Desmond A. Fraser
Feb 11, 20231 min
Printed Circuit Board (PCB) interconnects to meet all regulatory standards, including MIL-STD-461F, must be routed to reduce noise, provide a controlled low-impedance continuous return path to control reflection noise, and reduce the crosstalk in signals that use the same return conductor.
The lowest crosstalk configuration is a broad, continuous ground plane adjacent to a signal trace. Narrow planes will exhibit more crosstalk between signal paths sharing the same return conductor, which means one should never add a split or gap in the return path to avoid the risk of the trace inadvertently going over the discontinuity.
There are two effects if signal traces cross over a split ground plane, and they compound each other as follows:
Crossing a split (gap) creates a higher impedance path for return currents traveling the gap.
Return currents from multiple currents are forced to be superimposed on the same higher impedance common path.
Furthermore, three problems are created:
The return path creates discontinuities
The higher inductance return path creates ground bounce.
The two areas where the return currents flow creates electromagnetic interference (EMI) from their potential differences.
Thus, one should never add splits in the ground, or the risk of routing traces crossing the gaps in the plane will increase.
Split on a ground plane can solve serious resistive crosstalk problems if
Suppose the split or gap is parallel to the return signal paths, and return currents do not cross the gap. In that case, resistive coupling crosstalk is minimized.