3A across AC1 and J4 pin 3. I'm set up in a split reference configuration since I want to measure bidirectional current. The OUT voltage looks correct nominally sitting at 1.7V with no current thru the resistor, but when I went to energize, I would effectively see unity gain on the output instead of the 20X gain on the output of the amp.

I disconnected the load and power supply so the amp was biased but it's input terminals were floating and saw -7.4V sitting on the IN+/- pins. I thought this was particularly weird since there was only a 3.3V supply going to the device but it appears the amp was generating this voltage itself. Also seemed like this was exceeding the rated operating parameters of the device with IN+/- lower than -5V

I tried plugging the resistive load back in and tied J4 pin 3 to ground - this obviously dropped the voltage sitting on the IN+/- pins to 0V. I powered on the DC supply and was able to measure the proper gain output of the amp of 20x (.005 Ohm x 3A x 20V/V = 0.3 V on Vout).

I then played around with putting resistors from GND to IN+/-. I found that a 1MOhm dropped the voltage from -7.4V to -7.3V or so and gave a pretty noisy current output. Next value was 20kOhm which dropped the -7.4V to -1.3V or so and gave a pretty clean current output signal.

It seems like I may have a possible solution but this seems like very weird behavior to me - especially in the fully floating case - any ideas on why this is happening?

Thanks,

Chris

 

Schematic:

OUT voltage signal with 3A through sense resistor (no modifications to the schematic/board)

Measurement on IN- tied to GND then released so it was floating - you can see it pulls down to -7.4V within 100 us.

Adjusted circuit with the return of the resistive load tied to ground - good OUT voltage with appropriate gain value.

  • +1
    TI__Mastermind 32790 points

    Hello Chris,

    I don't understand the first issue with the unity gain unless your input common mode voltage is beyond the range of the device.  I normally see issues when this voltage is outside the common mode range reference to the gnd pin.

    The other issue of the device having a negative input voltage when left floating is know and is due to a charge pump internally created to be able to accept voltages below 0V down to -4V.  This is very week an cannot drive much current so any load would bring it to closer to the voltage driven on the pins.

     INA296A: maximum / minimum negative voltage appears on input pin 

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Thanks Javier, I realized I didn't have a proper ground between the supply and current sensor. I was further thrown off by the negative voltage and figured something was wrong but everything is working now.

    Thanks!

    Chris