Most Common EMI Myths and Their Realities

EMI/EMC is one area where many designers struggle, partially due to workflow and partially due to inexperience in solving EMC problems. Solving EMC problems is sometimes seen as somebody else's job, and yet some designers will still implement design practices that actually create EMI problems. The irony is that these practices are intended to prevent EMI, and they are based on old myths that just won't go away.

For new designers, this is an opportunity to learn which simple mistakes can be prevented in a potentially noisy design. Make sure you understand these myths and the context behind them, and hopefully you won't create an EMI problem in your PCB.

Top Emi and EMC Myths

Myth 1: Split Planes Prevent EMI

This myth comes in several versions, all of which relate to creation of radiated EMI, transfer of conducted EMI onto a cable where it can radiate, or reduction of EMI susceptibility. It tends to arise in mixed-signal PCB layout, where there is an attempt to eliminate crosstalk between analog circuits and digital circuits. It also tends to arise in power electronics, specifically isolated switching regulators.

There are a few reasons this practice arises:

• Perception that each analog and digital power rail needs to have its own ground

• Pin naming on ADCs and DACs creates an impression of separate grounds

• Misunderstanding of return currents in planes

• A perception that closed planes will be part of a ground loop

This non-isolated DC/DC converter board incorrectly attempts to enforce isolation with separate ground planes, creating a potential EMI problem.

When you have net and pin naming such as that shown above, it tends to create the impression that you need separate grounds too. This is not the case, as separated grounds can act like big antennas that emit radiation. They also interfere with routing as a route over the split region will also create radiation. A uniform ground plane should be the only option in most boards unless the system has a galvanic isolation specification.

Myth 2: Ferrites Are the Cure for Noise

It seems that anytime there is excessive noise in a design, someone tries to throw a ferrite at the problem. This does not solve the problem creating the excess noise. This tends to be seen in a few common instances:

• Ferrite on a power rail

• Ferrites on signal lines

• Ferrites connecting disconnected ground planes

When used as a filtering element on power rails for digital processors, ferrites could create noise.

Instead, the origin of the noise source should be investigated, and it may turn out that ferrites do not solve the root problem. In some of the cases mentioned above, ferrites create new EMI problems or signal Integrity problems.

Myth 3: Enclosure Should be Tied to Signal Ground

This deserves some additional context for a few reasons. Sometimes, an enclosure can be tied to ground, depending on the insulation around the chassis and how that ground connection is being used. Unfortunately, this idea that all grounds need to be connected together may cause a designer to connect their chassis, signal ground, and earth to the same connection. This usually then gets extended such that the system ground and enclosure are tied together at multiple points in an attempt to minimize ground loops.

Enclosures can be tricky because they can participate in the propagation of common-mode noise around a system, which could then lead to radiation or injection of common-mode currents onto a cable. Once the common-mode current gets onto a cable, it will radiate. The reason we don't want to connect earth to the closure and signal ground at multiple points is to prevent the enclosure and earth from carrying return current. Earth and enclosure are not intended to carry a return current, instead they are meant to be safety grounds for system faults.

When used as a path for return current, the chassis could allow some current to leave the system, such as passing through a user. This undesirable shock could be considered a safety hazard. When earth is present, understand how that earth is connected alongside the remaining power lines in your system. It's usually used in AC systems, which means it is connected to a panel or breaker.

Myth 4: Right Angle Traces Create Radiated EMI

There is a perception about right angle traces that persists to this day. Right angle traces are seen as creators of radiated EMI, particularly from the corner region of the trace. This is a long lasting signal integrity and EMI myth.

While it is true that a right angle corner on a trace adds some fringing capacitance, the impedance mismatch created by this is very low and it only becomes problematic at very high frequency ranges. Below dozens of GHz, it can be ignored and will not affect signal integrity. It certainly will not create so much EMI that a product fails EMC testing.

Myth 5: Copper Pour and Stitching Vias Prevent EMI

This is a misconception about copper pour and stitching villas. If EMI already exists in a board, copper pour and stitching vias might help suppress radiated emissions. But in reality, they do not prevent systems from exhibiting conducted emissions or radiated emissions. New designers will often fill in everything in a board with copper and connected everywhere with stitching vias, then assign these to the ground net. This provides clear ground in two layer boards, but it may not do anything beneficial in higher layer count boards.

Stitching vias are sometimes used around edges of PCBs to help confine radiated EMI. It’s better to investigate the source of the EMI first.

Instead, focus on finding the root cause of EMI and solve that problem. You may find that things like copper pour and stitching vias are not needed in every design.

High power systems with discrete semiconductors can be easily designed with the best PCB design features OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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