This paper presents a unified platform for design and verification of mixed-signal systems based on the SystemC AMS standard. The platform relies on a top-down design method- ology. 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, in order to achieve a unified and standard design environment. Moreover, a transient simulation method is proposed to simulate nonlinear dynamic behavior of complete mixed-signal systems. The unified platform is used to design and verify a pipeline ADC. The simulations results prove the effectiveness of the proposed methodology.