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Electromagnetic interference (EMI) filters are treated much as the effective equipments for controlling EMI. However, traditional EMI filter design methods are difficult, because the measurements of impedances are not easy due to the existences of parasitic parameters. In this paper, EMI filters are modeled by scattering parameters(S-parameter.) The paper also gives EMI digital filter design methods using S-parameters. Simulation is carried out to verify this design approach. After filtering, the conducted EMI produced by a DC/DC converter can meet the standard of GJB151A-91/CE102. The approaches here are very useful for the design and definition of EMI filters.