PCB Design

July 9, 2019 by Web Admin

PCB Design Tips — Use Ground Return Paths to Avoid Unwanted Currents

Electric current in a printed circuit board (PCB) is produced with a change in state — such as a logic high to logic low. This electric current generates an electromagnetic field near a conductor. Any change in electromagnetic fields can travel throughout the PCB at a fraction of the speed of light. 

Due to a delay in changes in the electromagnetic fields to propagate inside the circuit, the two ends of a trace can be in different states. So, it’s important that you provide a ground return path in the same layer or the adjacent one to avoid unwanted currents near the conductors. 

In this post, you will learn how to design a PCB with a ground return path. Following the tips mentioned here will result in designing a good quality circuit board. 

Using Ground Return Paths in Design of PCBs

Providing a ground return path is a PCB best design practice. The path can be provided on the same or adjacent layer for differential pairs, power planes, and single-ended signals. 

When designing a PCB, you should route fast-changing signals (<1ns) by creating a ground return path nearby. Surround fast rise and fall time signals with ground pours to minimize the electromagnetic noise. You can also create an uninterrupted ground plane below to minimize the noise. 

Another important advice for designing a PCB with ground return paths is to route fast-changing signals between ground planes and enclose them with via-stitching. This will help in FCC testing of the printed board. 

The image above shows how to reduce unwanted current using ground return paths. Make sure to keep differential pairs coupled together through the entire circuit. The ground return should be provided directly adjacent to or below the signal lines. This will help in the maximum reduction of electromagnetic induction noise. 

To provide an immediate ground return path, the ground return vias should be placed near the differential pair vias. This will provide a path of least impedance for signals as they propagate from one layer to another. 

When designing the PCB with ground return paths, you should layer of the circuit should stackup from above resulting in routing over a power plane. This is important as the signals that propagate overpower planes before arriving at the ground layers will have a common electric field with the power plane. This can result in noise in that signal lines. 

Final Remarks 

Remember that the signals follow the path of least impedance, and not resistance. Creating a ground return path can reduce unwanted currents inside the circuit. This will help in creating a high-quality circuit board with minimum noise. 

While we have been able to reduce operating voltage with smaller microchips. Noise margins have also increased as microchips have gotten smaller. This requires a rethinking in the way PCBs are designed. 

The ground return paths and return vias should be provided for all the signals, particularly fast switching signals. You also need to tie copper pours around the interconnects otherwise they can become radiating elements creating unwanted noise. 

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