WIZnet Developer Forum

W5500 Modular jack with integrated magnetics EMI

HI All,

I have designed a board (4-layer) which is using the w5500 and the modular jack 7499210121A from Wurth. We failed EMC pre-compliance because of a 225MHz and 275MHz frequency peaks. I identified that these emissions are coming from the ethernet part of our design. It looks we have impedance mismatching. We are re-designing the board and we’ve rerouted the differential pairs. The calculated differential impedance is 100.6 Ohm and the individual track impedance is 56ohm. I don’t think that the 56ohm will cause us any issue (50ohm wasn’t easy to achieve).
The total differential pair length for the RX is 12.93mm for RX+ and 12.88mm for RX- .
The total differential pair length for the TX is 20.12mm for TX+ and 20.11mm for TX-.
What is better to do with the ground plane? Should i route the differential pair to the modular jack having ground plane underneath it or I should stop the ground plane in the edge of the modular jack connector and continue the differential routing without it?
I have attached you a photo with the routing. Where is green there isn’t ground or power planes.
Thank you in advance.W5500 ethernet routing

Go here, locate icon “HW design guide” and download the archive. You will find W5100 layout guide inside, those general application best practices should also apply to W5500.

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