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I am working on a system prototype which includes two ethernet switches (KSZ9896) to provide Ethernet Gigabit connectivity between several devices (let's call them slaves, from now on); every slave has it own PHY IC (dunno about the P/N, it's from Marvell).

Every switch has at least one slave connected to it, and both switches are connected together to access all slaves from an external computer (connected to one of the switches)

Switches and slaves are assembled in the same PCB, so they share the same ground reference.

After some investigation on the internet, I still have many questions to answer about this:

  1. Is it needed to add Ethernet magnetics between switches? And from a switch to every slave connected to it?

  2. Microchip recommends to use HX5008NL discrete magnetics with this switch (Magjack is not an option) and I see that the magnetics has a Chip side (PHY) and a Media Side (RJ45... that we don't have on our system).

As you can read, I just need to connect two PHYs in the same circuit, does it mean that I have to add 2xHX5008NL to connect both media sides?

  1. If not, how can I connect both PHYs together (switch-slave or switch-switch) to work properly?

  2. In case of use discrete magnetics, could you recommend a circuit? Microchip shows a typical circuit (http://ww1.microchip.com/downloads/en/devicedoc/00002390a.pdf page 186), but it is slighly different from others I've seen on the internet. Many of them connect the common terminals capacitors (chip side) to 2.5V (I understand this is has to be the same voltage reference of PHY), and many others ties DP lines to GND through 50ohm resistors...

Thanks in advance.


marked as duplicate by Jasen, Edgar Brown, DoxyLover, brhans, RoyC Jan 13 at 9:26

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CMRR degrades with rising frequency and rising path length.

Thus unless it is <5% wavelength over controlled impedances away from SMPS and similar disturbances, you ought to use magnetics. It could be SMD CM chokes or jacks with built-in magnetics. CM chokes with pi and ground filter is critical for good eye-patterns and immunity at GHz rates.


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