HW LAN network: transformers, 75Ohm resistors and capacitors

[McShamrock]

Hi,

Is someone able to solve any doubt I have about the circuitry which must be set between the LAN PHY chip and the RJ45 connector?
I know it is necessary to set a transformer, and I guess it works as an isolation protector against voltage peak coming from the RJ45 connector, but actually I cannot explain some physical aspects of its working:
1) why the center tap of the transformer is connected to a 2.5V(roughly) source for both RX and TX lines?
2) why pins 4,5,7,8 and the center tap of transformer are connected to resistors of 75 Ohm. I also find a capacitor, which all resistors are connected to, and I guess it serves to ground any ESD coming from RJ45 pins 4,5,7,8 or pins 1,2,3,6 (thru the center tap of the transformer). I don't understand why ESD in pins 1,2,3,6 should pass thru the center tap of transformer and why the choice of 75 Ohm resistors...???
3) Does anyone have any application note or design guide where the working principle of LAN transformer+resistor network and design choices are dealt with?

Thank you a lot.

N.

 

[y_patch]

Hi,

Is someone able to solve any doubt I have about the circuitry which must be set between the LAN PHY chip and the RJ45 connector?
I know it is necessary to set a transformer, and I guess it works as an isolation protector against voltage peak coming from the RJ45 connector, but actually I cannot explain some physical aspects of its working:
1) why the center tap of the transformer is connected to a 2.5V(roughly) source for both RX and TX lines?
2) why pins 4,5,7,8 and the center tap of transformer are connected to resistors of 75 Ohm. I also find a capacitor, which all resistors are connected to, and I guess it serves to ground any ESD coming from RJ45 pins 4,5,7,8 or pins 1,2,3,6 (thru the center tap of the transformer). I don't understand why ESD in pins 1,2,3,6 should pass thru the center tap of transformer and why the choice of 75 Ohm resistors...???
3) Does anyone have any application note or design guide where the working principle of LAN transformer+resistor network and design choices are dealt with?

Thank you a lot.

N.

I was just wondering it (especially Q.2) myself...Do you know why by now?

 

[FvM]

75 ohm is a termination for unwanted common mode waves transmitted by the differential pairs, ideally absorbing their energy at both cable ends. Otherwise common mode waves would bounce for- and backward, deteriorating the signal quality by crosstalk to differential signals and different propagation speed.