With the increasing density of circuit board, signal crosstalk is always a problem that can not be ignored. Because it will not only affect the normal operation of the circuit, but also increase the electromagnetic interference on the circuit board.
Some high-frequency signals on the circuit board will crosstalk to MCU circuit or MCU I / O interface circuit, forming common mode voltage. As we all know, common mode voltage is the most annoying thing in circuit design. Therefore, we should avoid crosstalk of common mode voltage that may cause abnormal circuit operation when designing circuit board.
The design principle of reducing crosstalk on circuit board is simple
1. Make the connection between components as short as possible through reasonable layout.
2. Because the degree of crosstalk is directly proportional to the frequency of the interference signal, the high frequency signal line should be kept away from the sensitive signal line.
3. The interference signal line and the interference signal line should not only be far away, it is recommended to use the ground wire to isolate, and avoid parallel wiring.
4. In Multilayer PCB board, the interference signal line and the interference signal line or sensitive signal line should be isolated or separated by ground wire.
5. In the multilayer PCB, the interference signal line and the interference signal line are on the opposite sides of the ground wire or the layer, that is, the interlayer.
6. Try to use sensitive circuit with low input impedance, and use bypass capacitor to reduce the input impedance of sensitive circuit when necessary.
Finally, it should be noted that the ground wire plays an important role in suppressing the signal crosstalk when wiring. The crosstalk can be reduced by about 10dB if the ground wire is directly laid in the interference line and the disturbed line.
Famous 3W routing rules
There is a well-known 3W wiring rule (where W is the wiring width) in restraining the crosstalk of circuit board wiring signals. Its content is: for the signal line with the width of 3W, if the distance between the center of other wiring lines and its center is greater than 3W, crosstalk between signals can be avoided. As shown in the figure below.
According to this rule, 75% of the magnetic flux generated by the signal current is included in the 3W range. As long as the adjacent wires are outside this range, the crosstalk between the signals will not be very serious. It is worth noting that 98% of the magnetic flux generated by the signal current is included in the 10W range.