Ethernet equipment, through the PHY is connected to RJ45, the middle will add a network transformer. Some transformer center tap to ground. But when the power, power value can not be the same as, 3.3V, 2.5V, 1.8V are. The role of the transformer is analysed as follows:

1, the intermediate tap why some power? Some ground? This is the type with the use of the PHY chip UTP driver, the driver type has two kinds, driving voltage and current drive. A voltage driven need power supply; current drive is directly connected to a capacitor to ground! So for different chip, the center tap of the connection, and PHY are closely related, but also the specific see chip datasheet and reference design.

What level of decision, will be connected to the corresponding voltage. If 2.5V is pulled to 2.5V, if 3.3V is pulled to 3.3V. 3 the transformer is what role do, can not. 3 the transformer is what role do, can not.

From a theoretical point of view, can not need transformer, directly connected to the RJ45, but also can work normally. However, the transmission distance is limited, and when receiving the different level net mouth, also can have the effect. However, the transmission distance is limited, and when receiving the different level net mouth, also can have the effect. And external to the chip have great disturbance. When the network transformer, it is mainly used for signal level coupling. On the one hand, can enhance the signal, the transmission distance; second, the isolation chip end and the external, anti-interference ability is greatly enhanced, and increases great protective effect on chip (such as lightning); thirdly, when receiving the different level (such as some PHY chip 2.5V, PHY chip is 3.3V). Net export, will not affect each other equipment.