Ground Plane - Printed Circuit Boards

Printed Circuit Boards

A ground plane on a printed circuit board (PCB) is a large area or layer of copper foil connected to the circuit's ground point, usually one terminal of the power supply. It serves as the return path for current from many different components.

A ground plane is often made as large as possible, covering most of the area of the PCB which is not occupied by circuit traces. In multilayer PCBs, it is often a separate layer covering the entire board. This serves to make circuit design easier, allowing the designer to ground any component without having to run additional traces; component wire needing grounding is routed directly through a hole in the board to the ground plane on another layer. The large area of copper also conducts the large return currents from many components without significant voltage drops, ensuring that the ground connection of all the components are at the same reference potential.

However, in digital and radio frequency PCBs, the major reason for using large ground planes is to reduce electrical noise and interference being coupled from one part of the circuit to another through the ground (ground loops), and crosstalk between adjacent circuit traces. When digital circuits switch state, large current pulses flow from the integrated circuits through the ground circuit. If the power supply and ground wires have significant resistance, the voltage drop across them may create noise voltage pulses in the ground wires, which are applied to other parts of the circuit, The large capacitance of the ground plane allows it to absorb the current pulses without much change in voltage,

In addition, a ground plane under printed circuit traces can reduce crosstalk between adjacent traces. When two traces run parallel, an electrical signal in one can be coupled into the other through electromagnetic induction by magnetic field lines from one linking the other; this is called crosstalk. When a ground plane layer is present underneath, it forms a transmission line (stripline) with the trace. The oppositely-directed return currents flow through the ground plane directly beneath the trace. This confines the electromagnetic fields to the area between the trace and the ground plane, reducing crosstalk.

Ground planes can also be placed on adjacent layers to power planes, creating a large parallel plate decoupling capacitor that prevents noise from being coupled from one circuit to another through the power supply.

Ground planes are sometimes split and then connected by a thin trace. This allows the separation of analog and digital sections of a board or the inputs and outputs of amplifiers. The thin trace has low enough impedance to keep the two sides very close to the same potential while keeping the ground currents of one side from impacting the other.