This paper presents a unified platform for design of mixed-signal systems based on the SystemC AMS standard. The platform relies on a top-down design methodology. In this methodology, several hierarchical abstraction views of the system are considered. These abstraction levels are: system, functional, macromodel and circuit levels. We introduce a simple and efficient method to implement an interface between system level models and their circuit level realizations. Simulation tools such as SystemC AMS and Spice simulators are combined with a sizing tool such as CHAMS, in order to achieve a unified and standard design environment.