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Modern Printed Circuits

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. When we consider the return current path, more ‘ground’ is not always the ‘right’ answer. For example, on a recent design, there was a 144 pin connector with many high speed signals traveling from one board to the other. It was determined that 30 pins could be used for ‘power’ and ‘ground’ combined. At least five pins must be ‘power’ so there would not be an excessive DC voltage drop across the connector. How many of the remaining 25 pins should be ‘ground’? In this particular design, it turned out that about 2/3 of the total signal pins were referenced to the ‘power’ plane, and only 1/3 referenced against the ‘ground’ plane. This meant that of the total 30 possible power/ground pins, 2/3 should be ‘power’ and only 1/3 should be ‘ground.’ More ‘ground’ pins was NOT the best design for this case. Of course, once we consider both the ‘power’ and the ‘ground’ pins to be return current paths, it is obvious we should distribute them throughout the signal pins to keep the return current deviation as small as possible (compared to putting all the ‘ground’ pins at the ends of the connector, etc.).

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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. 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’ or ‘power’ or ‘carrots’. When traces cross a split in the return plane (for example if a trace is routed next to a power layer with multiple power islands), 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 get back to its source. It will get back to its source. The only question is whether it will be a path that is beneficial to you, or if it will cause problems. So, “Do you feel lucky today?” It is always best to design ‘on purpose’ rather than ‘by luck’. ●


Modern Printed Circuits: Sierra Circuits