Why do ethernet phy magnetics have center tap

[FreshmanNewbie]

I've a question with the ethernet phy connection to the rj45 connector.

Usually, with either discrete or integrated magnetics, either with Voltage mode PHY or current mode PHY, what is the purpose of the center tap of the magnetics.

In someplaces, with voltage mode PHYs, the centre tap is connected to the ground via a capacitor.

In current mode PHYs, the centre taps are pulled to Vcc via a termination resistor and connected to ground via a capacitor.

I want to understand the current/voltage flow across the magnetics and how does the capacitor on the centre tap is helpful for this? What happens if I don't have the capacitor on the centre tap?

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[KlausST]

Hi,

controller/driver side?
or cable side?

Did you do an internet search, check schematics, read magnetics datasheets, magnetics application notes, PHY driver datasheet, PHY driver application notes?

Klaus

 

[FreshmanNewbie]

Hi,

controller/driver side?
or cable side?

Did you do an internet search, check schematics, read magnetics datasheets, magnetics application notes, PHY driver datasheet, PHY driver application notes?

Klaus

Yes, I did search. The centre tap of capacitors on the PHY side of the magnetics.

 

[KlausST]

Hi,

please provide a link to the manufacturer page of the PHY IC you want to use.

Klaus

 

[FreshmanNewbie]

Hi,

please provide a link to the manufacturer page of the PHY IC you want to use.

Klaus

https://ww1.microchip.com/downloads/en/devicedoc/00002117f.pdf

 

[FvM]

Gigabit PHY implements analog hybrid circuit and is neither voltage nor current mode. The datasheet doesn't give any implementation details, it's effectively impossible to determine the purpose of the center tap capacitor. Provided the circuit is symmetrical, the sole purpose may be to drop common mode interferences capacitively coupled between transformer windings.

 

[KlausST]

Hi,

I expected the manufacturer to provide more details.
But here the original manufacturer was MICREL .. and somehow MICROCHIP seems to have acquired MICREL.
Maybe this is why there is a lack of detailed information.

***
I agree with FvM regarding common mode noise suppression.

Klaus