Input Offset Voltage

V_OS is the Direct Current (DC) error voltage that exists between the input terminals. To understand this problem, consider the following experiment: If the two input terminals of the op-amp are connected to ground, it will be found that a finite DC voltage exists at the output. It can be positive or negative. Refer to the accompanying image.

According to the transfer characteristic of an op-amp, V_OUT = A (V_IN+ - V_IN-), there does exist a DC error voltage between the two input terminals, and the value is V_OUT/A, which is called input offset voltage V_OS.

The input offset voltage arises as a result of the unavoidable mismatches present in the input differential stage inside the op-amp.
The accompanying table is an example of the MCP601 input offset voltage specification.