**How to Route a Good Differential Pair on a PCB Copy Board:**

1. Before delving into the importance of differential pair alignment, it is essential to first understand the factors that influence the impedance of a transmission line. Key factors include the line width, line length, line thickness, sidewall profile, resistive layer coverage, and the medium through which the transmission line passes. Additionally, the dielectric constant and dielectric thickness also affect the accuracy of impedance. For the exact calculation formulas, refer to specialized books on signal integrity analysis.

2. Understanding these factors is crucial for designing effective routing, especially when the goal is to achieve controlled impedance. By adjusting the line width and thickness using appropriate software tools, we can achieve a fixed impedance for the transmission line. A differential pair gets its name because the signal on each trace represents the difference between two complementary, mutually referenced signals, which significantly reduces external interference. The differential traces should be routed in a closely spaced, parallel manner. The term “appropriate proximity” refers to the spacing between the traces, which directly impacts the differential impedance — a critical parameter in differential pair design.


1. **Parallel Wiring Requirement**

The circuit design should implement parallel wiring. If all signal lines are single-ended, proper impedance design must be applied. Typically, if the signal lines at both ends have different ground potentials, the system may still operate normally, even if the potential gap is significant. However, this difference can lead to system malfunctions. Using differential routing or alignment is an effective solution to this issue.

2. **Advantages of Differential Line PCB Copy**

The PCB copy board ensures that the differential lines have equal length and impedance, while passing through the same environment. This provides inherent advantages for maintaining signal stability.

3. **Differential Signal Marking in Schematic Design**

In PCB schematic design, differential signals are typically labeled with “_n” and “_p” to indicate their backward loss. Differential pairs effectively address the lack of a solid reference connection between the signal source and the load, thus mitigating electronic interference and reducing the electromagnetic interference (EMI) emitted by the signal lines.

4. **Why Differential Lines Reduce Noise**

Why are differential lines effective in eliminating noise? First, let’s look at the general design requirements for differential lines: both lines should have equal length, typically within a 5% tolerance. The distance between the two differential lines should be 3 times their width (3w), and the surrounding cladding around the differential pair should be well-designed to minimize issues.

5. **Reducing EMI and Noise with Differential Pairs**

This design approach ensures that the magnetic fields generated by the two signal lines cancel each other out, thereby reducing EMI. Furthermore, if external noise is introduced to both lines simultaneously, the differential signal will effectively cancel out ground noise, similar to how traditional three-op-amp circuits function.

6. **PCB Differential Pair Routing Considerations**

When routing the PCB differential pair, try to keep the lines on the same layer as much as possible. Routing across different layers can introduce impedance discontinuities due to vias. Additionally, if the layer is switched, the loop current might not form a low-impedance path, potentially resulting in unwanted RF interference. As the differential pair length increases, common-mode RF energy can also have an impact. Another factor to consider is that different board layers have different signal transmission speeds. As seen in signal integrity analyses, microstrip lines generally transmit signals faster than stripline configurations, which can lead to signal delays.

7. **Considerations for Differential Pair Connectivity**

In terms of connectivity, it’s important to address potential issues with differential pair connections. For instance, if the load is capacitive rather than direct, additional attention is needed to ensure proper differential pair connection.

8. **Impedance Matching in PCB Copy Circuit Design**

In the PCB copy circuit design, special attention must be given to terminal impedance matching to avoid reflection and EMI issues. Differential signal transmission generally uses differential mode, whereas common mode is another transmission mode to consider.

9. **Reference Design for Terminal Impedance Matching**

The reference design for terminal resistance matching is outlined below. Typically, an impedance analyzer is used to measure and calibrate the matching resistance. Common values for differential terminal resistance are 50 ohms or 100 ohms, though further fine-tuning may be required in practice. Since differential mode signals are referenced to each other rather than to ground, common-mode RF energy is not present. It’s also beneficial to set the differential mode as the initial PCB design model, and then adjust based on different mode testing during the debugging phase.

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