ferrite bead resistance/inductance/part id for ethernet?

[Garyl]

Hey,
does a "FERRITE BEAD" part has a specific resistance/inductance value in the ethernet application?

I am making a circuit similiar to the one from attachment:
View attachment 1863901.pdf
On the page 9, there is L2 "Ferrite Bead" between VDD (3.3V) and Ethernet magjack circuitry.
I am looking for more information about this part, because I want to order it in SMD case.

In my prototype I have used a random THT ferrite bead and it worked, but right now I am making a "clear" version in SMD and I have only this part left to order...

PS: I checked the PIC datasheet but the example application schematic there is different and it has no such part at 3.3V....

 

[barry]

The ferrite bead is functionally a filter. What frequencies do you want to reject? That determines the characteristics of the bead. It looks like this bead is simply to keep noise from the ethernet signals getting onto the power supply line. It's probably not that critical. Maybe something with a high impedance at 100MHz, and low enough DC resistance to drive the pull-ups .

 

[FvM]

Ferrite beads are specified by its impedance at 100 MHz and at least DC current rating. Be aware that impedance reduction due to DC saturation starts far below the rated current. Respectively the ethernet supply filter bead should have at least 500 mA, better 1 - 2 A DC rating. Suitable impedance range is 120 to 600 ohms.

Reviewing recent datasheets, PIC18FxxJ6x series seems to have an EMI problem, thus additional Tx low pass filters are suggested that are missing in previous versions. Probably no problem for a hobby design.