Switching currents in active integrated circuits (ICs) generate noise in the power distribution network (PDN), which is one of the main sources for many signal/power integrity and electromagnetic interference issues in high-speed electronic devices. Accurate knowledge of the switching currents is the key to ensure a good PDN design. This paper proposes a measurement methodology, when IC information is not available, to obtain the equivalent switching current of each IC in the case where multiple ICs are connected to a common power island structure. Time-domain oscilloscope measurements are used to capture the noise-voltage waveforms at a few locations in the power island. Combining with the multi-port frequency-domain S-parameter measurement among the same locations, an equivalent switching current for each IC is calculated. The proposed method is validated at a different location in the power island by comparing the calculated noise voltage using the equivalent switching currents as excitations with the actual measured noise voltage.