Differential Pair PCB Design

PCB Design Points for Differential Line Pairs

A differential pair is a pair of coupled transmission lines. Differential signaling is done by using two output drivers to drive the differential pair, one carrying the signal and the other carrying its complement. All we need is the voltage difference between the differential pairs that carry the information to be transmitted.

1. Advantages of differential signal transmission

Differential signaling has many advantages over single-ended signaling:

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

(2) It can effectively suppress EMI. For the same reason, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less electromagnetic energy will be released to the outside world;

(3) Timing positioning is accurate. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the single-ended signal, it is judged by the high and low threshold voltages, so it is less affected by the process and temperature, and can reduce the timing error. , but also more suitable for circuit design of low-amplitude signals.

For PCB engineers, the most concerned thing is how to ensure that the advantages of differential signals can be fully utilized in actual wiring. Anyone who has been in contact with PCB design will understand the general requirements of differential wiring, that is, "equal length and equal distance". But all these rules are not used to apply mechanically, and many engineers seem to have not done an in-depth analysis of the actual design process of the differential line pair. The following will focus on discussing several common points in PCB differential signal PCB design.

2. Key points of differential line pair PCB design

2.1 Isometric

The purpose of equal length is to make the signal transmission delay on each line the same, so as to ensure that the polarities of the two differential signals are kept opposite at all times. Any difference in time delay between the two transmission lines will make part of the differential signal become a common-mode signal, seriously affecting the signal quality. Equal length is to make the wiring length of the two signal lines of the differential line pair the same as possible. Usually, the matching requirement for equal length of high-speed differential signals is within ±10 mils. Of course, this is a relatively high requirement. The real value can be calculated by allowing signal misalignment (skew, which can be found in the chip manual) and signal transmission delay (generally 180 picoseconds per inch).

Due to device layout, pin distribution, etc., most of the differential line pairs generated by direct wiring are of unequal length, which requires manual winding. Manual winding is generally performed at the chip pins to reduce the impedance discontinuity of the differential line pair.

2.2 Isometric

Equidistance is to ensure the continuity of differential impedance between differential line pairs and reduce reflection. Differential impedance is an important parameter for designing a differential pair, and if it is discontinuous, it will affect signal integrity. Differential impedance can be regarded as the equivalent impedance of two single-ended signal lines in series. Usually, the equivalent impedance of a single-ended signal line is 50Ω, so in general, the differential impedance should be kept at 100Ω. Equidistance is to keep the distance between the differential line pairs equal (that is, parallel wiring), so as to ensure that the differential impedance of the differential line pair does not change throughout the process.

The differential impedance is related to many parameters such as the line width, line spacing, printed board stacking sequence, and dielectric constant of the differential line pair. Some of these parameters can only be provided by the printed board manufacturer. Therefore, the printed board designer should contact Manufacturers negotiate together to determine parameters such as line spacing. It is worth noting that when a differential signal is transmitted on different layers of a multilayer PCB (especially when both inner and outer layers are routed), the line spacing must be adjusted in time to compensate for the characteristic impedance change caused by the dielectric constant change of the medium. Unequal distances have less impact on signal integrity than unequal lengths. When the equal length conflicts with the equal distance rules, the equal length should be satisfied first.

2.3 Lamination of differential line pairs and printed boards

The stacking setting of the PCB board is closely related to the coupling and shielding of the signal. There is a view that differential lines provide return paths for each other, so differential signals do not need a ground plane as a return path, which is a wrong understanding. Generally, the coupling between differential traces is small, often only accounting for 10% to 20% of the coupling degree, and more is the coupling to the ground, so the main return path of the differential traces still exists on the ground plane. In PCB design, differential signals are required to be close to at least one ground plane, and it is best to be close to the ground plane on both sides. The recommended stacking method is shown in Figure 2. The signal quality decreases from left to right, but all of them meet the basic requirements.

Figure 2 Common stacking methods

Like high-speed single-ended lines, differential pairs also have integrity requirements for the reference ground plane. That is, on the path that a differential line pair passes, its reference ground plane must be continuous and no divisions may occur.

2.4 The distance between the differential line pair and other signals

Controlling the distance between the differential line pair and other signals can effectively reduce the interference of other signals on the differential line pair and suppress EMI. We know that the energy of the electromagnetic field decreases with the square of the distance. Generally, when the distance between the differential line pair and other signals is greater than 4 times the differential line width or 3 times the differential line spacing (whichever is larger), the distance between them The influence of time is extremely weak and can basically be ignored. The formula is as follows:

L>4w and L>3d, wherein, L: the distance between the differential line pair and other signals; w: the line width of the differential line; d: the line spacing of the differential line pair.

Here, other signals include other differential lines, single-ended lines, signal planes, and the like. At the same time, the distance between the differential line pair and the edge of its reference plane should also be calculated according to the above method. The purpose of this is to ensure the symmetry of the two differential lines and reduce common mode noise.

2.5 Termination of Differential Pairs

Adding termination resistors to differential line pairs is an effective way to ensure impedance matching of differential transmission lines. The control of the terminal matching resistance should be based on different logic level interfaces, to select an appropriate resistance network and parallel connection with the load, so as to achieve the purpose of impedance matching. At present, the most commonly used differential signals are LVDS and LVPECL. The following will introduce the termination methods of these two signals respectively.

(1) LVDS signal: LVDS is a low-swing differential signal technology, and its transmission rate is generally above several hundred Mb/s. The driver for the LVDS signal consists of a current source that drives the differential line, typically at 3.5 mA. Generally, the termination resistor only needs to be connected in the middle of the positive and negative signals.

(2) LVPECL signal: LVPECL level signal is also one of the differential signal levels suitable for high-speed transmission, and its transmission rate can reach 1 Gb/s. Each of its single-channel signals has a DC potential that is 2 V lower than the signal driving voltage. Therefore, when applying terminal matching, you cannot connect a resistor between the positive and negative differential lines, but only perform single-ended matching on each channel. ,As shown in Figure 6.

It should be noted that with the development of microelectronics technology, many device manufacturers have been able to make the terminal matching resistor inside the device (you can find it in the chip manual) to reduce the work of PCB designers. At this time, the termination can no longer be performed, otherwise the signal quality will be affected instead.

2.6 Other issues to pay attention to

When designing the PCB of differential line pairs, you should also pay attention to the following issues: minimize the use of vias and other factors that cause impedance discontinuity; do not use 90° broken lines, and use arcs or 45° broken lines instead; Add ground plane isolation between differential line pairs to prevent crosstalk between each other;

Don't just ensure that the total length of the traces is equal, but try to make each section of the traces equal (for the division of impedance discontinuities, such as sockets); if not necessary, try not to add test pads on the differential lines.

Summarize

Due to its excellent performance, the differential line pair has gradually become a common method in the design of high-speed digital circuits. In high-speed digital PCB design, the use of differential line pairs to transmit high-speed signals, on the one hand, is of great benefit to the signal integrity and low power consumption of the PCB system, and on the other hand, it also puts forward a higher level for PCB designers. Require. As a PCB design engineer, we should have a deep understanding of the relevant concepts of transmission line theory, carefully analyze the causes of various distortion phenomena, and find out reasonable and effective solutions; we must continue to summarize some of the experience accumulated in the work in order to achieve satisfactory design results. .

The above are the key points of PCB design for differential line pairs introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.