Analog/Mixed-Signal Behavioral Modeling

From Cadence... (see the original)

Analog/Mixed-Signal Behavioral Modeling – When to Use What

By Richard Goering on February 20, 2011

So when to use what? The conservative style provided by Verilog-A and Verilog-AMS is useful when there are significant accuracy requirements. This approach can potentially provide a 50-100X speedup over SPICE, but it all depends on how good your modeling is. "If you're a poor modeler, there's a chance you could end up with a model that's as slow as SPICE simulation or even slower," Walter warned.
Real  number modeling, also available through Verilog-AMS with the wreal data type, brings real number values into event-driven digital simulation. It thus has the speed benefits of digital simulation and can leverage the metric-driven verification methodology that's increasingly used by digital engineers. It's good when there are hard performance requirements and limited accuracy requirements. For example, wreal is very useful for full-chip mixed-signal simulations.
The following chart shows the accuracy/speed tradeoff ranges provided by various analog/mixed-signal modeling alternatives. Note that the conservative modeling style has a broad possible range, depending on how good the modeling is.
Also important is the modeling effort. Here we can see that conservative models require the most amount of effort. "You can potentially spend days, weeks, months to develop good behavioral models," Walter said. Wreal models are relatively fast to develop because they're less detailed. An important rule of thumb: "Model what you need, not what you can."