safety isolation and the prevention of fault propagation. Internal ground loops in equipment Typically these are caused by poor design. Where there is mixed-signal technology on a printed circuit board (PCB), e.g. analog, digital and possibly RF, it is usually necessary for a highly skilled engineer to specify the layout of where the grounds are to be interconnected. Typically the digital section will have its own ground plane to obtain the necessary low inductance grounding and avoid ground bounce which can cause severe digital malfunction. But digital ground current must not pass through the analog grounding system, where voltage drop due to the finite ground impedance would cause noise to be injected into the analog circuits. Phase lock loop circuits are particularly vulnerable because the VCO loop filter circuit is working with sub-microvolt signals when the loop is locked, and any disturbance will cause frequency jitter and possible loss of lock. Generally, the analog and digital parts of the circuit are in separate areas of the PCB, with their own ground planes, and these are tied together at a carefully chosen star point. Where analog-to-digital converters (ADCs) are in use, the star point may have to be at or very close to the ground terminals of the ADC(s). Differential signal transmission, optical or transformer isolation, or fibre optic links, are also used in PCBs in extreme cases. In circuit design Ground and ground loops are also important in circuit design. In many circuits, large currents may exist through the ground plane, leading to voltage differences of the ground reference in different parts of the circuit, leading to hum and other problems. Several techniques should be used to avoid ground loops, and otherwise, guarantee good grounding: The external shield, and the shields of all connectors, should be connected. If the power supply design is non-isolated, this external chassis ground should be connected to the ground plane of the PCB at only one point; this avoids large current through the ground plane of the PCB. If the design is an isolated power supply, this external ground should be connected to the ground plane of the PCB via a high voltage capacitor, such as 2200 pF at 2 kV. If the connectors are mounted on the PCB, the outer perimeter of the PCB should contain a strip of copper connecting to the shields of the connectors. There should be a break in copper