In this paper, the EMI filter is first characterized by S-parameters. Based on this S-parameters model, the insertion voltage gain of EMI filters with arbitrary source and load impedances are derived. Experiments are carried out to verify this approach. The S-parameters with current bias are also measured to characterize the EMI filter under current bias conditions. The concept of waves is then used to interpret the impedance requirement of the filters. Finally, the predictions on filter insertion voltage gain with practical source impedances are compared with the results from the measurement and the effects of the source impedances are analyzed.