SMARC T335x Carrier Board Hardware Design Guide, Document Revision 1.2
2.2.2.4. LAN Ground Plane Separation
Isolated separation between the analog ground plane and digital ground
plane is recommended. If this is not implemented properly then bad
ground plane partitioning could cause serious
EMI
emissions and degrade
analog performance due to ground bounce noise.
The plane area underneath the magnetic module should be left empty.
This free area is to keep transformer induced noise away from the power
and system ground planes.
The isolated ground, also called chassis ground, connects directly to the
fully shielded RJ45 connector. For better isolation it is also important to
maintain a gap between chassis ground and system ground that is wider
than 60mils. For
ESD
protection, a 3kV high voltage capability capacitor is
recommended to connect to this chassis ground.
Additionally, a ferrite bead can be placed parallel to the capacitor.
Figure 7: LAN Ground Plane Separation