Calculating the input current that can be taken by an Operational amplifier

Question

Is there a way to calculate the input current that can be taken by an Operational amplifier, if we know its input Bias and input offset current?

Answer 1

Offset current is just the difference between bias currents on the two inputs and this can be regarded as fairly constant. For non inverting configurations, input current is bias current plus a dc current proportional to input voltage due to non infinite input resistance and, for higher frequencies, there is a noticeable ac current due to non infinitesimal input capacitance usually a few pico farads.

There are also noise currents if you want to consider those.

Answer 2

For calculation of these currents (caused by input signals) you need some inputs - either derived from data sheets (with very large tolerances) or from measurements. The latter can be done as follows: After determining the offset voltages and bias currents you can detect the additional output voltage which is created by the current-resistor product at the opamp`s input - of course, with resistive feedback.

Two alternatives: (a) Large resistor in series with the non-inv. input and low-resistive feedback (with gain); (b) Large resistor between the common node of the feedback resistors and the inv. opamp input.