As electronics developed, the common return paths of various circuits were also referred to as “ground,” regardless of whether or not they were eventually connected to earth. In addition, a single ground circuit most often serves, either intentionally or accidentally, more than one purpose. Thus, the very meaning of the term ground has become vague, ambiguous, and often quite fanciful. Some engineers have a strong urge to reduce these unwanted voltage differences by “shorting them out” with massive conductors — the results are most often disappointing.
Other engineers think that system noise can be improved experimentally by simply finding a “better” or “quieter” ground. Many indulge in wishful thinking that noise currents can somehow be skillfully directed to an earth ground, where they will disappear forever! Here are some common myths about grounding:
Earth grounds are all at zero volts — presumably with respect to each other and to some “mystical absolute” reference point. This leads to whimsical ideas about lots of ground rods making system noises disappear! In fact, the soil resistance between ground rods is much higher (often tens of ohms) than a wire between them.
Wires have zero impedance — and, therefore, can extend a zero-voltage reference to many locations in a system, eliminating voltage differences. In fact, wires are quite limited:
• The DC resistance of a wire applies only at very low frequencies and is directly proportional to its length. For example, the resistance of 10 feet of #12 gauge wire is about 0.015 S.
• The inductance of a wire is nearly independent of its diameter (gauge) but is directly proportional to its length and increases at bends or loops. Our 10 feet of #12 gauge wire has an impedance of 30 S at 1 MHz (AM broadcast band) as shown in the graph. Substituting a ½-inch diameter solid copper rod lowers the impedance only slightly to about 25 S.
• A wire resonates (becomes an antenna) when its physical length is a quarter wavelength. For a 10-foot wire, this means it will essentially become an open circuit at about 25 MHz.
Are EARTH grounds really necessary for low-noise system operation? Think about all the electronics in an airplane!