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I'm currently developing a device which require an ethernet switch on it (to provide Internet connection to a microprocessor and an iPad). One of the major goal is to make it very small.

I found an ethernet switch IC and a USB to Ethernet IC (both have MAC and PHY integrated), but I was wondering if it's necessary to put a magnetics between both of them or I just do some direct connection. Maybe an alternative with smallest passive component would be possible?

My microprocessor has an ethernet port on it, so I guess it would be the same connection as the USB to Ethernet IC?

Thank you all.

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It may not be impossible, but both devices must support transformer-less configuration, and be compatible in its signalling. If you want to connect two devices without magnetics, manufacturers of both devices must give you OK for this, otherwise you will be doing it on your own risk.

It will be easier if you have devices to connect developed by one manufacturer. Here's typical answer of WIZnet chip manufacturer.

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Yes it is required, if you don't these things will happen:
1) You will be susceptible to ground loops, the magnetics isolate the ground between devices and prevent current flow at DC.
2) You will not get the benefit of the common mode choke to reduce noise which acts like a filter.
3) The circuit will be more susceptible to ESD as you will no longer be isolated from the cable

These are some physical reasons why you wouldn't want to do this. Here is another big reason, it violates the IEEE spec:

An isolation-magnetic for each RJ45 connector is required by the IEEE standard.

From A Beginner’s Guide to Ethernet 802.3

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My microprocessor has an ethernet port on it, so I guess it would be the same connection as the USB to Ethernet IC?

Don't guess, verify.

Many processors don't have a direct BASE-T ethernet connection. Instead they have some variant of MII and expect you to use an external PHY. You can get switch chips that connect directly to a MAC over MII avoiding the need for a PHY, for example the LAN9303 from Microchip.

Unfortunately I haven't seen a switch chip with two MII ports on it, so you will probablly end up needing to connect at least one of the devices to the switch over BASE-T

As to connecting two BASE-T phys without magnetics it is possible, various vendors have appnotes on it but if you are trying to connect parts from two different vendors in this way you are likely to be on your own analysing the circuits and trying to design a transformerless equivilent.

In general the transformer provides three functions.

  • It provides isolation
  • It converts a pair of single ended drives to a differential signal on transmit.
  • It establishes the desired common mode voltage on receive.

The first is not needed for two phys on the same board but the second and third points will need to be addressed in some other way.

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