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This paper focuses on the electromagnetic emissions of smart-power integrated systems that include analog, digital, and power sections. With reference to common high-voltage CMOS technology processes, it is shown that the switching noise of digital core blocks can propagate to the power sections through the silicon substrate they share. Such disturbances feed the printed circuit board traces and cables connected to the IC power section, which behave like parasitic antennas, and unwanted electromagnetic emissions are experienced. A generic integrated system fabricated with a planar CMOS technology process is considered and the propagation of digital switching noise is analyzed referring to a system level equivalent circuit, which comprises a substrate model and an IC package model. Based on this, the circuit parameters that affect the propagation of the switching noise are highlighted and a new grounding scheme, which reduces the substrate parasitic coupling in smart-power system-on-chip, is proposed. Its effectiveness is proved through computer simulations and experimental tests.