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In Ethernet equipment, when PHY chip is connected to RJ45, network transformer is usually added. Some network transformer center tap grounding. Some are connected to the power supply, and the power supply value can be different, 3.3V, 2.5V, 1.8V. How to connect the transformer intermediate tap (PHY end)?

A. Why are some middle taps connected to power supply? Some grounding?

This is mainly determined by the UTP driver type of phy chip. The driving type is divided into voltage driving and current driving. When driving by voltage, it is connected with power supply; when driving by current, it is connected with capacitor to ground. Therefore, the connection method of the center tap is closely related to the UTP port driver type of phy chip, as well as the datasheet and reference design of the chip.

Note: if the middle tap is connected incorrectly, the network port will be extremely unstable or even blocked.

B. Why are different voltages connected to the power supply?

This is also determined by the UTP port level specified in the PHY chip data used. The level must be connected to the corresponding voltage, that is, if it is 1.8V, pull up to 1.8V, if it is 3.3V, pull up to 3.3V.

Center tap effect:

1. By providing a low impedance return path of common mode noise on the differential line, the common mode current and common mode voltage on the cable are reduced;

2. Provide a DC bias voltage or power source for some transceivers.

The integrated RJ45 common mode suppression can do better, and the influence of parasitic parameters is relatively small; therefore, although the price is relatively high, it is also very popular with engineers because of its high integration, small space occupation, common mode suppression, parasitic parameters and other advantages.

3. What is the function of network transformer? Can we not take it?

Theoretically, it can be directly connected to RJ45 without network transformer, and it can also work normally. However, the transmission distance will be limited, and when connected to different level network interface, it will also have an impact. And the external interference to the chip is also very big. When connected to the network transformer, it is mainly used for signal level coupling. 1、 Enhance the signal, so that the transmission distance is longer; second, make the chip end and external isolation, enhance anti-interference ability, and increase the chip protection (such as lightning); third, when connected to different levels (such as some PHY chip is 2.5V, some PHY chip is 3.3V) of the network port, will not affect each other’s equipment.

Generally speaking, network transformer has the functions of signal transmission, impedance matching, waveform repair, signal clutter suppression and high voltage isolation.


Post time: Jan-12-2021