opamp offset voltage simulation and measurement
OP Amps have very high differential gains and any small offset voltage can saturate an OP Amp to the positive or negative supply rails. OP Amp simulation in SPICE can be difficult especially in the open loop as you have to find the exact offset voltage before any kind of open loop test can be performed.
Offset voltage is an external voltage applied at the OP Amp input which keeps the DC level of the output at the nominal common mode value. If we do not apply proper offset voltage at the input terminals, the OP Amp devices may not biased correctly and any AC analysis will use models extracted in the wrong device operation regions.
There are many different simulation methods to find offset vo ltage. Fig. . shows one of them, which will be used for grading. Apply differential voltage sources at the OP Amp inputs and sweep their DC values until your output voltage becomes exactly the common mode output voltage (2 V in our case). Note that you should sweep your DC sources with enough resolutions since the offset voltages are in milli/micro volt ranges. This method is not practical for lab use, however, it works well for simulations.
Do not forget that every design change, no matter how simple it may be, can change your offset voltage completely. Try to develop a script that can extract your offset voltage quickly, and make sure that you find your offset voltage after every single design change.