In the above calculations it is assumed that the input impedance is infinite and there is no current flowing into the inverting input (–). But actually a small amount of current must flow into the inputs due to bias requirement or parasitic leakage in real op-amp. The input bias current can either flow into or out of the inputs.

If the input bias current is considered in Equation 2 and Equation 5, they can be revised as follows:

Then ISO_POS and ISO_NEG can be obtained including input bias current error respectively.