In high-speed PCB design, differential signals are increasingly used, and the most critical signals in the circuit often have to be designed with differential structures. Why is this so? Compared with ordinary single-ended signal routing, differential signals have the advantages of strong anti-interference ability, effective EMI suppression, and precise timing positioning.
Requirements for differential signal PCB routing
On the circuit board, the differential routing must be two lines of equal length, equal width, close proximity, and on the same layer.
1. Equal length: Equal length means that the lengths of the two lines should be as equal as possible, in order to ensure that the two differential signals always maintain opposite polarities. Reduce common mode components.
2. Equal width and equal distance: Equal width means that the routing widths of the two signals need to be consistent, and equal distance means that the spacing between the two lines should remain unchanged and parallel.
3. Minimum impedance change: When designing a PCB with differential signals, one of the most important things is to find the target impedance of the application and then plan the differential pairs accordingly. In addition, keep the impedance change as small as possible. The impedance of differential lines depends on factors such as trace width, trace coupling, copper thickness, and PCB materials and stacking. Consider each of these when you try to avoid anything that changes the impedance of the differential pair.
3 Common Misunderstandings in PCB Differential Signal Design
Misunderstanding 1: Thinking that differential signals do not need a ground plane as a return path, or that differential traces provide return paths for each other.
The reason for this misunderstanding is being confused by surface phenomena, or not understanding the mechanism of high-speed signal transmission deeply enough. Differential circuits are insensitive to noise signals such as ground bounce and other noise signals that may exist on the power supply and ground planes. The partial return offset 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 signal return analysis, the mechanism of differential traces and ordinary single-ended traces is the same, that is, high-frequency signals always return along the loop with the smallest inductance. The biggest difference is that in addition to the coupling to the ground, the differential lines also have mutual coupling. The one with the stronger coupling becomes the main return path.
In PCB circuit design, the coupling between differential traces is generally small, often accounting for only 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 in the ground plane. When the ground plane is discontinuous, the area without the reference plane, the coupling between the differential traces will provide the main return path. Although the discontinuity of the reference plane does not have a serious impact on the differential traces as on ordinary single-ended traces, it will still reduce the quality of the differential signal and increase EMI, so it should be avoided as much as possible.
In addition, some designers believe that the reference plane under the differential trace can be removed to suppress some common-mode signals in differential transmission, but theoretically this approach is not advisable. How to control impedance? Not providing a ground impedance loop for the common-mode signal will inevitably cause EMI radiation, which is more harmful than beneficial.
Misunderstanding 2: It is believed that maintaining equal spacing is more important than matching line length.
In actual PCB wiring, the requirements of differential design cannot often be met at the same time. Due to factors such as pin distribution, vias, and routing space, proper winding is necessary to achieve the purpose of line length matching, but the result is that some areas of the differential pair cannot be parallel. The most important rule in the design of PCB differential routing is to match the line length, and other rules can be flexibly handled according to design requirements and practical applications.
Misunderstanding 3: Thinking that differential routing must be very close.
Bringing differential routing close is nothing more than enhancing 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 the 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 and not interfered by the outside world, then we don’t need to achieve the purpose of anti-interference and EMI suppression through strong coupling with each other.
How can we ensure that differential routing has good isolation and shielding? Increasing the spacing between other signal traces is one of the most basic ways. The electromagnetic field energy decreases in a square relationship with the distance. Generally, when the line spacing exceeds 4 times the line width, the interference between them is extremely weak and can be basically ignored.
In addition, the isolation of 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 CPW structure, which can ensure strict differential impedance control (2Z0).