PCB News - Main points of PCB design for differential pair

Differential signal transmission has many advantages compared with single-ended signal transmission:

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 is only concerned with 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. The tighter the coupling, the less the 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 the single-ended signal, which depends on the high and low threshold voltages, it is less affected by the process and temperature, which can reduce the error in the timing. It is also more suitable for the circuit design of low-amplitude signals.

For PCB engineers, the most concern is how to ensure that the advantages of differential signals can be fully utilized in actual wiring. Anyone who has been exposed to 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 mechanically apply, and many engineers seem to have not done an in-depth analysis of the actual design and processing of differential line pairs. The following will focus on discussing several common points in PCB differential signal PCB design.

1 equal length

The equal length is to make the signal transmission delay on each line the same, to ensure that the two differential signals keep the opposite polarity at all times. Any delay difference between the two transmission lines will cause part of the differential signal to become a common mode signal, which will seriously affect the signal quality.

Equal length is to make the wiring length of the two signal lines of the differential pair as the same as possible. Generally, the matching requirement for high-speed differential signals with equal length is within ±10 mils. Of course, this is a higher requirement. The real value can be calculated by allowing signal misalignment (skew, which can be found on the chip manual) and signal transmission delay (generally 180 picoseconds per inch).

Due to the device layout, pin distribution, etc., the differential lines generated by direct wiring are not of equal length in most cases, which requires manual winding. Manual winding is usually carried out at the chip pins, the purpose is to reduce the impedance discontinuity of the differential line pair. Figure 1 shows two commonly used winding methods.

2 Isometric

The equidistance is to ensure the continuity of the differential impedance between the differential line pairs and reduce reflection. Differential impedance is an important parameter for designing a differential pair. If it is not continuous, 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 50 Ω, so in general, the differential impedance should be kept at 100 Ω. Equidistance is to keep the distance between the differential line pairs equal (that is, parallel routing) to ensure that the differential impedance of the differential line pair does not change throughout.

Differential impedance is related to the line width of the differential line pair, line spacing, printed board stacking order, dielectric constant of the medium and many other parameters. Manufacturers jointly negotiate and determine parameters such as line spacing. It is worth noting that when a differential signal is transmitted on different layers of a multi-layer PCB (especially when the inner and outer layers are routed), the line spacing must be adjusted in time to compensate for the characteristic impedance change caused by the change of the dielectric constant of the medium. Compared with unequal length, unequal distance has less influence on signal integrity. When the equal length conflicts with the equal distance rule, the equal length should be satisfied first.

3 Stacking of differential pair and printed board

The stacking of the PCB board is closely related to the signal coupling and shielding. There is a view that differential line pairs provide a return path for each other, so differential signals do not need a ground plane as a return path. This 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 trace still exists on the ground plane. In PCB design, differential signals are required to be adjacent to at least one ground plane, and it is best that both sides are adjacent to the ground plane. The recommended stacking method is shown in Figure 2. The signal quality decreases from left to right, but it can meet the basic requirements.

Like high-speed single-ended lines, differential pairs also have integrity requirements for the reference ground plane. That is, on the path that the differential pair passes through, its reference ground plane must be continuous and cannot be divided, as shown in Figure 3.

4 The distance between the differential 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 [1]. We know that the electromagnetic field energy decreases with the square of the distance. Generally, the distance between the differential line pair and other signals is greater than 4 times the differential line width or 3 times the distance between the differential line pairs (whichever is greater), whichever is greater. The influence of time is extremely weak and can basically be ignored. The formula is as follows:

L>4w and L> 3d,

Among them, 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, etc. At the same time, the distance between the differential line pair and the edge of its reference plane should also be calculated in the above manner. The purpose of this is to ensure the symmetry of the two differential lines and reduce common mode noise.

5 Termination of differential pair

Adding termination resistance to the differential line pair is an effective way to ensure the impedance matching of the differential transmission line. The control of the terminal matching resistance should be based on different logic level interfaces, to select an appropriate resistance network and load in parallel, in order to achieve the purpose of impedance matching .

At present, the most commonly used differential signals are LVDS and LVPECL. The following describes the termination methods of these two signals.

(1) LVDS signal

LVDS is a low-swing differential signal technology, and its transmission rate is generally above several hundred Mb/s .The driver of the LVDS signal consists of a current source that drives the differential line, usually with a current of 3.5 mA. Termination resistors generally only need to be connected across 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 less than the signal drive voltage. Therefore, when terminal matching is applied, resistances cannot be connected between the positive and negative differential lines, but each channel can only be single-ended matching.,As shown in Figure 6.

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

6 Other issues to pay attention to

When designing the PCB of the differential pair, you should also pay attention to the following issues: try to reduce the use of vias and other factors that cause impedance discontinuity; do not use 90° fold lines, can be replaced by arcs or 45° fold lines; if necessary, use different Add ground plane isolation between differential line pairs to prevent crosstalk between each other;

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