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can input offset be negative for single supply mosfets ?

Krishna Kishore Nedunuri
Prodigy 80 points

Hello All,

I have implemented  current measurment using shunt and LMC6482A in differential mode. I have supply voltage from 12 V.  some how the measured out put value of the Opamp is far less then calculated value. At 0.5 amps current through a shunt of 2 mOhm, and differential amplifier gain of 133.45 I expect op amp out put of 133.45 mV. But I am measuring around 64mV.  I checked out data sheet of LMC6482A, 1.35mv of input offset is menctiond. I am bit surprised. I would like to know whetherthi offset could alo be negative draging my measurmen!!

Thanks & Regards,

Krishna

  • 0
    TI__Guru 69566 points

    Hello Krishna,

    Yes. The offset can be negative.

    The datasheet offset number is actually a "±" number. 1.35mV means the offset can range from -1.35mV to +1.35mV.

    In your case, it seems the amp has a -520uV offset, which would "drag" your output down by 69.45mV..

    So...+1mV + (-520uV) = +480uV.

    +480uV * 133.45 = +64.05mV.

    It is common for op-amps to have ± offsets, so you need to take this into account during designs. Never assume the offset will always be the same direction.

    Regards,