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OCTOBER 2025 I DESIGN007 MAGAZINE 17 B E YO N D D ES I G N Bulk bypass capacitors are effective up to approxi- mately 10 MHz, offering low impedance in that range (Figure 3). For higher fre- quencies, ceramic capaci- tors extend decoupling per- formance into the hundreds of megahertz. Beyond this, only on-die capacitance and the intrinsic planar capaci- tance between tightly cou- pled power and ground planes can effectively sup- press PDN impedance. This inter-plane capaci- tance acts as an ideal high- frequency capacitor, free from lead loop inductance and exhibiting minimal equivalent series resistance (ESR), making it highly effective at mitigating noise in the gigahertz range. Achieving tight coupling—typically less than 5 mils— between these planes significantly enhances this capacitance, providing a robust high-frequency energy reservoir. While signal trace lengths are often meticulously matched, the return current path is frequently over- looked, yet it plays a critical role in timing and signal integrity. Any delay in the return displacement cur- rent path, such as detouring around the plane gap, can introduce skew between timing-critical signals. The return current path also plays a pivotal role in power integrity. Discontinuities and high-impedance paths, such as splits in ground or power planes, poor return path geometry, and insufficient return vias, can severely impact signal integrity and power delivery in high-speed designs. These issues increase loop inductance, degrade decoupling capacitor effective- ness, and allow high-frequency transients to propa- gate, resulting in voltage drops, ground bounce, EMI, crosstalk, and common mode radiation. The primary aim in designing a high-performance power distribution network is to suppress impedance peaks below the target threshold and shift resonant frequency components beyond the signal bandwidth. Achieving this requires strategic mitigation of cavity resonances and electromagnetic emissions. Key Design Strategies for AC Impedance Optimization • Minimize Cavity Impedance With Thin Dielectrics: Using a thin dielectric layer between power and ground planes is one of the most effective methods for damping modal resonances. It lowers spreading inductance, reduces cavity impedance, and attenuates high-frequency peaks. A smaller plane separa- tion also reduces the equivalent magnetic cur- rent area at the plane edges, thereby shrinking the local fringing field volume and minimizing emissions for a given field strength. • Select High-Dk Dielectrics for Enhanced Planar Capacitance: Contrary to conven- tional high-speed design practices that favor low-Dk materials, the interplane dielectric should have a high dielectric constant (Dk) to increase planar capacitance. Since this dielectric lies between planes and has a min- imal influence on signal propagation, you can optimize it independently for power integrity. • Elevate Resonant Frequencies via Geome- try and Stitching: To push parallel cavity res- onances above the signal bandwidth, reduce the plane dimensions, and incorporate stitch- ing vias between similar planes. These vias disrupt standing wave patterns and help sup- press localized resonances. F i g u re 3 : Fu n d a m e nt a l t a rg et i m p e d a n c e, V R M , c a p a c i to rs , a n d p l a n e p rof i l e s . ▼