PCB Tech - Advantages of PCB wiring in circuit boards

Differential signal pairs with very close wiring will also be tightly coupled to each other. This mutual coupling will reduce EMI emission. In PCB layout, the main disadvantage of differential signal lines is that they increase the area of the PCB. This article introduces the circuit The layout strategy of differential signal line routing is adopted in the board design process.

As we all know, the signal has the characteristic of transmitting along the signal line or under the PCB line. Even though we may not be familiar with the single-ended mode wiring strategy, the term single-ended distinguishes this transmission characteristic of the signal from the differential mode and common mode signal transmission methods. In the future, the latter two signal transmission methods are usually more complicated.

Differential and common mode

The differential mode signal is transmitted through a pair of signal lines. One signal line transmits the signal we usually understand; the other signal line transmits a signal of equal value but opposite direction (at least in theory). Differential and single-ended modes are not much different when they first appear, because all signals have loops.

The signal in single-ended mode is usually returned via a zero-voltage circuit (or called ground). Each signal in the differential signal must be returned through the ground circuit. Since each signal pair is actually equal and reversed, the return circuits simply cancel each other out, so there will be no return components of the differential signal on the zero-voltage or ground circuit.

Common mode means that the signal appears on two signal lines of a (differential) signal line pair, or appears on the single-ended signal line and ground at the same time. The understanding of this concept is not intuitive, because it is difficult to imagine how to generate such a signal. This is mainly because we usually do not generate common mode signals. Most common-mode signals are noise signals generated in the circuit according to hypothetical conditions or coupled in by adjacent or external signal sources. Common mode signals are almost always "harmful", and many design rules are designed to prevent common mode signals from appearing.

Routing of differential signal lines

Usually (with some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply to the routing of differential signals, especially when designing signal lines such as transmission line 1. This means that we must carefully design the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous and constant along the signal line.

In the layout and routing process of the differential pair, we hope that the two PCB lines in the differential pair are exactly the same. This means that in practical applications, the greatest effort should be made to ensure that the PCB lines in the differential pair have exactly the same impedance and the length of the wiring is exactly the same. Differential PCB lines are usually routed in pairs, and the distance between them is kept constant at any position along the line pair direction. Under normal circumstances, the placement and routing of differential pairs is always as close as possible.

In PCB design, the advantages of differential signals

Single-ended signals always refer to some kind of "reference" level. This "reference" level may be a positive voltage or ground voltage, the threshold voltage of a device, or another signal somewhere else. On the other hand, the differential signal always refers to the other side of the differential pair. In other words, if the voltage on one signal line (+ signal) is higher than the voltage on another signal line (- signal), then we can get a logic state; and if the former is lower than the latter, then we can Get another logic state, see Figure 1.

Differential signals have the following advantages: 1. The timing is precisely defined, because the intersection of the control signal line pair is simpler than the absolute voltage value of the control signal relative to a reference level. This is also one of the reasons why it is necessary to accurately realize the equal-length wiring of the differential pair. If the signal cannot reach the other end of the differential pair at the same time, then any timing control that the source can provide will be greatly compromised. In addition, if the signal at the far end of the differential line is not equal in the strict sense but reversed, common mode noise will appear, which will cause problems in signal timing and EMI. 2. Since differential signals do not refer to any signals other than their own, and can more strictly control the timing of signal crossing points, differential circuits can usually work at higher speeds than conventional single-ended signal circuits.

Since the operation of the differential circuit depends on the difference between the signals on the two signal lines (their signals are equal but opposite), compared with the surrounding noise, the signal obtained is twice the size of any single-ended signal. Therefore, under all other conditions being the same, the differential signal always has a higher signal-to-noise ratio and thus provides higher performance.

The differential circuit is very sensitive to the difference between the signal levels on the differential pair. But compared to some other references (especially ground), they are not sensitive to the absolute voltage value on the differential line. Relatively speaking, differential circuits are insensitive to problems like ground bounces and other noise signals that may exist on the power and ground planes, while for common-mode signals, they will appear exactly in each case. One signal line.

Differential signals also have certain immunity to EMI and crosstalk coupling between signals. If the wiring of a pair of differential signal pairs is very compact, any externally coupled noise will be coupled to each signal line in the pair to the same degree. Therefore, the coupled noise becomes "common mode" noise, and the differential signal circuit has perfect immunity to this signal. If the wire pair is twisted together (such as twisted pair), the signal wire will be more immune to coupled noise. Since it is impossible to easily twist the differential signals on the PCB, it is a very good way to put their wiring as close together as possible in practical applications.

Differential signal pairs that are routed very close to each other are also tightly coupled to each other. This mutual coupling will reduce EMI emissions, especially when compared to single-ended PCB signal lines. It can be imagined that the external radiation of each signal line in the differential signal is equal in magnitude but opposite in direction, so they will cancel each other, just like the signal in a twisted pair. The closer the differential signals are routed, the stronger the coupling between them, and the smaller the external EMI radiation.

The main disadvantage of the differential circuit is the addition of PCB lines. Therefore, if the advantages of differential signals cannot be used in the application process, it is not worth increasing the PCB area. But if there is a major improvement in the performance of the designed circuit, the price paid for the increased wiring area is worth it.