Author Topic: KSZ8081 PHY center tap and magnetics  (Read 486 times)

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Offline radar_macgyver

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KSZ8081 PHY center tap and magnetics
« on: July 01, 2021, 05:14:49 pm »
I'm using an off the shelf SOM with a KSZ8081 Ethernet PHY on it. The PHY datasheet mentions that the center taps on the TX/RX magnetics must be separately AC grounded with a 0.1uF capacitor. Yet, on the SOM's eval board, they short together the center taps and then AC ground them. I have had some occasional issues with the Ethernet connection on the SOM eval board (will establish a link if the cable is plugged in prior to powering up, but sometimes won't establish a link if plugged in after powering up).

Could anyone with some experience with this or related PHYs let me know which way is correct? I'm designing my own base board for the SOM.

Another question: There seem to be several different configurations of magnetics. Some have the common mode chokes on the cable side of the isolation transformer, and this is what's shown in the KSZ8081 data sheet. Others, like the Molex 480250002 show the CMC on the RX pair on the PHY side. Why the difference? The documentation I could find online mostly talks about differences between magnetics that do and don't include the CMC.

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