Several Misunderstandings of Differential Line in PCB Design

Differential signal (Differential Signal) is more and more widely used in high-speed circuit design, the key signal in the circuit is often designed by differential structure, what makes it so popular?

With these two questions in hand, we will proceed to the next part of the discussion. In layman's terms, the drive end sends two equivalent, inverted signals, and the receiving end determines whether the logic state is "0" or "1" by comparing the difference between the two voltages ". The pair of traces that carry the differential signal is called a differential trace.

Compared with ordinary single-ended signal traces, differential signals have obvious advantages in the following three aspects:

A strong anti-interference ability, because the coupling between the two differential traces is very good, when there is noise interference from the outside, it is almost coupled to the two lines at the same time, and the receiving end is only concerned about the difference between the two signals, so the external common mode noise can be completely canceled.

B. can effectively suppress EMI, the same reason, because the polarity of the two signals is opposite, their external radiation electromagnetic field can cancel each other, the closer the coupling, the less electromagnetic energy released to the outside world.

C. timing positioning is accurate, because the switching change of differential signal is located at the intersection of two signals, unlike ordinary single-ended signal, which depends on the judgment of high and low threshold voltages, it is less affected by process and temperature, which can reduce the error in timing, and is also more suitable for circuits with low amplitude signals. The current popular LVDS(low voltage differential signaling) refers to this small amplitude differential signal technology.

For PCB engineers, the concern is still how to ensure that these advantages of differential routing can be fully used in the actual routing. Perhaps anyone who has come into contact with Layout will understand the general requirements of differential routing, that is, "equal length and equal distance". The equal length is to ensure that the two differential signals maintain opposite polarity at all times and reduce the common-mode component; the equidistance is mainly to ensure that the two differential impedances are consistent and reduce reflections. The principle of "as close as possible" is sometimes one of the requirements of differential routing. But none of these rules are meant to be applied mechanically, and many engineers don't seem to understand the nature of high-speed differential signaling. The following focuses on several common misunderstandings in PCB differential signal design.

Myth 1: Think that differential signals do not need a ground plane as a return path, or think that differential traces provide a return path for each other. The reason for this misunderstanding is that it is confused by the surface phenomenon, or the understanding of the mechanism of high-speed signal transmission is not deep enough. The differential circuit is insensitive to noise signals like ground bounce and others that may be present on the power and ground planes. The partial backflow cancellation of the ground plane does not mean that the differential circuit does not use the reference plane as the signal return path. In fact, in the signal backflow analysis, the mechanism of differential wiring is consistent with that of ordinary single-ended wiring, that is, high-frequency signals always flow back along the circuit with small inductance. The big difference lies in the coupling between the differential wiring in addition to the coupling to the ground, which coupling is strong, which becomes the main return path. In PCB circuit design, the coupling between the general differential trace is small, often accounting for only 10 ~ 20% of the coupling degree, more or the coupling to the ground, so the main return path of the differential trace still exists in the ground plane. When there is a discontinuity in the ground plane, the coupling between differential traces will provide the main return path in the area without the reference plane. Although the discontinuity of the reference plane has no more serious influence on the differential traces than the ordinary single-ended traces, it will still reduce the quality of differential signals and increase EMI, which should be avoided as much as possible. Some designers also believe that the reference plane below the differential trace can be removed to suppress part of the common-mode signal in differential transmission, but theoretically this approach is not desirable, how to control the impedance.

Myth 2: think that maintaining equal spacing is more important than matching line length. In the actual PCB wiring, it is often not possible to meet the requirements of differential design at the same time. Due to the existence of pin distribution, vias, and routing space, the purpose of line length matching must be achieved through appropriate winding, but the result must be that some areas of differential pairs cannot be parallel. The important rule in the design of PCB differential traces is the matching line length, and other rules can be flexibly processed according to the design requirements and practical applications.

Misunderstanding 3: think that the differential line must be very close. Let the differential line is nothing more than to enhance their coupling, which can not only improve the immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset the electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, but it is not, if we can ensure that they are fully shielded from external interference, then we do not need to let each other through strong coupling to achieve the purpose of anti-interference and EMI suppression. How to ensure that the differential trace has good isolation and shielding? Increasing the spacing with other signal traces is one of the basic ways, the electromagnetic field energy is decreasing with the square of the distance, the general line spacing of more than 4 times the line width, the interference between them is extremely weak, the basic can be ignored. In addition, the isolation of the ground plane can also play a good shielding effect. This structure is often used in high-frequency (10G or more) IC package PCB design, which is called CPW structure and can ensure strict differential impedance control (2Z0).

Differential traces can also walk in different signal layers, but this method is generally not recommended, because the differences in impedance and vias generated by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the adjacent two layers are not tightly coupled, the ability of the differential trace to resist noise will be reduced, but if the proper spacing with the surrounding trace can be maintained, crosstalk is not a problem. At general frequencies (below GHz),EMI will not be a very serious problem. Experiments show that the attenuation of radiated energy beyond 3 meters has reached 60dB for differential traces with a distance of 500Mils, which is sufficient to meet FCC's electromagnetic radiation standard. Therefore, designers need not worry too much about electromagnetic incompatibility caused by insufficient coupling of differential lines.