Return Current Topologies in PCB Design
A current loop is a complete path of current sent from a signal source to receiver to ground and back to signal source. The signals are transmission of current pulses that starts from a source terminal, runs through traces to a receiving device, consumes current and exits the ground pin of the receiving device that return current back to the source device. The current loop exists in every electronics circuit. The current always tries to travel on the shortest path and it reduces the noise propagation if optimized carefully.
The unnecessary longer current loops emit noise and cause various signal integrity problems. The return paths or ground are difficult to layout. It is good approach to layout the signal traces so that each signal trace would have ground return path underneath it. In this way the signal traces radiate minimum noise.
A bad ground loop or return path loop is a poorly routed ground connection so that it causes multiple orderless connected ground paths in the PCB. A lengthy loop causes undue voltage drop in the entire path due to current flow and copper resistance i.e. IR loss in the copper. The ground return path, to avoid noise propagations, should be optimized by shortening the length. A number of tips are summarized to lay down and route the return current path in PCBs.
1.In the single layer or double layer PCBs the ground should be routed in star topology to avoid ground loops. The mesh connected routing topology will create a lengthy return path.
2.In PCB routing keep signal track width 1x, power supply tracks 2x and return path tracks 4x.
3.Laydown the ground planes in solid copper area layer form and very adjacent or underneath the signal layers.
4.Route all high current power supply grounds, analog grounds, digital grounds and RF circuit grounds separately and then combine at a single thin point within the PCB.
5.If the dc-dc converters or power supply design is non-isolated then external ground or body ground should be connected with the PCB ground directly or through a small value capacitor.
6.The PCB should have strip of copper around the perimeter of PCB and connected to PCB ground through a capacitor or ferrite bead and connected to the connector at a single point.
7.The high current devices should be located near the power supply to make very short return current path. Whereas low power and low amplitude signals processing devices may be placed away from the power supply.
The high speed digital signal processing devices should be placed closer to each other but away from analog signal devices.