One of the popular design methodologies for power distribution networks (PDNs) is to identify a target impedance to be met across a broad frequency range. The methodology is based on the assumption that the ratio of the timedomain maximum output voltage noise to the multiplication of target impedance and time-domain maximum input current is no more than one. In this paper, the ratios for different impedance profiles are analyzed, and the assumption is proved to be not necessarily true. Particularly, for secondorder impedances, the maximum ratio can be two. Several cases with real PDN structures are investigated to support our analysis. A real case of the complete PDN path with