Advantages and strategies for differential signal line routing

It is well known that signals exist along signal lines or under PCB lines. Even though we may not be familiar with single-ended mode routing strategies, the term single-ended distinguishes this transmission characteristic from that of differential and common-mode signals. The latter two signal transmission methods are usually more complicated.

1. Differential and common mode

The differential mode signal is transmitted through a pair of signal lines. One signal line transmits the signal we normally understand; the other signal line transmits a signal that is equal and opposite in direction (at least in theory). Differential and single-ended modes initially differed little because all signals have loops.

Signals in single-ended mode are typically returned via a zero voltage circuit (or ground). Each of the differential signals is returned through the ground circuit. Since each signal pair is actually equivalent and inverted, the return circuits simply cancel each other out, so that the components of the differential signal return do not occur on the zero voltage or ground circuit.

Common mode means that the signal appears on two signal lines of a (differential) signal pair, or both on a single-ended signal line and on the ground. 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 generated in the circuit according to imaginary conditions or coupled by adjacent or external signal sources. Common-mode signals are almost always "harmful" and many design rules are designed to prevent the appearance of common-mode signals.

2. Wiring of differential signal lines

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

During the placement and routing of differential pairs, we want the two PCB lines in the differential pair to be exactly the same. This means that in practical applications, every effort should be made to ensure that the PCB lines in the differential pairs have exactly the same impedance and that the length of the wiring is exactly the same. Differential PCB lines are usually always wired in pairs, and the distance between them remains constant at any position along the direction of the pair. In general, the layout of differential pairs is always as close as possible.

3. 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 threshold voltage of a device, or another signal elsewhere. On the other hand, the differential signal always refers to the other of the differential pairs. That is, if the voltage on one signal line (+ signal) is higher than the voltage on the other signal line (-signal), then we can get a logic state; if the former is lower than the latter, then we can An additional logic state is obtained.

The differential signal has several advantages: 1. The timing is precisely defined because the intersection of the control signal pair is simpler than the absolute voltage of the control signal relative to a reference level. This is also a reason to accurately implement differential wire pair equal length routing. 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 pair is not exactly equivalent in the opposite direction, then common mode noise will occur, which will cause signal timing and EMI problems. 2. Since differential signals do not refer to any signal other than themselves and the timing of signal crossings can be more tightly controlled, differential circuits can typically operate 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 inverted), the resulting signal is twice the size of any single-ended signal compared to the surrounding noise. Therefore, differential signals always have a higher signal-to-noise ratio and thus provide higher performance under all other conditions.

The differential circuit is very sensitive to the difference between the signal levels on the differential pair. But relative to some other references (especially ground), they are not sensitive to absolute voltage values on the differential line. In contrast, differential circuits are insensitive to problems such as similar bounce reflections and other noise signals that may be present on the power and ground planes, and for common mode signals, they appear exactly the same in each A signal line.

The differential signal also has some immunity to crosstalk coupling between EMI and the signal. If the wiring of a pair of differential signal pairs is very compact, then any externally coupled noise will be coupled to each of the pairs in the same degree. Therefore, the coupled noise becomes "common mode" noise, and the differential signal circuit has perfect immunity to this signal. If the pairs are twisted together (such as twisted pairs), the signal lines are more immune to the coupling noise. Since it is not possible to easily twist the differential signals on the PCB, it is a very good idea to put their wiring close together as much as possible.

The differential signal pairs that are in close proximity 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 each of the signal lines in the differential signal is equal in magnitude and opposite in direction, and therefore cancels each other as if the signal were in a twisted pair. The closer the differential signal is to the wiring, 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 the differential signal cannot be utilized 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, then the cost of the increased cabling area is worthwhile.

4. Summary of this article

The differential signal lines are coupled to each other. This coupling affects the external impedance of the signal line, so a termination matching strategy must be used (see discussion in Note 2 and calculation of differential impedance). The calculation of differential impedance is difficult, and National Semiconductor provides some references in this field. Polar Instruments also offers a separate differential impedance calculator that can calculate many different differential signal structures3 (some cost). The high-end design kit also calculates the differential impedance.

However, it should be noted that the mutual coupling between the differential lines will directly affect the calculation of the differential impedance. The coupling between the differential lines must be guaranteed to remain constant along the entire differential line or to ensure continuity of impedance. This is why the "constant spacing" design rule must be maintained between the differential lines.