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Beyond Design: Return Path Discontinuities

Simultaneous switching noise (SSN) is a major problem in high-speed systems. But the underlying issue is really the management of transmission line return currents that flow on the nearby reference planes, causing the planes to bounce. High-speed design is not as simple as sending a signal from the driver to the receiver, over an interconnect. Rather, one should also consider the presence and interaction of the power distribution network (PDN) and how and where the return current flows. A logic schematic diagram masks details crucial to the operation of unintentional signal pathways vital to understanding signal performance, crosstalk and electromagnetic emissions.

PCB designers generally take great care to ensure that critical signals are routed exactly to length from the driver to the receiving device pins, but take little care of the return current path of the signal. Current flow is a “round trip” and the critical issue is delay, not length. If it takes one signal longer for the return current to get back to the driver—around a gap in the plane for instance—then there will be skew between the critical timing signals. Return path discontinuities (RPDs) can create large loop areas that increase series inductance, degrade signal integrity and increase crosstalk and electromagnetic radiation.

Four factors must be considered in order to mitigate the RPDs:

1. Recognize the impact of RPDs.

2. Understand the importance of referencing.

3. Identify the location of the RPDs–path of least inductance.

4. Take corrective action to mitigate the RPDs. 

1. Recognize the impact of RPDs

Ground impedance is at the root of virtually all signal and power integrity problems–low ground impedance is mandatory for both. This is readily achieved with a continuous ground reference plane, but becomes increasingly difficult with the addition of more and more plane layers on a multilayer PCB. A ground plane serves well as a signal return, provided the ground is continuous under the signal path. But even with a continuous return path, there may be enough voltage drop across the plane to generate a common-mode voltage. And if left unchecked, may escape as electromagnetic emissions via the signal or power/ground conductors. RPDs have a huge impact on supply bounce of single-ended signals. Fortunately, differential signaling dramatically reduces this affect. Serial interfaces also significantly reduce the number of interconnects, which is another advantage over the use of parallel buses for high-speed design.

To read this entire column, which appeared in the April 2017 issue of The PCB Design Magazine, click here.