The Circuit Designer's Companion - diagramas.diagram...

Grounding and wiring 17Unit AUnit BEarth0V0VEarthFigure 1.15 Inter-unit ground connection via the mainsV nmains wiringlive and neutral conductors in the equipment mains cable. But this configuration cannotbe prescribed: it will be possible to use outlets some distance apart, or even on differentdistribution rings, in which case the ground connection path could be lengthy and couldinclude several noise injection sources. Absolute values of injected noise can vary fromless than a millivolt rms in very quiet locations to the several volts, or even tens of volts,mentioned in section 1.1.6. This noise effectively appears in series with the signalconnection.In order to tie the signal grounds in each unit together you would normally run aground return line along with the signal in the same cable, but then• noise currents can now flow in the signal ground, so it is essential that theimpedance of the ground return (R s ) is much less than the noise sourceimpedance (R n ) − usually but not invariably the case − otherwise theground-injected noise will not be reduced;• you have created a ground loop (Figure 1.16, and compare this with section1.1.4) which by its nature is likely to be both large and variable in area, andto intersect various magnetic field sources, so that induced ground currentsbecome a real hazard.V g = V n · (R s /[R n + R s ])Unit AUnit BEarth0V0VEarthground loopV nmains wiringFigure 1.16 Ground loop via signal and mains earths