Printed Circuit Board Design and Layout Questions

4. If I have to route a trace over a gap in a ground plane, what precautions should I take?

The short answer is, "Never route a trace over a gap in the ground plane." We've evaluated a lot of board layouts and have yet to see an example of a situation where this is a good idea. If your design appears to require routing a trace over a gap in the ground plane, then chances are you have put a gap where it is neither necessary nor desirable.

in the event that your signal is conveying information between two locations with "grounds" that require low-frequency isolation, the signal should be differential and/or a high-frequency low-impedance connection should be established between the two grounds that permits the uninhibited flow of high-frequency return (or common-mode) currents.

References

[1] R. L. Hill, T. P. Van Doren, T. H. Hubing, J. Drewniak, and F. Gisn, “Common mode currents induced on wires attached to multilayer printed wire boards with segmented ground planes,” Proc. of the 1994 IEEE International Symposium on Electromagnetic Compatibility, Chicago, IL, Aug. 1994, pp. 116‑120.

[2] J. A. Roden, B. Archambeault,R. Lyle,"Effect of stitching capacitor distance for critical traces crossing split reference planes," Proc. of the 2003 IEEE International Symposium on Electromagnetic Compatibility, Aug. 2003, pp. 703-707.