Hello-
I am having an issue with a simple inverting op-amp configuration. I am trying to demonstrate to a colleague the effects of resistor tolerances on the gain of the inverting amplifier including real opamp parameters. If I use an ideal opamp model with practical parameters for things like open-loop gain, input impedance, input bias current, etc., the Worst-Case Analysis using an AC Analysis does the correct thing by skewing R2 HIGH while skewing R1 LOW (or vice versa). However, if I use a SPICE model for a real world opamp (in this example, the NCV33074ADTBR2G), then for some reason, the Worst Case Analysis is skewing both resistors in the same direction (either both higher or lower) and saying that is the worst case scenario. I have tried so many permutations of doing the simulation with the parameters. I have also confirmed that if I just do regular AC analysis, but skew the resistors manually, the circuit behaves as I would expect.
Does anyone know what is causing this anomaly? And if so, how I can prevent it from happening?
Without bogging down with too much detail, I am doing AC analysis with my output variable set to V(out), and the collating function is a specific frequency (10kHz).
These are set to be appropriate for the given circuit, and like I said, I have tried dozens upon dozens of permutations of fiddling with these parameters (even changing which resistor is listed first in the tolerance list).
Please help,
Brian
