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INA138: error rate caused by VOS

Fred Cheng
Expert 5470 points
Part Number: INA138

Hi team,

below is customer's spec,  RL=516kohms

you can see the 21% error rate is expected, and yes they now report that they see output 0.38V which ideally should be 0.516V

However, they've doing this design for generations ,why they didn't encounter this until now?

is VOS random scenario?

  • 0
    Guru 140200 points

    Hi Fred,

    1A times 5mR gives a voltage drop across the shunt of 5mV. The maximum input offset error voltage of INA138 is 1mV, which is 20% of 5mV. The typical input offset error voltage of INA138, on the other hand, is only 0.2mV, which is only 4% of 5mV. So 20% is the worst case error here and 4% is the typical error (in both cases only the input offset error voltage of INA138 is considered in this error calculation).

    But why are you using such a small shunt? Section 8.2.1.2.1 of datasheet says:

    "For most applications, best performance is attained with a shunt resistor value that provides a full-scale shunt voltage range of 50 mV to 100 mV. Maximum input voltage for accurate measurements is 500 mV."

    From this point-of-view your shunt would be way too small.

    Kai

  • +1 in reply to kai klaas69
    TI__Mastermind 30790 points

    Fred,

    I agree with Kai.  You just happened to have mostly units that were in the typical specification.  VOS will vary from lot to lot and one should always use the datasheet maximum as a guide of worst case.