I was going through data sheets of op amps and saw that bias currents usually add unwanted voltages due to voltages generated from the bias current flowing through the source impedances.
Bias currents are the dc currents for biasing the input differential stage of the opamp. Because these currents are very small (<1µA) these currents, in many cases, are neglected during calculation of the feedback network. However, there are some cases requiring rather large resistors (some hundreds of kOhms) which cause a dc voltage drop that might be unacceptable. (It acts like a dc offset and causes a dc shift of the output operational point).
In those cases, we can make something like balancing/compensating these unwanted dc voltages at the input. That means: We try to make the dc voltage drops equal at both opamp input terminals.
Example: Inverting amplifier with a feedback resistive chain 500k-1k (gain of -500). In this case, we could ground the non-inv. opamp terminal via an additional compensating resistor Rc of app. (1k||500k~1k).
In this example, it was assumed that the signal source has an internal source resistance Rs=0. If this is not the case, this resistance Rs has to be taken into account (in series to the 1k resistor) - for computing the gain value as well as for finding the proper compensation resistor Rc.