The application of differential signal becomes wider in PCB designs, and differential signal design is used for key signals.
Compared with common signal wiring of one end, differential signal has strong anti interference and accurate timing positioning, and can effectively suppress EMI.
Wiring requirements
On circuit board, differential wires must be two wires on the same layer with same length and width, and the two wires should be close.
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The length of two wires should be as same as possible to ensure that the two differential signals are always of opposite polarity. And common-mode components can be reduced.
The width of two signal wires should be as same as possible, and the spacing of the two wires should be kept same, and the two wires should be parallel.
Minimal change of impedance: when PCB with differential signals is designed, applied target impedance should be discovered, and differential pairs are then planned accordingly. Also, keep the impedance change as small as possible. The impedance of a differential wire depends on factors such as wire width, wire coupling, copper thickness, and PCB materials and layers.
Misunderstanding 1: Consider that differential signals do not require the ground as a reflow path, or that differential wires provide a reflow path for each other.
The reason for this misunderstanding is that the knowledge toward high speed signal transmission is not full and deep. Differential circuits are insensitive to similar and other noisy signals that may exist on the power supply and ground planes.
Partial reflow of ground does not represent that differential circuits do not use a reference plane as the signal return path. The principle of differential wiring is same as the principle of common wiring of one end. That is, the high-frequency signal is always re-flowed along the loop with the least inductance. The biggest difference is that in addition to the coupling to the ground, the differential wires also have coupling with each other, and whichever coupling is stronger, which becomes the main reflow path.
In PCB designs, the coupling between differential wires is small, and accounts for 10%--20% of total coupling. And the coupling to the ground is the most. Therefore, main reflow path of differential wires is the ground plane.
When there is a discontinuity in the ground plane, the coupling between the differential wires in the area without a reference plane will provide the main reflow path. Although the effect of discontinuous reference plane on differential wires is not very serious, but discontinuous reference plane can still lower differential signal quality, and increase EMI.
Some designers believe that reference plane below differential wires can be removed to suppress some common-mode signals in differential transmission. But this method cannot be adopted theoretically. Failure to provide an impedance loop for common-mode signals will inevitably result in EMI radiation, which shows the shortcoming of this method.
Misunderstanding 2: Same spacing is more important than matching wire length
During actual PCB wiring, requirements for differential wires cannot be completely satisfied. Due to the factors such as pin distribution, vias, and wiring space, proper winding should be adopted to match wire length, so some regions of the differential pair cannot be parallel.
The most important rule is matching wire length in PCB differential wiring design. Other rules can be adjusted according to design requirements and application.
Misunderstanding 3: Differential wires must be very close
The purpose of making differential wires close is to increase their coupling, improve the immunity to noise, and make full use of the opposite polarity of the magnetic field to cancel the electromagnetic interference from the outside.
This approach has its advantage in most cases, but it is not a must. If differential wires are fully shielded from external interference, then we no longer need to achieve anti-interference and EMI suppression through strong coupling with each other.
How to ensure that differential wires have good isolation? One of basic way is to increase the spacing between differential wires and other signal wires. The electromagnetic field energy decreases in a squared relationship with distance. When the spacing between differential wires and other signal wires exceeds the 4 times the wire width, the interference between them is extremely weak and basically negligible.
In addition, ground plane can also play a good shielding role, and this structure is often used in high-frequency (more than 10G) IC package PCB design, known as CPW structure, which can ensure strict differential wire impedance control.
