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This paper presents a systematic, hierarchical, optimization based semi-formal equivalence checking methodology for large analog/mixed signal systems such as PLLs, ADCs and I/O's. We verify the equivalence between a behavioral model and its electrical implementation over a limited, but highly likely, input space defined as the Constrained Behavioral Input Space. Further, we clearly distinguish between the behavioral and electrical domains and define mappings between the two domains to allow for calculation of deviation between the behavioral and electrical implementation. The verification problem is then formulated as an optimization problem which is solved by interfacing a SQP based optimizer with commercial circuit simulation tools. The proposed methodology is then applied for equivalence checking of a PLL as a test case.