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Ethernet Capacitive Coupling Design and Layout?


Hi all, has anyone got an example of a PCB layout which uses capacitive coupling instead of discrete magnetics when connecting ethernet from a cable to a generic PHY? I was thinking of following an approach like this https://wiki.analog.com/_media/resources/eval/user-guides/adin1300-and-adin1200/tvs_and_cmc.jpg?w=600&tok=55ed1b.

1. Do you need Bob Smith termination on the connector side to the chassis as you would with discrete magnetics?
2. Do you need to have a keepout plane on all layers underneath the connector to the capacitors?
3. Do these capacitors need to have HV specifications?


Ethernet 10baseT..100Baset, ghz requirements include transformers for CMRR and to break ground current loops.

Use in telecom, broadcast, pro audio and pro cinéma all use the transformers

See the PHY IC mfg app notes and eval bds.

Depending on the requirements, EMI and transient susceptibility and environmental you can certainly try your scheme but 8t will not be Ethernet.

The transformers are available in single..dual, quad a etc in DPI and SMD package

Some RJ45 have the magnetics built in.

Suggest that you read the Ethernet PHY standard and specs for the desired baud rate.

Bon chance


is it worth risk if you need an rj45 anyway and can get those with build in transformers?

You had another thread about ethernet but it did not go further into your design.

Are you 100% sure you want to go to such special snowflake design? What is the reasoning for wanting to do capacitive isolation instead of the usual transformer thing? With transformers, you have plenty of off the shelf components and reference designs which are tested and used everywhere.

From your 3rd question I also deduce you are probably fine with the usual 1kV or so functional isolation provided by the usual ethernet transformers.

Read this application note from microchip https://ww1.microchip.com/downloads/en/Appnotes/ANLAN120-UNG.pdf

I am doing a design now with 5 port embedded ethernet switch KSZ8795, one port will have a RJ45 with integrated magnetics for external cable connection, all other Ethernet units connecting to the other ports only need capacitive coupling as shown in figure 3 as long as they implement 10BASE-T/100BASE-TX PHY layer

But it will not work with capacitive coupling  above 100mbit or for long distance, only shorter distances with lower speed

Not 100% sure, but at least this is my uncerstanding at the moment, I will post a reply after testing with diffferent ethernet devices :)

Kind regards


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