Shielding, Grounding, Noise Suppression, etc.

Grounding

Types of Grounds

There are generally three types of gounds: earth ground, signal ground, and chassis ground. Their respective symbols are shown below:

Each of these symbols represents a common node, but there are some differences.

The earth ground denotes a physical connection to the earth, which is conventially defined to be 0 volts. On electrical appliances, this is the third pin on the power plug. However, many circuit diagram will use this symbol to designate a 0V reference even if there is no physical connection with the ground.

The second symbol, the signal ground, is used as a reference for data transmission lines. The signal ground is generally not connected to the earth to isolate it from electrical noise and interference from electric currents and fields in the earth. Thus, there is often a significant voltage difference between the signal ground and the earth ground.

The third symbol is known as the chassis ground, and is usually connected to the conducting shell or chassis of that particular piece of equipment. For example, the negative termial of a car battery is wired to the metallic shell of the car. This provides a reference voltage that may or may not be connected to the earth, although for most appliances it usually is. Not connecting the chassis ground to the earth ground can result in a voltage difference between the chassis ground and the earth ground, which leaves the risk of electrical current (including static electricity) finding a path between the chassis and earth through something...or someone. This is the reason why fuel trucks must be grounded while dispensing fuel—the voltage difference can result in a spark and explosion.

Grounding Considerations

Because of the resistance of our conductors, it is possible for our ground wire to have difference voltages at different points along the wire when current is flowing through it. To keep wire resistance and voltage differences at a minimum, there are several things we can do:

• Use large ground wires and thick PCB traces, and physically attach the grounds close to each other.
• The ground wire should also not be connected in a loop with itself; changing magnetic flux through the loop can induce a current in the loop.

Power circuitry often causes electrical noise on the power supply. The power supply may also see voltage spikes from commutator brushes or other circuitry.

• Put a capacitor across power and GND. A large capacitor (10's or 100's of uF) can filter out low frequency changes in the voltage—it acts as a sink that can absorb large changes in voltage (remember, the voltage across a capacitor cannot change quickly, but the current can). Capacitors often used for this purpose are polarized tantalum electrolytic capacitors.