I am designing a PCB that will use a Microchip KSZ9896C 6-port Gigabit Ethernet Switch. In the datasheet (section 7.2) they state that :
"the four transformer center tap pins on the KSZ9896C chip side should not be connected to any power supply source on the board; rather, the center tap pins should be separated from one another and connected through separate 0.1μF common-mode capacitors to ground. Separation is required because the common-mode voltage could be different between the differential pairs, depending on the connected speed mode."
I want to connect the switch to a 2×2 Gigabit modular jack. The confusion I am having is that most, if not all, ganged modular jacks have the transformer center tap pins from the common mode filter connected together and sent out one pin (for each individual port). Even if the pin is left unconnected, all of the center taps are tied together in the jack and, therefore, not isolated. Correct?
I am totally new to Ethernet magnetics so I am trying to figure out if a 2×2 ganged jack will work. Any help?
KSZ9896C : http://ww1.microchip.com/downloads/en/DeviceDoc/00002390A.pdf
BelFuse 2×2 Gigabit MagJack : https://belfuse.com/resources/drawings/magneticsolutions/dr-mag-0879-2c2r-54.pdf
Best Answer
You need to change the magnetics your using or the Ethernet physical layer IC to one that uses the center tap for bias current. The KSZ9896C uses a differential output and a common center tap could make it not work.
I realize these parts are not cheap, but you cannot mix apples and oranges. Check the datasheets first before buying. They must be an exact match.