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[SOLVED] Ethernet magnetics between embedded devices.. do i need them?

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thunderdantheman

thunderdantheman

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G'day,

I have two OEM Ethernet devices that I will embed on a motherboard. On the bench I connect the two devices together via the standard RJ45 crossover with magnetics on each device interfacing the PHY to the cable. When I embed these two devices, am I able to connect them PHY to PHY (cross over)? i.e. without the magnetics?
 

Possibly. It depends on the receiver and transmitter design details. In some cases, at least inductors may be needed.
 

G'day,

If anyone else is interested I found this app note from Texas Instruments - TLK100 Ethernet PHY Transformerless Operation and was available from https://www.ti.com/lit/an/slla310/slla310.pdf It suggests using capacitors as the interface. Whilst it is based on the TLK100 Phy, it should be applicable to others.
 
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    FvM

    FvM

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As said, some PHYs may require an output inductor to avoid output stage saturation, particularly in 10 MBPS operation. But I guess, the method will work with most PHYs.

As a problematic point, the application report doesn't address the risk of PHY damage by common mode voltages in non-embedded applications. Obviously it's not sufficient to use capacitors with high voltage rating, because the capacitors will be charged/discharged when plugging the network cable. The energy stored in a 33 to 100 nF capacitor is far beyond the handling capability of built-in chip ESD protection.

While the solution is O.K. for interchip communication, it shouldn't be suggested for other cases in my view.
 

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