Grounding and shielding; inductive and capacitive shielding

Grounding

Grounding is the provision of a low-impedance path to a reference point (earth/chassis) for return currents, fault currents and noise, giving a common reference potential and safety.

Shielding

Shielding is surrounding a circuit, cable or component by a conductive (or magnetic) material to block or reduce the transfer of electric, magnetic and EM fields between the source and the receiver.

Capacitive (electrostatic) shielding

• Noise passes through stray capacitance: I = C·dV/dt.
• A grounded conductive shield (foil, braid, metal box) between source and victim splits the stray capacitance. The noise current goes to ground through the shield and not into the victim.
• The shield must be properly grounded (to the reference of the receiver circuit) and cover the receiver conductor fully. Lower coupling capacitance results.

 Source --C1--[shield]--C2-- Victim
                 |
                GND

Inductive (magnetic) shielding

• Noise voltage induced is V = M·dI/dt.
• At low frequency (below about 100 kHz), use a high-permeability material (mu-metal, steel) that provides a low-reluctance path to divert the flux around the victim. A thick shield is required, and grounding does not matter.
• At high frequency, a nonmagnetic conductor (copper, aluminium) works: eddy currents induced in the shield produce an opposing field, cancelling the incident field.
• Other ways: reduce loop area, twisted pair, increase distance, cross the wires at right angles.