Ground

From HvWiki

Ground

To a beginner "ground" can be a confusing concept. There are two main types of ground: the actual ground or earth (often called RF ground) and a region of 0 Volts in a circuit. Many books define it this way, but a more accurate definition is "a point in a circuit from which other voltages are referenced."

In most cases, "ground" is simply the return lead on the battery or power supply. "Ground" is then just a standard reference voltage level, which is usually negative. (This should not be taken for granted, as in many circuits, ground is positive. Many disposable camera circuits are positive grounded.)

However, sometimes, ground is actually more positive than other nodes in the circuit.

One common mistake is to think of the "ground" node of a circuit as being a sort of ocean of zero impedance, so all parts of the circuit connected to ground sit at exactly 0 volts all the time. This is generally not true, because the stuff the ground connections are made of will have finite resistance and inductance. Usually, the resistance causes trouble in high-current circuits, while the inductance causes trouble in circuits that handle high frequencies or fast edges.

Good circuit design tries to get the ground impedance as close to this ideal "ocean of zero" as practical (that's why PCB designers use double-sided and multi-layer boards with whole layers devoted to "groundplanes") and if this isn't good enough, carefully thought-out splitting of the ground network is used to prevent large currents from getting near to sensitive nodes.

A whole book could be written on grounding, but if you use star grounding in high power amplifiers and the like, and prototype high-speed circuits on copperclad PCB with one whole side left untouched as a groundplane, you shouldn't go too far wrong.

Fig 1. Common symbols for ground. The first is "ground", second is used as "RF ground" or earth, and the third is another symbol for "ground"

Ground reference Fig 2.

Ground reference Fig 3.