LAN PHY to connector design: transformer?

[McShamrock]

75 ohm center tap

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 for both RX and TX lines?
2) why pins 4,5,7,8 are connected to 4 resistors of 75 Ohm to the center tap at the other part of the transformer? I guess the capacitor I find between the first 2 and second two resistors 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...???

Please, anyone could help me? I would really appreciate.


Thank you,
N.

 

[arnarendra]

bob smith termination

If you are talking about the resistors used, they are the differential termination resistors and normally they will be 50 ohms (in your case it is 75 ohms which gives 150 ohm differential impedance).

 

[McShamrock]

lan phy transformer

Thank you.
But let me understand the purpose of those resistor:
Is 150 Ohm differential impedance due to any constraint of LAN protocol?
Why exactly 150 Ohm? Any relation with PoE?

Thank you in advance.

N.

 

[Iouri]

phy to phy without transformer

did you lookk in the Ethernet spec? Also if it is backplane application sometimes you don't need transformer, look at micrel.com

 

[dennis zhang]

bob smith lan

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 for both RX and TX lines?
2) why pins 4,5,7,8 are connected to 4 resistors of 75 Ohm to the center tap at the other part of the transformer? I guess the capacitor I find between the first 2 and second two resistors 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...???

Please, anyone could help me? I would really appreciate.


Thank you,
N.

Transformer is used as an isolation protector to protect the circuit on the secondry side of transformer, in case there is a voltage potential between the UTP and the circuit after transformer. Normally, the rated isolation voltage is 1.5kv.
1) Applying 2.5V to RX/TX pair, I think it 's due to the mechanism of PHY transciever. Most 1000Base-T PHY needs this.
2) For the termination, please check Bob smith termination through internet. The caps provide an AC connection between UTP and the shield or the I/O ground. 2KV is not for ESD, but the maybe the surge between UTP and shied of RJ45, or the frameground.

 

[cucmi]

lan differential impedance

Hi, I found that in some designs the centre taps of transformer are connected to GND through a cap, in other designs the centre taps are connected to 2.5v or 3v. Is there a guideline in such design ?

Take the HR901103A as example (the schematic attached), What signal should P6 and P3 connect to? 2.5V or GND?

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 for both RX and TX lines?
2) why pins 4,5,7,8 are connected to 4 resistors of 75 Ohm to the center tap at the other part of the transformer? I guess the capacitor I find between the first 2 and second two resistors 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...???

Please, anyone could help me? I would really appreciate.


Thank you,
N.

Transformer is used as an isolation protector to protect the circuit on the secondry side of transformer, in case there is a voltage potential between the UTP and the circuit after transformer. Normally, the rated isolation voltage is 1.5kv.
1) Applying 2.5V to RX/TX pair, I think it 's due to the mechanism of PHY transciever. Most 1000Base-T PHY needs this.
2) For the termination, please check Bob smith termination through internet. The caps provide an AC connection between UTP and the shield or the I/O ground. 2KV is not for ESD, but the maybe the surge between UTP and shied of RJ45, or the frameground.

 

[sxb416]

Hi, I found that in some designs the centre taps of transformer are connected to GND through a cap, in other designs the centre taps are connected to 2.5v or 3v. Is there a guideline in such design ?

Take the HR901103A as example (the schematic attached), What signal should P6 and P3 connect to? 2.5V or GND?

I have the quention long time,anybody can help me ?

 

[FvM]

The rather trivial answer is: You can't determine the required connection from a ethernet magnetics datasheet. It has to be made according to the chip manufacturer requirements. You can always find clear instructions in the datasheet as far as I experienced.

 

[devarajs]

Hi,
Please browse the pages from 8-12 which gives the details of the the high voltage capacitor and the 75 Ohm resistors.

Regards,
Deva.