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Connecting an ENC28J60 to a RJ45 10/100 connector w/internal magnetics

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TiagoSilva

Member
Hi, here's the problem, the ENC28J60 datasheet says that I need 2x 49,9 Ohm resistors per pair, well, the RJ45 connector I have already have the resistors (and magnetics), but with different values...
Here are the links to both parts datasheets:
The ENC28J60:
https://www.electro-tech-online.com/custompdfs/2010/01/39662c.pdf (page 9 on acrobat)
The Tyco 5-6605758-1 RJ45 connector:
https://www.electro-tech-online.com/custompdfs/2010/01/0900766b807c5bb1.pdf
Can I use this connector, should I even try?
The datasheet doesn’t say anything about these 49.9 Ohm resistors, only about the RBIAS resistor, which influences the TPOUT levels.
Should I risk using the RJ45 connector with internal 75 Ohm resistors instead?
My connector also has common mode chokes for noise attenuation and the ENC28J60 datasheet doesn’t recommend using common mode chokes…
Err…
 

DirtyLude

Well-Known Member
I'm not certain what you are looking at, but both diagrams show the 75ohm resistors between the plug and the magnetics. Your ENC28J60 spec shows the 49.9's between the magnetics and the chip, plus the 75's between the magnetics and the plug. The 49.9's the connector does not replicate or have anything for.
 

TiagoSilva

Member
I'm not certain what you are looking at, but both diagrams show the 75ohm resistors between the plug and the magnetics. Your ENC28J60 spec shows the 49.9's between the magnetics and the chip, plus the 75's between the magnetics and the plug. The 49.9's the connector does not replicate or have anything for.

You’re right… I’m seeing ghosts, I had imagined the connector had some resistors of 75 ohms in place of the 49,9’s… My brain is asking for some valium.
Anyway, what about the common mode chokes? What do you think?
 

TiagoSilva

Member
I was also misinterpreting the RJ45 connector datasheet… The common mode chokes will probably won’t matter (right?) as they are before the magnetic…
Thanks anyway.
 

Wp100

Well-Known Member
Hi,

Can anyone say how critical the resistor values are in the ENC circuit - microchip use Rbias as 2.32k 1% yet that last diagram uses 2k, and the 49.9R have changed to 51.1R - could a pair of more common 100R 1% be used instead.

Would assume microchip chose to use those unusual values for a reason ?
 
Hi,

Can anyone say how critical the resistor values are in the ENC circuit - microchip use Rbias as 2.32k 1% yet that last diagram uses 2k, and the 49.9R have changed to 51.1R - could a pair of more common 100R 1% be used instead.

Would assume microchip chose to use those unusual values for a reason ?

The 49.4 to 51.1 ohm resistors are the bus termination resistors, they should be 50 ohms, 100 ohms will not work in their place. The value of the Rbias resistor is dependent upon the characteristics of the internal circuitry of the ENC chip. As refinements have been made in the design the value of the resistor has changed. That resistor controls the strength of the carrier signal and is important to reduce cross-talk and misshaped signals on the cable that other devices will not recognize as valid.

Mike
 

Wp100

Well-Known Member
Hi Mike,

Thanks for you quick and informative reply - I should have said two 100r 1% in paralllel to make the 50r

Will stick with microchips specified value for rbias.
 
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