So, where is that mixed-signal behavioral model I ordered?

It has been said many time that SPICE, the analog engineers tool of choice, is virtually the same as it was 20 years ago, while digital engineers have been happily zooming up the evolutionary chain. There have been a number of attempts to prod analog designers into closing the gap with the introduction of behavioral modeling languages, and more abstract system modeling solutions. However, they are not widely adopted, much to the disappointment of the EDA vendors that provide them. So why not?

Analog designs are, by their very nature, multidimensional, and any assumptions you make to model one circuit are completely inappropriate to model another. Modeling requires that you make some decisions on which characteristics are important and which are not, with more decisions needed on which conditions to model over, and which to ignore. So standardized modeling techniques go out the window, leaving every model as unique as the design it replicates.

This isn't all doom and gloom, since there are many ways to leverage behavioral modeling. In the early years of the behavioral languages, users discovered that they could leave the design alone and instead model the test fixtures. Since only the design under test remains in SPICE, the simulations run faster and the test fixtures are more readily reusable

Today, there are more tools coming out to support the model creation process and they adopt a variety of different strategies, from fully automatic to designer assisted. Also the modeling languages themselves are getting faster and more efficient, such that there is more impetus to push for modeling expertise within a company.

My own personal opinion is that automated tools can be very effective on some circuits, and useless on most, leaving many engineers without support. So a company has to invest in some level of modeling expertise to get the best out of the behavioral simulation technology.

Cadence is taking a number of approaches in this arena. The first is to provide a multi-disciplinary simulation solution to handle the broad range of requirements. So with MMSIM 7.X from the Virtuoso platform, you can happily simulate at the SPICE or fast-SPICE level and move up to the behavioral level as it makes sense.

For model creation, Virtuoso Analog Design Environment GXL in IC 6.X includes a Model calibration option. This enables experienced modelers to create a library of adaptable models that are flexible enough to adapt to a number of design variants. The end user can automatically configure them to their specific designs, making reuse more practical for the majority of engineers and across many design types. In both cases the control is left in the hands of the design engineer.

This is an interesting time, as the majority of designs now have both analog and digital content. The need for an effective AMS solution with modeling technology in support has never been more necessary and the EDA vendors are rising to the challenge.

What are your thoughts on this?