Looking at the datasheet, the following guidelines are given about the magnetics (see section 9.2.2.2.1 - MDI Layout Guidelines)
9.2.2.2.1 MDI Layout Guidelines
• MDI traces must be 50 Ω to ground and 100 Ω-differential controlled impedance.
• Route MDI traces to transformer on the same layer.
• Use a metal shielded RJ-45 connector, and connect the shield to chassis ground.
• Use magnetics with integrated common-mode choking devices.
• Void supplies and ground beneath magnetics.
• Do not overlap the circuit and chassis ground planes, keep them isolated. Instead, make chassis ground an
isolated island and make a void between the chassis and circuit ground. Connecting circuit and chassis
planes using a size 1206 resistor and capacitor on either side of the connector is a good practice.
But then look at the PCB Design Layout Checklist (SNLA387.pdf) in the product folder and it says the following under Magnetic Isolation:
No metal should be under the magnetics on any layer. If metal is needed under the magnetics, it must be separated by a ground plane at the least. Metal under the RJ45 connector with integrated magnetic is allowed. Figure 2-4 shows a layout example with no metal below the magnetics.
So the question is, how critical is the voiding of power and ground layers under the magnetics? It seems like you don't have to void any and are able to route signals underneath as long as there is a ground layer between the magnetics and the other metal (signals or power). What is the recommendation here?
