RJ45 with integrated magnetics - different configurations

[thm_w]

You must have been using an RJ45 without integrated magnetics.

Sure, but it does illustrate why integrated magnetics are a good thing: one less thing to worry about.

I think the point being that some of us like to stay on topic 

[Nominal Animal]

Okay.  Me stray.  Sorry.

Back on topic: Am I the only one who finds it odd that some of the integrated magnetics RJ45s have common mode chokes on the cable side (before the isolation), whereas others have them on the PHY side (after the isolation)?

In particular, of the aforementioned Abracon ARJM11 series, -009- (100Base-T) and -502- (1000Base-T) have them on the cable side, but -104- (100Base-T POE+), -114- (100Base-T POE), -805- (2.5GBase-T), -809- (2.5GBase-T), and -811- (5GBase-T) have them on the PHY side.

Why would you have a common mode choke on the cable side anyway, prior to the isolation, instead of on the PHY side?

[nctnico]

I've seen LAN transformers with common mode at the primary (PHY) side and some at the secondary side. On the secondary side I guess you have the advantage that whatever RF leaks from the ground / power connections from the primary into the secondary side, gets filtered.

[T3sl4co1l]

Instinctively to me, media side is better because if you have RF currents coming in, they're choked away from ground, whereas without, that flows right into whatever termination you have (HoJo resistor + ESD cap).  You're relying more on the CMRR of the transformer, something the primary side CMC can't address.

It's less common for PoE, I think because CMC on pairs would trivially saturate; it's only meaningful for pair-pairs at a time, or the whole cable.  So a more complex winding is needed.  Maybe this isn't a hard limit after all -- good working examples of PoE apparently exist* -- but maybe it's harder to do than just tossing a power tap at the line and assuming everything else is normal.

*Mind, I haven't done anything with PoE yet, so my experience there is limited.

Also, especially with respect to GBASE's, there are full balanced vs. push-pull drivers (the latter needs VDDA to center tap), generally GBASE being the former type, so the transformer can be simpler, permitting higher bitrates.

Tim