1. Differential signal pairs with closely spaced wiring will be tightly coupled to each other. This mutual coupling helps to reduce EMI emissions. In PCB layout, the main disadvantage of differential signal lines is that they increase the overall area of the PCB. This article discusses the layout strategies for routing differential signal lines during the board design process.
2. As is well known, a signal typically travels along the signal trace or beneath the PCB trace. While the single-ended wiring approach might be less familiar to some, the term “single-ended” distinguishes this transmission method from differential and common mode signal transmission methods, which are often more complex.
3. **Differential vs. Common Mode**
The differential mode signal is transmitted via a pair of signal lines. One line carries the signal as we usually understand it, while the other line transmits an equal but opposite signal (at least in theory). Initially, differential and single-ended signals may appear similar, as all signals involve loops.
In single-ended mode, the signal is typically returned through a zero-voltage circuit (or ground). In differential mode, each signal is returned through its own ground circuit. Since the two signals in a differential pair are equal and opposite, their return currents cancel each other out, meaning there are no return currents in the zero-voltage or ground plane.
Common mode refers to a signal that appears simultaneously on both lines of a differential pair or on a single-ended signal line and ground. This concept can be non-intuitive, as it is difficult to imagine how such a signal arises. Common mode signals are generally the result of noise, either internally generated under certain conditions or coupled from adjacent or external sources. Common mode signals are almost always undesirable, and many design rules are aimed at preventing their appearance.
4. **Routing Differential Signal Lines**
Typically, differential signals are high-speed signals, and therefore high-speed design rules apply to their routing, especially when designing transmission line traces. This means that careful attention must be given to the signal routing to ensure the characteristic impedance remains consistent and continuous along the signal path.
During the layout of differential pairs, it is important that the two PCB traces are as identical as possible. This means the routing should aim to ensure that the impedance of each line in the differential pair is the same, and that the lengths of the traces are also matched. Differential PCB traces are usually routed in pairs, with the spacing between the two traces kept constant along the entire length. Typically, differential pairs are placed and routed as close together as possible.
5. **Advantages of Differential Signals in PCB Design**
Single-ended signals always reference a “ground” or other “reference” level. This reference could be a positive voltage, ground voltage, a device threshold voltage, or another signal elsewhere. In contrast, differential signals reference the opposite signal in the pair. In other words, if the voltage on one signal line (+) is higher than the voltage on the other signal line (-), a specific logic state is achieved. Conversely, if the voltage on the first line is lower than on the second line, the opposite logic state occurs, as shown in Figure 1.
Differential signals offer several advantages:
1. **Precise Timing**: The timing of a differential signal is more accurately defined because the interaction between the signal pair is simpler to track than the absolute voltage value of a signal relative to a reference. This is why maintaining equal trace lengths in differential pairs is so critical. If the signal reaches the far end of the differential pair at different times, the timing accuracy provided by the source is compromised. Furthermore, if the signal at the far end is not strictly equal but inverted, common-mode noise can arise, leading to timing issues and increased EMI.
2. **Higher Speed**: Since differential signals do not rely on an external reference and can more tightly control the timing of signal transitions, differential circuits typically operate at higher speeds than traditional single-ended circuits.
Because the operation of differential circuits depends on the difference between the signals on the two lines (which are equal in magnitude but opposite in direction), the signal strength is twice that of any single-ended signal in the presence of external noise. As a result, differential signals inherently offer a higher signal-to-noise ratio, resulting in superior performance.
1. The differential circuit is highly sensitive to the voltage difference between the signal levels on the differential pair. However, compared to other references (especially ground), it is less sensitive to the absolute voltage value on the differential line. In general, differential circuits are less affected by issues like ground bounces and other noise signals that may be present on the power and ground planes. For common-mode signals, they will appear identically on both signal lines.
2. Differential signals also offer some immunity to electromagnetic interference (EMI) and crosstalk coupling between signals. When the wiring of a differential pair is compact, any externally coupled noise will affect each signal line in the pair equally. As a result, the coupled noise becomes “common-mode” noise, and the differential signal circuit is immune to this type of interference. If the wires are twisted together (as in twisted pair configurations), the signals become even more resistant to external noise coupling. While it may not be feasible to easily twist the differential signals on a PCB, it is highly beneficial to keep their traces as close together as possible in practical applications.
3. Differential signal pairs that are routed closely together experience strong mutual coupling. This mutual coupling reduces EMI emissions, especially when compared to single-ended signal lines on the PCB. The external radiation from each signal line in the differential pair is equal in magnitude but opposite in direction, effectively canceling each other out—much like a twisted pair. The closer the differential signals are routed, the stronger the coupling between them, resulting in reduced external EMI radiation.
4. The primary disadvantage of differential circuits is the additional PCB traces required. Therefore, if the benefits of differential signals are not essential for the application, it may not justify the increased PCB area. However, if using differential signals leads to a significant improvement in circuit performance, the trade-off for the added routing area is well worth the cost.
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