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A method for passivity analysis of a class of nonlinear systems is described. For linear plants controlled by sampled-data controllers, the resulting interaction admittance is nonlinear. It is shown that satisfying the linear requirements for passivity is insufficient to guarantee passivity of the nonlinear system. To analyze such plants, we first show that if a waveform constituting a counter-example of passivity exists, then a counter-example may also be found within a limited class of functions. By restricting attention to this class of functions, a numerical passivity evaluation is feasible. Examples using the method are presented.