Signal Integrity and Radiated Emission of High-Speed Digital Systems

10

Grounding in PCBs

In this chapter the concept of ground as reference for signals and power distribution is introduced. Some ground strategies to avoid common-mode coupling among circuits are outlined. An example of grounding for high-power, analog, and digital circuits in the same PCB is given.

Techniques for distributing power and ground planes in multilayer PCBs are considered. It is shown that the return current of a trace is not uniformly distributed in the return plane but is concentrated near to the trace. Some design rules such as splits, moats, and stitches with power and ground planes are qualitatively discussed. Crosstalk and common-ground noise at connector level are investigated. The concept of transfer impedance of a connector is introduced. Grounding solutions to mitigate radiated emissions when an I/O cable is attached to a PCB are compared experimentally and by numerical simulations. Numerical simulations quantify fixes such as a split in a ground plane, common-mode EMI filters, and PCBs in a shielded box with stitch connections.

Three test boards having power and ground planes are used to investigate the problem of partitioning the power plane distribution. Measurements, circuit simulations, and numerical simulations are compared for the first test board with a split in the power plane. The effects of a bridge or ferrite bead across the moat with capacitors are studied by numerical simulations in the case of the second test board having an island in the power ...