The current popular low voltage differential signaling (LVD) refers to this small amplitude differential signaling technology. For PCB engineers, the main 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”. Equal length ensures that the two differential signals maintain opposite polarity at all times and reduce the common mode component. Equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. “As close as possible” is sometimes a requirement of differential wiring. However, these rules should not be mechanically applied, as many engineers seem to still misunderstand the essence of high-speed differential signal transmission. The following focuses on common misunderstandings in PCB differential signal design.
Misunderstanding 1: Differential wiring must be very close. Keeping the differential traces close enhances their coupling, improving immunity to noise and utilizing the opposite polarity of the magnetic field to offset electromagnetic interference. While beneficial in most cases, it is not absolute. If full shielding from external interference is guaranteed, strong coupling is unnecessary for anti-interference and suppressing EMI. Ensuring good isolation and shielding in differential traces can be achieved by increasing the distance between other signal traces. The electromagnetic field energy decreases with the square of the distance, and a line spacing exceeding 4 times the width weakens interference between them. Ground plane isolation can also provide good shielding, commonly used in high-frequency IC package PCBs for strict differential impedance control (2Z0).
Misunderstanding 2: Equal spacing is more important than matching line length. In actual PCB layout, it can be challenging to meet differential design requirements simultaneously. Line length matching must be achieved through proper winding due to factors like pin distribution, vias, and wiring space, resulting in some areas where the differential pair cannot be parallel. In such cases, it is essential to prioritize line length matching over spacing consistency. Simulation results show that while inconsistent spacing may slightly impact differential impedance, it has minimal effect due to insignificant coupling between the pair. On the other hand, line length mismatch significantly affects timing and introduces common mode components into the signal, reducing its quality and increasing EMI. Matching line length is the most important rule in PCB differential wiring design, while other rules can be adapted according to design requirements and practical applications.
Misunderstanding 1: Differential wiring must be very close. Keeping the differential traces close enhances their coupling, improving immunity to noise and utilizing the opposite polarity of the magnetic field to offset electromagnetic interference. While beneficial in most cases, it is not absolute. If full shielding from external interference is guaranteed, strong coupling is unnecessary for anti-interference and suppressing EMI. Ensuring good isolation and shielding in differential traces can be achieved by increasing the distance between other signal traces. The electromagnetic field energy decreases with the square of the distance, and a line spacing exceeding 4 times the width weakens interference between them. Ground plane isolation can also provide good shielding, commonly used in high-frequency IC package PCBs for strict differential impedance control (2Z0).
Misunderstanding 2: Equal spacing is more important than matching line length. In actual PCB layout, it can be challenging to meet differential design requirements simultaneously. Line length matching must be achieved through proper winding due to factors like pin distribution, vias, and wiring space, resulting in some areas where the differential pair cannot be parallel. In such cases, it is essential to prioritize line length matching over spacing consistency. Simulation results show that while inconsistent spacing may slightly impact differential impedance, it has minimal effect due to insignificant coupling between the pair. On the other hand, line length mismatch significantly affects timing and introduces common mode components into the signal, reducing its quality and increasing EMI. Matching line length is the most important rule in PCB differential wiring design, while other rules can be adapted according to design requirements and practical applications.
