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Delivering power to a digital load is an AC function, not DC. That simple statement may be obvious, but the implications of how electricity travels to an electronic load are complicated. Dynamic loads with rapidly changing currents create electric and magnetic fields that adhere to Maxwell’s equations. Ground can be misleading and is probably better used for describing where one grows potatoes and carrots1.
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 Telegrapher’s Equations. This enabled engineers to design systems that could deliver power/signals over long distances with the understanding that impedance is critical, and that reflections can destroy a circuit. Combining this with the classic power integrity (PI) equation for target impedance provides the basic rules of thumb for delivering 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 current di/dt load times the impedance of the power delivery network.
How does an engineer know that the return path is where they expect? It can be expensive to wait until a design is complete, like the design in Figure 2, to see if crosstalk and radiation from the return currents are causing EMI/EMC test failures. How does the engineer know if the power delivery network is impedance-matched to the load? Multiple resonances caused by poor impedance matching can result in tough-to-troubleshoot field failures from rogue voltage waves.
To continue reading this article, which originally appeared in the October 2025 edition of Design007 Magazine, click here.
