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INA240-Q1: Input Bias Current at Vs=0V

Koji Hayashi
Intellectual 955 points
Part Number: INA240-Q1

Hi all,

When Vs=0V,IB(Input Bias Current) is indicated on Fig.15 of DataSheet

In order to suppress this current, we want to suppress the current by adding an FET between GNGPIN and GRANDPLANE.

Is it possible to suppress the IB in this way?

Also is there any influence on the electric characteristics when FET is put in?

Best Regards
Koji Hayashi

  • 0
    TI__Genius 17990 points
    Hello Hayashi-san,

    Yes, it is possible to suppress IB by adding a FET between GNDPIN and GNDPLANE. Just make sure to note that the voltage at GNDPIN will be slightly different from the GNDPLANE, so make sure to measure the output with respect to REF or GNDPLANE, but not GNDPIN.

    For you second question, although there will be a slightly different voltage at GNDPIN, the devices electrical characteristics should not see much change while running because the device has a high PSR. However, I recommend making sure that you set the Vs to 0 before you open the FET and disconnect GND from the device.
  • 0 in reply to Mitch M
    Intellectual 955 points

    Hi Mitch-san

    Thak you for four support.

    When adding FET, does REFPIN connect to GNDPLANE?

    Or is it connected to GNDPIN?

    Written on P.12 of datasheet.

     NOTE Do not connect the REF1 pin or the REF2 pin to any voltage source lower than GND or higher than VS.

    Best Regards
    Koji Hayashi

  • 0 in reply to Koji Hayashi
    TI__Genius 17990 points

    Hello Hayashi-san,

    Either would be fine to do, but there are some trade-offs. For example, if you connect the REFPIN to GNDPLANE then you will still have a path to GND when your FET is open, although this would be very high resistance and would probably only leak on the order of micro amps. However, with this setup you would have a more accurate measurement than connecting the REFPIN to GNDPIN because when connecting to GNDPIN whatever voltage offset your FET introduces will show in your measurement. On the other hand, connecting to GNDPIN allows you to completely remove GND from your device when the FET is open, avoiding the small current leak. So it really depends on what is more important to your design.

  • 0 in reply to Mitch M
    Intellectual 955 points
    Hi Mitch-san

    Thak you for four support.

    I was understood.
    When Vs=0V,it becomes goal is to less than 1uA.
    Therefore, I would like to connect REFPIN to GNDPIN.

    Best Regards
    Koji Hayashi