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PoE Ground Noise Issue

Category: Hardware
Product Number: LT4295

We are using an LT4321 + LT4295-based solution in our design. We are using this to power a Microcontroller SOM module. PoE output is configured at 12V and is designed to support 30W load.

We were able to get the 12V up and is also validated up to 30W. However, the 100Mbps Ethernet link is not working. We observed that the link is UP as shown both in the Microcontroller and external PC side. But when we do a ping test, it is failing, there is no response. We did a quick exercise of powering the Microcontroller from an external 12V DC supply. When powered with an external DC supply, we can ping between PC and microcontroller, which confirms the PC and Microcontroller ethernet configuration.

While investigating further, we could see that there is lot of Ground noise in the PoE ground which happens to be propagated to ethernet signals. I have uploaded a zip file containing three pictures to Gdrive (link provided below). One is the capture of PoE ground w.r.t digital ground. Other two are of ethernet signal while powered with DC and PoE.

Can you please let me know what is the reason for this ground noise.

I have uploaded  the schematics and layout file to Gdrive(link below)for your reference. Please let me know if you need any other information from my side. Awaiting for a reply.