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Combating EMI Head On!

Electromagnetic compatibility (EMC) and its associated troublesome pal, electromagnetic interference (EMI) are and have been part of your PCB design issue package since day one. With the introduction of shrinking PCBs and smaller packaging, EMC and EMI are working overtime to cause you larger headaches.

For refresher purposes, EMC is related with the generation, propagation, and reception of electro-magnetic energy. That energy results due to a mixed combination of energy producers when different circuits, traces, vias, and PCB materials operate in unison.

Fig 1 -- Solid GND Plane

EMI, on the other hand, are the unwanted, damaging effects EMC or the undesirable energy generates. In such an electromagnetic environment, the PCB designer’s goal is to assure that various energy elements are reduced to maintain minimal interfering effects.

There are a number of considerations when minimizing or resolving EMC/EMI. For more details, read our EE Times PCB Designline article.

In the meantime, here are some tips and hints that’ll provide you a good understanding of how to minimize these issues.

  • Increase as much as possible a PCB’s ground area within the board’s total area. This reduces emissions, crosstalk, and noise.
  • Split ground planes at three to five different places for different ground signals. Multiple ground sections can be accommodated in one ground layer to reduce fabrication costs in check and reduce EMC and EMI.
  • It’s best to have a solid ground plane vs. copper thieving or hashed ground planes in a multi-layer PCB (Fig. 1 above)
  • Length of signal returns to ground is critical; amount of time a signal takes to and from the source must be compatible; otherwise, an antenna-like phenomenon is created that radiates energy as part of EMI.
  • Separately focus on and treat analog circuitry differently than digital circuitary.
  • At high-speed levels, signals and clocks should be kept as short as possible and adjacent to the ground plane.

Traces, themselves, are especially important to ensure current is properly propagated from one place to another. This is particularly true if current is coming from an oscillator or some other similar device to keep the current separated from the ground plane or another trace not running in parallel. Two high-speed signals running in parallel create EMC and EMI, in particular crosstalk. Also to be kept in mind is maintaining the return current path as short as possible with the least resistive path. Return path traces should be about the same length as the transmit traces.