Behavioural modelling of mixed-signal circuits using PWL waveforms

The development of mixed signal modelling languages such as VHDL-AMS and Verilog-A allows the behaviour of analogue and mixed signal circuits to be described at a higher level than is possible with conventional circuit-level (e.g. SPICE type) simulators. Mixed-signal simulators for these new languages can process such behavioural models far more efficiently than was previously possible. The non-linear characteristics of many circuits require differential equations to be used in the behavioural descriptions. Unfortunately, the methods used to solve these equations in a mixed-signal simulator are more computationally demanding than those required to evaluate the logic expressions used to describe digital circuits. Consequently, the simulation of analogue and mixed-signal circuits is still far less efficient than the simulation of a pure digital circuit. This paper presents an alternative approach to modelling the behaviour of analogue and mixed-signal circuits based upon piecewise linear (PWL) waveforms. Since the signals processed by these models are linearised, direct methods (i.e. non-iterative) can be used to simulate their behaviour. Several alternative methods are discussed. Examples showing the application of this approach to commonly used circuits are developed and the results compared against corresponding models written for conventional simulators. Finally, suggestions are given for implementing these techniques in a VHDL simulators