Reducing PCB Emissions Low-Noise Design Practices

It is generally desirable to have reduced emissions with your printed circuit boards. Reducing emissions comes down to having the right design approach. Some manufacturers may not realize they are not stuck with the level of emissions their printed circuit boards currently register. If you are looking to improve printed circuit board performance, you should always start with a proper approach to design. It may be possible to reduce PCB emissions with low-noise design practices.

Here are some ideas for low-noise PCB design, which you may wish to integrate into your own board designs in order to reduce emissions.

  1. Use an Adjacent Pair Stack-up Design

One low-noise circuit layout option that may be useful relates to how you design your stack up. A more desirable design plan may be to stack up the layers with signal pairs adjacent to their respective image planes. If you stack up in a configuration that separates the signal pairs, it will cause the return current to transition between two image planes by passing over other image planes. This creates a larger loop and hence greater emissions.

  1. Reduce Splits

While you may need to create moats in your design to isolate your circuits, routing signals over ground or power planar splits will result in a larger return loop. As with a poorly designed stack-up, you can expect this larger loop to result in greater emissions.

  1. Network Decoupling Capacitors

You will have to use decoupling capacitors to reduce dynamic current switching noise. You may have better luck controlling emissions if you use a network of decoupling capacitors with a range of frequencies, rather than the same frequency capacitor on every circuit.

  1. Add Series Termination Resistors

If you’re having trouble with signal reflections and other methods are not giving you the results you need, consider adding series termination resistors to the output of the ADCs.

  1. Separate Digital and Analog Ground Planes

By keeping your digital and analog ground planes separate, connected only at the ADCs and DACs, you should have less current spreading than if you simply staple two ground planes together across the board’s surface.

  1. Use a Guard Fence

An exposed, grounded guard fence around the edge of the board that keeps out signal traces and power planes provides a discharge path for ESD and can help with emissions.

  1. Segregate I/O

Make sure you have a segregated I/O area on your printed circuit boards. This area can minimize noise that is coupled between signals on the PCB and off board signals.

While it is unlikely that you can design a virtually noise-free circuit board, use of all these techniques can reduce emissions dramatically, up to 20 dB or more. Of course, the necessities of your specific design needs may prevent you from implementing all of these techniques. This is fine. The reason that we are presenting multiple techniques is so that if your design requirements don’t allow for one or two low-noise design features, you have plenty of other options to choose from to keep your printed circuit boards’ noise down and within EMC specifications.

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