Ground - A Path for Current Flow

A signal ground is normally defined as an equipotential point or plane that serves as a reference potential for a circuit or system. This definition, however, does not emphasize the importance of the actual path taken by the current in returning to the source. It is often important that the design engineer know the actual path taken by the ground current. Only by knowing this can the designer accurately estimate the radiated emission from a circuit, or the susceptibility of a circuit to electromagnetic energy. An alternative definition for a signal ground is: A low impedance path for currrent to return to the source. This definition emphasizes the importance of the current flow in the ground system. It implies that since current is flowing through some log: but finite, impedance there will be a difference in potential between the two ends. This "current" concept of a ground is also useful in order to determine where decoupling capacitors should be connected, and explains why, in some cases, eliminating a ground may make a circuit less susceptible to electromagnetic interference.