Operational Amplifiers, voltage references, and other complex analog devices are modeled in Spice using
a sub-circuit. The device manufacturer commonly supplies this model, which comes with the simulator
software or is downloaded by the user. These sub-circuits do not participate in the Monte Carlo analysis,
or if they do, the sub-circuit does not have identified components whose values correspond to the
physical device parameters you would like to vary. If all sub-circuit component values are varied, the
model will produce behavior that does not correspond to the physical device.