Return Path for Bipolar Signals (EMC)

[dannybeckett]

Hello,

I found an excellent guide on how to set your layers for good EMC performance:



Can't recommend giving this a watch more. However, there is one case that isn't really covered. How should you route bipolar signals on a 4 layer board? If my 4 layer stackup goes....

• +12v/sig
• gnd
• 1mm FR4 core
• -12v/sig
• gnd

....do I end up in a situation where a bipolar signal trace on L1 cannot couple to its desired reference plane (L4) when in the negative direction and field spread occurs? Or will L2 act as a "surrogate" reference plane for -12V when called upon (even though we don't have access to the beneficial inter-plane capacitance between L3 and L4 for for -ve rail power delivery)?