Most simulators cannot currently take advantage of all the functionality afforded by complex behavioral models. They are typically layout-dependent and require other simulation tools for evaluation. They are not critical during functional simulations. When doing a mixed-signal simulation, the digital domain models functionality and variations in timing delays as a function of supply voltage, temperature and process corners. With this in mind, even though analog cells are dependent upon other variables such as noise, coupling, and frequency, the behavior of the analog model should be limited to functionality and timing.
Timing is dependent upon the cell s internal structure as well as external loading conditions.
Analog behavioral models consist of mathematical expressions, control logic, and analog operators to describe functionality of the cell. These equations and analog operations are constructed with analog cell characteristics such as gain, offset voltage, and propagation delay. These characteristics are measured when the actual transistor-level cell design is being characterized for each variation in supply voltage, temperature, and process corner. These parameterized expressions determine the intrinsic delays for the analog model and are dependent upon the same design variables as their digital model counterparts. Modeling load dependencies To model load dependencies, each analog model is created with an input and output impedance.