In the device, when the PHY is connected to RJ45, a network transformer is added in the middle of the RJ45 connector. Some transformer center tap received. And power supply, the power value can not be the same as 3.3V, 2.5V, 1.8V. The function of the transformer is as follows:

1, the middle tap why some power? Some ground? This is primarily driven by the use of the PHY chip UTP port driver type, which has two types, voltage driven and current driven. The voltage is driven by the power supply; the current is driven directly by a capacitor to the ground! So for different chips, the center tap of the connection method, and PHY is closely related to the specific but also see the chip datasheet and reference design.

2, why the power supply, and then take a different voltage? This is also determined by the UTP port level specified in the PHY chip data. To determine the level of the level, you have to pick up the corresponding voltage. If it is 2.5V on the pull to 2.5V, if it is 3.3V on the pull to 3.3V.

3 what is the role of the transformer in the end, it can not be connected to it. In theory, it can not need to connect to the transformer, directly to the RJ45, but also to work properly. However, the transmission distance is very limited, and when the network received a different level of network, it will have an impact. And the external interference to the chip is also very big. When the network is connected, it is mainly used for signal level coupling. One, can enhance the signal, so that its transmission distance; second, the chip and the external isolation, anti-interference ability is greatly enhanced, and the chip has increased a lot of protection (such as lightning); third, when receiving different levels (such as PHY chip is 2.5V, PHY chip is 3.3V), will not affect each other.