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In Ethernet equipment, when the PHY chip is connected to RJ, a network transformer is usually added. The center tap of some network transformers is grounded. Some are connected to the power supply, and the power supply value can be different, including 3.3V, 2.5V, and 1.8V. Then how to connect the middle tap (PHY end) of the transformer?

A. Why are some of the center taps connected to power? Some are grounded?

This is mainly determined by the UTP port driver type of the PHY chip used. Drive types are divided into: voltage drive and current drive. Connect the power supply when driving with voltage; connect the capacitor to ground when driving with current. Therefore, the connection method of the center tap is closely related to the UTP port drive type of the PHY chip. At the same time, please refer to 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.

In Ethernet equipment, when the PHY chip is connected to RJ, a network transformer is usually added. The center tap of some network transformers is grounded. Some are connected to the power supply, and the power supply value can be different, including 3.3V, 2.5V, and 1.8V. Then how to connect the middle tap (PHY end) of the transformer?

B. 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. The level must be connected to the corresponding voltage, that is, if it is 1.8v, it will be pulled up to 1.8v, if it is 3.3v, it will be pulled up to 3.3v.

The role of the center tap of the LAN transformer:

1. Reduce the common mode current and common mode voltage on the cable by providing a low impedance return path for common mode noise on the differential line;

2. For some transceivers, provide a DC bias voltage or power source.

The integrated RJ common mode suppression can be better, and the parasitic parameters are also less affected; therefore, although the price is relatively high, it is also very popular due to its high integration, small space, common mode suppression, parasitic parameters and other advantages. welcome.

What is the role of network transformer? Can you not pick it up?

Theoretically speaking, it can work normally without connecting the network transformer and directly connecting to the RJ. However, the transmission distance will be limited, and it will also be affected when it is connected to a network port of a different level. And the external interference to the chip is also great. When the network transformer is connected, it is mainly used for signal level coupling. 1. Strengthen the signal to make the transmission distance farther; 2. Isolate the chip end from the outside, enhance the anti-interference ability, and increase the protection of the chip (such as lightning strike); 3. When connected to different levels (such as Some PHY chips are 2.5V, and some PHY chips are 3.3V), it will not affect each other’s devices.
In general, the network transformer mainly has the functions of signal transmission, impedance matching, waveform repair, signal clutter suppression and high voltage isolation.