What is the role of the network transformer filter on the center tap?

Regarding the connection problem of the center tap (some grounding, some power supply, the level value may also be different), there is a Q&A on the Internet: Q: In the Ethernet device, when the RJ45 is connected through the PHY, one will be added in the middle. Network transformer. Some transformers are connected to the power supply at the center of the transformer, and some are connected to the ground. And when connected to the power supply, the power supply value can be different, 3.3V, 2.5V, 1.8V. What is the role of this network transformer filter?

A: Here is a rough answer:

1. Why is it connected to a different voltage when it is connected to the power supply? This is also determined by the UTP port level specified in the PHY chip data used. To determine what level, you have to pick up the corresponding voltage. That is, if it is 2.5v, it will be pulled up to 2.5v, if it is 3.3v, it will be pulled up to 3.3v.

2. Why is the intermediate tap connected to the power supply? Some grounding? This is mainly determined by the type of UTP port driver used by the PHY chip. There are two types of driving, voltage driving and current driving. The voltage drive should be connected to the power supply; if the current is driven, the capacitor can be directly connected to the ground!

For different chips, the center tap connection is closely related to the PHY. For details, please refer to the chip's datasheet and reference design.

3. What is the role of this transformer? Can you not pick it up? In theory, it can be connected to the RJ45 without a transformer, and it can work normally. However, the transmission distance is very limited, and it will also have an impact when connected to different level network ports. Moreover, external interference with the chip is also large. When connected to a network transformer, it is primarily used for signal level coupling. First, the signal can be enhanced to make the transmission distance farther; secondly, the chip end is isolated from the outside, the anti-interference ability is greatly enhanced, and the chip is greatly protected (such as lightning strike); When the network ports of different levels (if the PHY chip is 2.5V and some PHY chips are 3.3V), it will not affect each other's devices.

In general, network transformers mainly have signal transmission, impedance matching, waveform repair, signal clutter suppression and high voltage isolation. Looking for the PHY chip data, there is no description of the UTP port level, so there is still some doubt in this place.

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