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8 DESIGN007 MAGAZINE I OCTOBER 2025 D elivering power to a digital load is an AC func- tion, not DC. That simple statement may be obvious, but the implications of how electricity trav- els to an electronic load are complicated. Dynamic loads with rapidly changing currents create electric and magnetic fields that adhere to Maxwell's equa- tions. Ground can be misleading and is probably better used for describing where one grows pota- toes and carrots 1 . Electrical currents have "return paths," and the energy is traveling in the fields between the power rail and the return path. Oliver Heaviside was able to provide a conceptual approach with his famous F E AT U R E A RT I C L E by H e i d i B a r n es , Keys i g ht Te c h n o l o g i e s Everyone's Talking About Power Telegrapher's Equations. This enabled engineers to design systems that could deliver power/sig- nals over long distances with the understand- ing that impedance is critical, and that reflections can destroy a circuit. Combining this with the clas- sic power integrity (PI) equation for target imped- ance provides the basic rules of thumb for deliver- ing DC to GHz of dynamic power to a digital load, as shown in Figure 1. Target impedance simply says that power rail ripple voltage should not exceed the maximum specification limits for any dynamic cur- rent di/dt load times the impedance of the power delivery network. F i g u re 1 : C l a s s i c p owe r i nte g r i t y t a rg et i m p e d a n c e e q u at i o n a n d t h e d ef i n i t i o n of m atc h e d i m p e d a n c e d e r i ve d f ro m R F/u W te l e g ra p h e r 's e q u at i o n s . ▼