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54 DESIGN007 MAGAZINE I JULY 2019 Article by Chang Fei Yee KEYSIGHT TECHNOLOGIES This article discusses the impact of intercon- nection inductance on the impedance of the decoupling capacitor, which influences the power integrity of the PCB. The investigation is performed with 3DEM simulation by vary- ing the trace length and height of stitching vias that connect the decoupling capacitor across the power rail and ground. On a PCB, a power distribution network (PDN) with low impedance across the wide- band is required to transfer power with low switching noise and high stability from the supply to the digital and analog ICs. Each de- coupling capacitor—together with its intercon- nection inductance—are the major factors that contribute to the impedance of the PDN on a PCB. As shown in the cross-sectional view of the PCB depicted in Figure 1, interconnection inductance is formed by the traces and stitch- ing vias hooking up the decoupling capacitor across the power rail and ground (e.g., Loop 1, Loop 2, and Loop 3). This parasitic induc- tance is directly proportional to the stitching via height and trace length, as governed by Equations 1 and 2, respectively. Furthermore, referring to the directly propor- tional relationship between impedance and in- The Impact of Inductance on Impedance of Decoupling Capacitors Figure 1: Interconnection inductance associated with a decoupling capacitor.

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