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Split Planes in Multilayer PCBs

Creating split planes or isolated islands in the copper planes of multilayer PCBs at first seems like a good idea. Today’s high-speed processors and FPGAs require more than six or seven different high-current power sources. And keeping sensitive analog circuitry isolated from those nasty, fast, digital switching signals seems like a priority in designing a noise-free environment for your product. Or is it?

Many analog-to-digital converter (ADC) manufacturers recommend the use of split ground planes. "The analog ground (AGND) and digital ground (DGND) pins must be connected together externally to the same low impedance ground plane with minimum lead length.” This has been the age-old method for audio design. However, this approach has the potential of creating a number of additional problems in high-speed digital circuits.  A much better way to connect AGND and DGND together, through a low impedance path, is to use only one ground plane to begin with.

When both analog and digital devices are used on the same PCB, it is usually necessary to partition (not split) the ground plane.  The components should be grouped by functionality and positioned so that no digital signals will cross over the analog ground, and no analog signals will cross over the digital ground. Precise partitioning will minimize the trace lengths, improve signal quality, minimize the coupling and reduce radiated emissions and susceptibility. This is traditionally done by using keep-out zones whereby no trace can cross through the keep-out area. But this also creates issues in that control signals need to go into and out of these sensitive areas.

Particular care needs to be taken with oscillators and switch mode power supplies that may generate high frequency electromagnetic fields. If space permits, keep these circuits 10mm from any critical signals to avoid parasitic coupling.

To read this column in its entirety from The PCB Design Magazine March 2015 issue, click here.