The differential impedance of the USB signal line is

USB is a fast, bidirectional, synchronous transmission, cheap, easy-to-use, hot-swappable serial interface. USB devices are widely used due to their fast data transmission, convenient interface, and hot-swappable support.

Currently, most products on the market use USB2.0 as the interface. However, many hardware novices encounter many problems in USB applications. The USB interface often appears after the PCB is assembled. There are various problems, such as unstable communication or failure to communicate. If there are no problems after checking the schematic diagram and welding, perhaps it is necessary to suspect that the PCB design is unreasonable at this time. (Recommended learning: web front-end video tutorial)

Drawing a PCB that meets USB2.0 data transmission requirements plays an extremely important role in the performance and reliability of the product.

The signal line width and line spacing of the differential line of ordinary USB devices can be consistent with the signal line width and line spacing of the entire board. However, when the USB device works at 480 Mbits/s, it is not enough to do the above points. We also need to control the impedance of the differential signal. Controlling the impedance of the differential signal line is very important to the integrity of the high-speed digital signal. Important because differential impedance affects the eye diagram of the differential signal, signal bandwidth, signal jitter and interference voltage on the signal line.

The differential line impedance is generally controlled at 90 (±10%) ohms (refer to the chip manual for specific values). The differential line impedance is inversely proportional to the line widths W1, W2, and T1, and is inversely proportional to the dielectric constant Er1 It is inversely proportional to the line spacing S1 and directly proportional to the distance H1 of the reference layer. The following figure is a cross-sectional view of the differential line.

The picture below shows the reference stackup of a four-layer board. The middle two layers are reference layers. The reference layer is usually GND or Power, and the reference layer corresponding to the differential line It must be complete and cannot be divided, otherwise the impedance of the differential line will be discontinuous.

If you design a four-layer board based on the stacking shown in Figure 2, usually the differential line uses a line width of 4.5mil and a line spacing of 5.5mil to meet the differential impedance of 90Ω.

However, 4.5mil line width and 5.5mil line spacing are only our theoretical design values. In the end, the circuit board manufacturer will determine the line width, line spacing, and reference layer based on the required impedance value and the actual production situation and board materials. Make appropriate adjustments to the distance.

The wiring rules described above are based on USB2.0 devices. During the USB wiring process, keep the differential lines as short as possible, tightly coupled, equal in length, consistent in impedance and pay attention to USB. As long as the current carrying capacity of the power cord is mastered and the above principles are mastered, USB devices will basically run without problems.

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