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Data TR Termination and layout of DC2476A-A

To whom may it concern. 

HI, I'm HS.

This is screenshot of DATA Transformer in DC2476A-A.

I managed to find out that those RCs around TR are called "Bob Smith Termination" but I'm not quite sure about the actual name and purpose.

My questions are...

1. Shield and Termination

I assume "shield" means the chassis of RJ45 connector.

It'd be lovely if you enlighten me about the connection of C30 here.

1-1. Does the connection go like Shiled(Connector, pin 9&10) ---- C30 ---- Secondary GND ??

1-2. Do we need to make Chassis GND area when we do layout? Then what would be the purpose of chassis GND?

1-3. What's the purpose of C30?? 

1-4. Why does not J2 attach Cap. like C30? Why do you attach Pin 9 &10 to Secondary GND?

1-5. What's the purpose of 'termination' in data transformer??

2. Layout

There are several Transformers in this EVB.

I've heard that, by the rule of thumb, you don't put ground under the transformer. 

But some designers put ground under the DATA transformer.

Would it be find if i put ground under the data transformer?? Or I should not?

Is there any good guideline or tips when it comes to RJ45 and DATA Transformer? (Also for Transformer in flyback and gate driver, if you don't mind)

Thank you for your hard work.

All the best

  • Hi HS,

    1. For review, the ethernet cable must be isolated from earth ground, per the IEEE 802.3 standard. For this isolated PD circuit, earth ground and secondary-ground can be the same net. Shield does refer to the chassis of the RJ-45. Usually the chassis is connected to earth ground. This could poses a problem for shielded ethernet cables, where the cable is enshrouded with a conductive material that connects to the RJ-45’s chassis at both ends of the cable. To prevent any issues or ground loops, the RJ-45 chassis is connected to earth ground / secondary ground with an isolation capacitor. This prevents a DC current path while providing an AC path to ground.

    1.1. Yes.

    1.2. I would recommend having a separate Chassis ground net, that connects to secondary ground via an isolation capacitor. Chassis ground, referring to pins 9 and 10 on the RJ-45 provides mechanical stability for the RJ-45 and allows for C30 to AC ground the RJ-45’s chassis / shield of a shielded ethernet cable.

    1.3. C30 prevents any issues or ground loops that could arise with a shielded ethernet cable.

    1.4. J2 is a data only port that is just for data reliability testing on our demobard. Isolating its chassis is unnecessary.

    1.5. The termination at the transformer provides a matched termination to the common mode impedance of the transmission lines. You should terminate the secondary-side of the ethernet transformer however your PHY vendor suggests.

    1. Yes, you should avoid routing “ground” under the transformer, either primary or secondary. This applies to the data and the power transformers. Remember, there needs to be 1.5kV isolation between the either ends of the transformer. Ideally, the primary and secondary grounds should be physically separated. Routing either ground under the transformer may compromise the isolation barrier. For flyback and gate driver layout suggestions, please check out the LT3825 datasheet, page 26 for some guidelines on layout.

    Best Regards,