I'm currently working on a design which contains gigabit ethernet. Due to some weird size constraints, I can't use a magjack or one of those transformer ICs, so I'm using discrete magnetics. I've managed to pick a transformer (actually three... would be nice if they stayed in stock long enough to lay out the board), but am not exactly sure how to choose a common mode choke. In this marketing copy from Bourns, the SRF2012A-801Y is mentioned as a CMC for gigabit ethernet. But I don't know why (or even if!) this is a good choice. Is it the impedance (seems kinda high to me)? DC resistance/max current? Will any old common mode choke work for gigabit ethernet, or is there something to it? What about wire wound vs multilayer chip ones like this?

  • \$\begingroup\$ I'd be interested too in a real answer, but at least make sure that the choke's differential mode impedance is negligible wrt the line impedance (i.e. << 10 Ohm) at up to maybe 5*frequency of operation. Tbh, I can't really follow the necessity of the CMC in the ethernet magnetics, as the cable is anyways isolated from the board. \$\endgroup\$
    – tobalt
    Commented Nov 16, 2021 at 13:35

1 Answer 1


If you are plannig to go a test laboratory with your finished product, I strongly suggest you to buy an off-the-shelf common mode filter suited for Gigabit ethernet.

The common mode you mentioned, Panasonic EXC14CG/EXC14CE, might not work because the datasheet doesn't say anything about Gigabit ethernet. Yet, it's too little for filtering ethernet currents.

Stick with what datasheets state. If that particular feature is not expressively mentioned in the datasheet, than that product is not meant for what you got in mind.

  • \$\begingroup\$ Half the reason I asked this question is that I had a really hard time finding CMCs for ethernet, much less gigabit. Feel free to share if you can ;) One exception- I found a load of automotive gigabit ethernet chokes, but I’m not designing for that, and would prefer not to rip myself off over an extended temp range. Plus, it’s nice to know why, especially if a part goes OOS \$\endgroup\$ Commented Nov 17, 2021 at 10:07
  • \$\begingroup\$ Many parts are OOS nowadays. Depending on how many chokes you need, if you don't want to buy those automotive parts, you may look for a supplier of custom magnetics. In the past I used to purchase custom flyback transformers from prisource.com.tw. They are reliable. I purchased lots of 3k parts per order. \$\endgroup\$ Commented Nov 17, 2021 at 11:09

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