PCB Tech - Misunderstandings in PCB Differential Signal Design

Differential signal and PCB design several common misunderstandings in differential signal

Differential signal (DifferenTIal Signal) is more and more widely used in high-speed circuit design. The most critical signal in the circuit is often designed with a differential structure. What makes it so popular? How to ensure its good performance in PCB design? With these two questions, we proceed to the next part of the discussion. What is a differential signal? In layman's terms, the driving end sends two equal and inverted signals, and the receiving end judges the logical state "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces.

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

1. Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference from the outside, they are almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely canceled.

2. It can effectively suppress EMI. For the same reason, due to the opposite polarity of the two signals, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less electromagnetic energy vented to the outside world.

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

For PCB engineers, the most concern is how to ensure that these advantages of differential wiring can be fully utilized in actual wiring. Perhaps anyone who has been in touch with Layout will understand the general requirements of differential wiring, that is, "equal length and equal distance". The equal length is to ensure that the two differential signals maintain opposite polarities at all times and reduce the common mode component; the equal distance is mainly to ensure that the differential impedances of the two are consistent and reduce reflections. "As close as possible" is sometimes one of the requirements of differential wiring. But all these rules are not used to mechanically apply, and many engineers seem to still not understand the essence of high-speed differential signal transmission.

The following focuses on several common misunderstandings in PCB differential signal design.

Misunderstanding 1: It is believed that the differential signal does not need a ground plane as a return path, or that the differential traces provide a return path for each other. The reason for this misunderstanding is that they are confused by superficial phenomena, or the mechanism of high-speed signal transmission is not deep enough. Differential circuits are insensitive to similar ground bounces and other noise signals that may exist on the power and ground planes. The partial return 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 return analysis, the mechanism of differential wiring and ordinary single-ended wiring is the same, that is, high-frequency signals are always Reflow along the loop with the smallest inductance. The biggest difference is that in addition to the coupling to the ground, the differential line also has mutual coupling. Which kind of coupling is strong, and which one becomes the main return path. In PCB circuit board design, the coupling between differential traces is generally small, often only accounting for 10-20% of the coupling degree, and more is the coupling to the ground, so the main return path of the differential trace still exists on the ground flat. When there is a discontinuity in the ground plane, the coupling between the differential traces in the area without a reference plane will provide the main return path, although the discontinuity of the reference plane has no impact on the differential traces on the ordinary single-ended traces It is serious, but it will still reduce the quality of the differential signal and increase EMI, which should be avoided as much as possible. Some designers believe that the reference plane under the differential trace can be removed to suppress some common mode signals in differential transmission. However, this approach is not desirable in theory. How to control the impedance? Not providing a ground impedance loop for the common-mode signal will inevitably cause EMI radiation. This approach does more harm than good.

Misunderstanding 2: It is believed that keeping equal spacing is more important than matching line length. In the actual PCB layout, it is often not possible to meet the requirements of differential design at the same time. Due to the existence of factors such as pin distribution, vias, and wiring space, the purpose of line length matching must be achieved through proper winding, but the result must be that some areas of the differential pair cannot be parallel. The most important rule in the design of PCB differential traces is the matching line length. Other rules can be flexibly handled according to design requirements and actual applications.

Misunderstanding 3: Think that the differential wiring must be very close. Keeping the differential traces close is nothing more than to enhance their coupling, which can not only improve immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, then we do not need to use strong coupling to achieve anti-interference. And the purpose of suppressing EMI. How can we ensure good isolation and shielding of differential traces? Increasing the spacing with other signal traces is one of the most basic ways. The electromagnetic field energy decreases with the square of the distance. Generally, when the line spacing exceeds 4 times the line width, the interference between them is extremely weak. Can be ignored. In addition, isolation by the ground plane can also play a good shielding role. This structure is often used in high-frequency (above 10G) IC package PCB design. It is called a CPW structure, which can ensure strict differential impedance. Control (2Z0).

Differential traces can also run in different signal layers, but this method is generally not recommended, because the differences in impedance and vias produced 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, it will reduce the ability of the differential trace to resist noise, but if you can maintain a proper distance from the surrounding traces, crosstalk is not a problem. At general frequencies (below GHz), EMI will not be a serious problem. Experiments have shown that the attenuation of radiated energy at a distance of 500 mils away from the differential trace has reached 60dB at a distance of 3 meters, which is enough to meet the FCC electromagnetic radiation standard, so The designer does not have to worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling.