See Ground Return & Common Impedance Coupling
When two circuits share a common ground, the ground voltage of each one is affected by the ground current of the other circuit.
When two circuits share a common power supply, current drawn by one circuit affects the voltage at the other circuit.
27. January 2020 09:26 by Christian
in EMC/EMI, Load Simulator
Understanding the role of return ground is essential in EMC testing.
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See Differential Mode vs Common Mode Current
Christian Rosu, Flexautomotive.net, 2020-01-27