Facebook
Google
GitHub
LinkedinJust How Accurate Is a SPICE Model?
The quality of device models from wafer fabs varies greatly. A few are outstanding, unerringly accurate, and complete. Others are so bad that they will almost guarantee major flaws in your design. The majority of them are incomplete for analog design.
Figure 3-18. Analog simulation model quality can vary greatly between wafer fabs. Image used courtesy of Adobe Stock
It pays to examine the models before starting to simulate. If the NPN transistor model is not a subcircuit, use it with caution—behavior in saturation is going to be different in the real circuit. If the lateral PNP model is not a subcircuit, it doesn't make much sense to use it at all.
A set of device models isn't really ready for use until it has been tested in actual circuits. Unfortunately, models are commonly put together by people who are not designers—especially not analog designers—so they tend not to be verified in real-world applications.
This is especially true for bandgap references, which demand uncommon accuracy from the bipolar transistor models (see the Bandgap References chapter for more info). Even a small error in VBE (the SPICE parameter for the basic diode voltage) and its temperature coefficient will cause intolerable errors in the reference voltage. Here, it is in fact preferable to set such parameters as IS and its modifiers so that they fit several designs existing in silicon.
You should also check the models for the presence of Monte Carlo parameters. If there aren't any, you're going to be seriously handicapped for an analog design.
