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54 DESIGN007 MAGAZINE I JUNE 2020 A high-speed digital signal crossing a split in the reference plane impacts at least three as- pects of design integrity: signal quality, cross- talk, and EMI. The problem is the impedance discontinuity in the return signal path cross- ing the split. Any discontinuity in impedance reflects energy back toward the source, par- ticularly the higher-frequency components of the signal. At high frequencies, the return current follows the path of least inductance, which is directly above and/or below the sig- nal trace, but that path is broken by the split. In this case, the return current has to find an alternative path back to the source, creating a larger loop area. In this month's column, I will review the two common solutions to this is- sue, plus introduce a third optimal solution for high-speed design. 1. The Pass-Through Gap Low-frequency circuits in the audio range can often benefit from splitting the ground planes, whereas digital circuits cannot be effectively isolated from analog circuits using this method. Many years ago, wiring the ground connections to one common point was a great way to eliminate noise in a guitar amplifier, particular- ly value-based amplifiers. But as frequency increases, the parasitic capacitive and inductive compo- nents dominate, and distance or electromagnetic shielding is the only solution. The key to a successful mixed analog/digital design is function- al partitioning, understanding the current return path, and routing Split Planes: Reprise control and management—not carving up the ground planes. Analog and digital grounds should be connected together through a low impedance path. It is always better to have just one single reference (ground) plane for a system. 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 functional- ity and positioned such that no digital signals will cross over the analog ground, and no an- alog 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 data and control signals need to go into and out of these sen- sitive areas. Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Route fences used to control routing and isolation.