I'm currently designing a PCB that's powered through Power-over Ethernet. It's my first time working with ethernet on a PCB. I've based the schematic on a Development board by Espressif (Link : https://docs.espressif.com/projects/esp-idf/en/latest/esp32/hw-reference/esp32/get-started-ethernet-kit.html).

I know that the data line (pin 1 ,2 ,3 & 6 on RJ45) on Ethernet must have impedance matching at 100Ω differential. When it come to the other pin used for the PoE, I didn't find if they need impedance matching (differential) or not?

Thanks for the help

  • 1
    \$\begingroup\$ In passing it's worth noting that the POE 802.3af standard requires that your powered device PD must accept both powering mode A (1+2 and 3+6) and mode B (4+5 and 7+8), with any polarity, and that the choice belongs to the Power Supplying Equipment PSE (802.3 33.2.3, 33.3.1, table 33-13). So-called "passive POE" is really just "power-over-CAT5" and vendors pretty much do as they please, especially as regards voltage. \$\endgroup\$
    – jonathanjo
    Jul 26, 2023 at 13:44

1 Answer 1


Does PoE require impedance matching

PoE can feed power into the ethernet magnetics via the centre taps of the line-side transformer windings. This is a type of phantom-power arrangement and doesn't require any additional impedance matching. You still need the normal impedance matching of course.

There are also other arrangements of wires that make use of exclusive (non-data) wires and these also don't need impedance matching: -

enter image description here


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.