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AD8638 Input Bias vs. Offset-Current

In the datasheet for the AD8638 I don´t understand how the input offset current can be much larger than the inbut bias current.

E.g. on page 4 of the datasheet at the specs for V(sy)=16V there is 1pA/75pA (typ./max.) input bias current at 25°C, but 20pA/70pA (typ./max.) offset current.

How is that possible and why is it so?

  • Hi,

    This part is a auto-zero CMOS input opamp which bias and offset current is largely influenced by leakage from ESD protection components and other circuits connected to the inputs. Also, the input architecture has switches that chop’s and autozero’es.

    May I know what application are you working on and why are you concerned considering the small values of this parameter?

  • Basically it is used for driving a precision current source by measuring the voltage over a 10 Ohms shunt-resistor on the inverting input and a set-voltage on the noninverting input derived from a voltage divider with up to 1 Megohm resistance. The offset-current of typ. 20pA would generate an error of up to 20µV and thus nearly ten times as high as the error from the offset voltage. This is too high for my application where the MAX4238 has much better specs but only works with up to 5 or 6V supply voltage.

    But still I can´t understand how the input offset current (defined as the max. difference between IB+ and IB-) can be greater than twice I-Bias (and still think that it´s a typo error)

    Compare this to the companion chip ADA4638. There everything is as it should be. At a supply voltage of 10V (page 4) I-Bias and I-Offset are of the same order of magnitude. And figures 10...14 (which arre missing from the datasheet for the ADA8638) show the typical behaviour of I-Bias.