Smart designs consider the return current path to ensure EMI/EMC performance.

Electrical engineers are quite familiar with schematic drawings. While schematics are accurate for the intended signal flow, there is a very important piece of the signal path missing. Signal voltages are usually referenced to “ground”, and the schematic assumes that these signals have a common reference point. However, this is not true for high-speed signals.

The concept of ground requires that the entire area (a PCB or metal chassis, for example) be at the same potential/voltage. This is easily achievable at low frequencies, and “ground” has a definite meaning. However, at today’s high-speed data rates, the frequency content of signals is significantly higher, resulting in wavelengths that are much smaller than the ground area we use. This yields a voltage difference across the ground plane, or chassis, and makes the concept of ground at high frequencies completely meaningless. When dealing with high frequency signals, it is important to remember, “Ground is a place for potatoes and carrots!”

For EMI/EMC we need to consider the current, not the voltage, in our signal paths. Since current must always flow in a loop back to its source, the return current path must be considered as well as the intended signal path along a PCB trace. Any interruptions to the return current path can have serious negative effects to the EMI/EMC performance of a PCB. Even a very slight deviation in return current path can cause enough inductance to dramatically increase emissions.

When we consider the most important concerns for good EMI/EMC design, the schematic is not as important as the physical layout of the signal path and the return current. Therefore the layout of high-speed signal traces becomes the most important aspect of the PCB for EMC purposes. Since today’s high-speed PCBs have many layers and are very complex, it is difficult for an engineer to examine each critical signal path for a good return current path. Automated EMC rule checking tools can examine each net in turn, regardless of the PCB complexity. The key to selecting an automated rule checking tool is to make sure it can interface well with your existing design process, it is easy to use, and it can display rule violations in a graphical and easy to understand manner.

The most important EMC design rules for high-speed PCBs concern the return current path. Since the return current will always find a path that minimizes the inductance of that path, the return current will always flow on the nearest plane, whether it is called ground, power or apples. When traces cross a split in the return plane (if a trace is routed next to a power layer with multiple power islands, for example), the return current’s path is interrupted. Changing layers within the PCB so that the return current must also change planes will also interrupt the return current path. Remember, the return current must always return to its source. The only question is whether it will use a path that is beneficial to you, or if it will cause problems. It is always best to design on purpose rather than by mistake. PCD&M

Dr. Bruce Archambeault is IBM Distinguished Engineer at IBM in Research Triangle Park, NC. He can be contacted at This email address is being protected from spambots. You need JavaScript enabled to view it..