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58 The PCB Design Magazine • March 2015 by Barry Olney in-CiRCuiT Design PTy lTD BEyOnD DESIGn Split Planes in Multilayer PCBs coulmn Creating split planes or isolated islands in the copper planes of multilayer PCBs at first seems like a good idea. Today's high-speed pro- cessors and FPGAs require more than six or sev- en 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 environ- ment 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 con- nected 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 addi- tional 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 be- gin 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 com- ponents 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 parti- tioning will minimize the trace lengths, improve signal quality, minimize the coupling and reduce radiated emissions and susceptibility. This is tra- ditionally 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 oscil- lators 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. Route fences, rather than route keep-outs, are useful to control the routing. Controlled routing is the key to a successful mixed sig- nal design. The planes should not be split, but rather a pass-through gap is left in the plane so that control signals can enter and leave that area as seen in Figure 1. Route fences are also very effective is controlling an autorouter. They can be set up for each router pass and then moved to a different location. This is best done with interactive cross-probing from schematic to PCB, controlling functional sections of the design one-by-one, building up the route to completion. At low frequencies, current fol- lows the path of least resistance. But at high frequencies, return cur- rent follows the path of least induc- tance—which happens to be direct- ly under the signal trace on a plane (power or ground) that is closest to Figure 1: Route fences used to control routing and isolation.

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