What's in a SPICE Model?

What's in a SPICE Model?

Working with customers is one of the great pleasures of my job. Whether in a meeting or at a tradeshow, it’s always fun to chat with engineers to see how they use simulation in their design flows – and maybe use the opportunity to discuss how SystemVision might help their design processes. Among my favorite interactions with customers, however, is teaching simulation and modeling training classes.

Mentor Graphics offers two training classes for System Vision: Introduction to SystemVision, and Introduction to VHDL-AMS. Each class keeps students and instructor busy for a couple of days. While the Introduction to VHDL-AMS class deals specifically with modeling using the IEEE standard VHDL-AMS modeling language, the Introduction to SystemVision class, by its very nature, is a bit broader in scope. The main theme is learning how to setup and run simulations using the SystemVision environment. But since simulation depends on models, modeling topics are a common thread weaving in and out of class discussions.

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Earlier this week I taught the Introduction to SystemVision class at a customer’s site. Several engineers from different groups within the company attended. One of the drivers behind the class was a recent management mandate that all designs be simulated as part of the development process. Apparently one or two designs in the not so distant past had end-user problems that could have been found, pre-production, with simulation.

During a class section focused on modeling, one of the engineers asked a specific question about SPICE models (along with VHDL-AMS-based models, SystemVision can directly use SPICE models in a simulation). The engineer wanted to know how to relate the parameters in a SPICE model file to information in the component manufacturer’s datasheet. As part of his design process, he wants to verify the model and then vary some of the parameters for more advanced statistical and sensitivity analyses. All good goals. But it isn’t always clear what parameters in a SPICE model are responsible for specific data and curves in a datasheet. Without the ability to easily change parameters to do extended analyses, the engineer saw little value in simulation – even though it is specifically required for every product design in his company. Despite the simulation requirement, he would rather just build a prototype board and head directly to the lab to make measurements.

Unfortunately, I didn’t have an easy answer for his question. I imagine most engineers share similar views: they don’t want to be model(ing) experts — they just want to use a model to analyze their design in a way that seems best to them. I certainly understood the engineer’s dilemma, but couldn’t give him a simple point-and-click solution. While there are ways to figure out the parameter-to-datasheet connection, most require a pretty good understanding of the underlying structure of the SPICE model. But what if, like the engineer in my class, you just want a straightforward way of changing parameters to help you verify and investigate datasheet information? I’m sure the question is not unique, and that there is more than one solution. So I ask you – what’s the easiest method you’ve found for connecting SPICE model parameters to component datasheet specifications?

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