Input Current – Operational Amplifier

A certain amount of bias current is required at each input. The input bias current, IIB, is computed as the average of the two inputs,
IIB = (IN+IP)/2
Input offset current, IIO, is defined as the difference between the bias currents at the inverting and non-inverting inputs,
IIO = IN-IP
Bias current is of concern when the input source impedance is high. Usually offset currents are an order of magnitude less than bias current so matching the input impedance of the inputs helps to nullify the effect of input bias current on the output voltage.