Spice is a powerful tool for circuit analysis. But it often surprises users with
. holes in its abilities and strange definitions
. lack of industry-wide standards
. the need for the user to model the physics of their circuit, not just draw the schematic
Monte Carlo analysis Limitations
but is it useful with designs based on discrete components?
The idea is
1. the user assigns a tolerance to each circuit component's value
2. Spice varies the values of all components in a random fashion within the tolerance limits
3. run the simulation
4. repeat steps 2 and 3 a large number of times, recording the circuit's output from each simulation.
The designer can then review the saved circuit outputs to get a statistical idea of how the circuit's output will vary in production (due to component tolerances).
Op amps, voltage references, power MOSFETs and other complex analog devices are modeled in Spice using a subcircuit. The device manufacturer commonly supplies this model, which comes with the simulator software or is downloaded by the user. These subcircuits do not participate in the Monte Carlo analysis, or if they do, the subcircuit does not have identified components whose values correspond to the physical device parameters you would like to vary. If all subcircuit component values are varied, the model will produce behavior that does not correspond to the physical device.
Original Spice did not have Monte Carlo so there has been no Spice standard to guide model designers and Spice software developers in making Monte Carlo work with subcircuits.
If your simulator offers this analysis and you use subcircuits, proceed with caution!
Run Monte Carlo in your simulator using a Maxim or Linear Tech op amp subcircuit and 0% tolerance gain setting resistors. Does the DC offset voltage change? Does the AC gain's high frequency rolloff change? Add a comment here with your results.