What are the advantages of using differential signal line wiring in circuit boards?
Differential signal pairs that are very close to each other will also be closely coupled to each other. This mutual coupling will reduce EMI emissions. The main disadvantage of differential signal lines is that it increases the area of the PCB. This article introduces the use of Wiring strategy for differential signal line routing.
As we all know, the signal has the characteristics of transmission along the signal line or PCB line, even if we may not be familiar with single-ended mode wiring strategy, the term single-ended distinguishes this transmission characteristic of the signal from differential and common mode signal transmission In the future, the latter two signal transmission methods are usually more complicated.
Differential and common mode
Differential mode signals are transmitted through a pair of signal lines. One signal line transmits what we usually understand; the other signal line transmits a signal of equal value and opposite direction (at least in theory). Differential and single-ended modes did not differ much when they first appeared because all signals have loops.
Signals in single-ended mode are usually returned via a zero-voltage circuit (or ground). Each of the differential signals is returned through a ground circuit. Because each signal pair is actually equivalent and reversed, the return circuits simply cancel each other out, so no component of the differential signal return will appear on the zero-voltage or ground circuit.
Common mode means that the signal appears on the two signal lines of a (differential) signal line pair, or 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 of the common-mode signals are noise signals that are generated in the circuit according to the imaginary situation or are coupled by nearby or external signal sources. Common-mode signals are almost always "harmful," and many design rules are designed to prevent common-mode signals from appearing.
Wiring of differential signal lines
Usually (with some exceptions, of course) differential signals are also high-speed signals, so high-speed design rules are usually applicable to the wiring 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 at all points along the signal line.
During the layout of the differential pair, we want the two PCB lines in the differential pair to be completely identical. This means that, in practical applications, every 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 always routed in pairs, and the distance between them is kept constant at any position along the direction of the line pair. In general, the layout of the differential pair is always as close as possible.
Advantages of differential signals
Single-ended signals are always always referenced to some "reference" level. This "reference" level may be a positive voltage or a ground voltage, a device's threshold voltage, or another signal somewhere else. Differential signals, on the other hand, always refer to the other of the differential line pairs. That is, 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; 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. This is because the crossing point 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 a reason for the need to accurately implement the equal-length wiring of differential line pairs. If the signals 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 reduced. In addition, if the signal at the far end of the differential pair is not strictly equivalent and reverses, then common-mode noise will occur, which will cause problems with signal timing and EMI. 2. Because differential signals do not refer to any signal other than themselves, and they can control the timing of signal crossing points more strictly, differential circuits can usually operate at higher speeds than conventional single-ended signal circuits.
Because the operation of a differential circuit depends on the difference between the signals on two signal lines (their signals are equal and reversed), compared to the surrounding noise, the resulting signal is twice the size of any single-ended signal. Therefore, under the same conditions in all other cases, the differential signal always has a higher signal-to-noise ratio and thus provides higher performance.
Differential circuits are very sensitive to differences between signal levels on differential pairs. But relative to some other references (especially ground), they are not sensitive to the absolute voltage value on the differential line. In contrast, differential circuits are insensitive to problems such as ground bounce and other noise signals that may exist on the power and ground planes, while for common-mode signals, they appear completely uniformly in each On a signal line.
Differential signals are also immune to crosstalk coupling between EMI and signals. If the wiring of a pair of differential signal pairs is very compact, any externally coupled noise will be coupled to every signal line in the pair to the same extent. So the coupled noise becomes "common mode" noise, and the differential signal circuit has perfect immunity to this signal. If the pair is twisted together (such as a twisted pair), the signal line is more immune to coupling noise. Since it is impossible to easily twist the differential signals on the PCB, it is a very good method to put them as close together as possible in practical applications.
Differential signal pairs that are very close to each other are also tightly coupled to each other. This mutual coupling reduces 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 size and opposite in direction, so they will cancel each other out, just like the case of signals 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 differential circuits 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. However, if there is a significant improvement in the performance of the designed circuit, the cost of the increased wiring area is worth it.
Summary of this article
Differential signal lines are coupled with each other. This coupling affects the external impedance of the signal line, so a termination matching strategy must be used (see the discussion in Note 2 and the calculation of differential impedance). The calculation of differential impedance is difficult, and National Semiconductor provided some references in this area. Polar Instruments also provides an independent differential impedance calculator that can calculate many different differential signal structures3 (with some cost). High-end design kits can also calculate differential impedance.
However, it should be noted that the mutual coupling between differential lines will directly affect the calculation of differential impedance. Coupling between differential lines must ensure that a constant is maintained along the entire differential line or impedance continuity is ensured. This is why the "constant spacing" design rule must be maintained between the differential lines.