Interactive AC modeling and characterization of analog circuits via symbolic analysis

An advanced symbolic analyzer, called ASAP, has been developed for the automatic ac modeling of analog integrated circuits. ASAP works on a data base of model primitives and provides error-free symbolic expressions for the different system functions of analog circuits composed by the primitives. Both complete and simplified expressions can be calculated. Two simplification criteria have been implemented. The basic one is based on pruning the least significant terms in the different system function coefficients. This may yield important errors in pole and zero locations. To avoid that, an improved criterion has been developed where pole and zero displacements are forced to remain bounded. Also implemented are routines for symbolic pole/zero extraction and parametric ac circuit characterization. ASAP uses the signal flow graph method for symbolic analysis and has been written in the C language for portability. Together with portability, efficiency and ability to manage complexity have been fundamental goals in the implementation of ASAP. These features are demonstrated in this paper via practical examples.