With the proliferation of high-speed interfaces such as USB 3.0, HDMI, and PCIe, differential signal transmission has become the mainstream standard. Controlling differential impedance is more complex than controlling single-ended impedance; it involves not only the impedance of the individual traces but also the coupling effects between them.
1. The Coupling Mechanism of Differential Pairs
Differential impedance refers to the impedance between two conductors when they carry signals of opposite polarity. When the two traces of a differential pair are placed in close proximity, electric and magnetic field coupling occurs between them. This coupling alters the impedance characteristics of the individual traces.
The smaller the spacing between the traces, the stronger the coupling, and consequently, the lower the differential impedance. Therefore, when designing differential pairs, both trace width and spacing are variables that must be optimized simultaneously. Typically, we require the traces within a differential pair to be "equal in length and spacing" to maintain impedance continuity.
2. Common-Mode Impedance and EMC
In addition to differential impedance, there is a concept known as common-mode impedance—the impedance to ground when both traces carry the same signal simultaneously.
Although differential signals primarily rely on differential-mode transmission, common-mode signals are inevitably generated in practical circuits due to driver imbalance or asymmetry in the transmission path. Common-mode signals are a primary source of electromagnetic interference (EMI).
If the common-mode impedance is too low, common-mode currents can easily return through the ground plane, causing radiation. Therefore, in design, we must not only ensure differential impedance matching (e.g., 90 ohms) but also pay attention to common-mode impedance to avoid antenna effects.
3. Integrity of the Reference Plane
The return path is equally critical for differential pairs. Many people mistakenly believe that differential signals do not require a reference plane because the currents supposedly form a loop between the two traces. In reality, high-frequency signals always seek the path of least inductance.
If the reference plane is incomplete (such as crossing a split or gap), the return path is forced to detour, leading to a drastic increase in loop inductance and abrupt impedance changes. This not only disrupts the balance of the differential signal but also converts differential-mode signals into common-mode signals, which then radiate interference outwards.
Therefore, a complete ground plane must be maintained beneath differential traces; routing across splits is strictly prohibited.
4. The Impact of Vias on Differential Impedance
When differential signals need to change layers, vias must be used. Vias introduce parasitic capacitance and inductance, causing localized impedance discontinuities. For high-speed differential signals, such discontinuities cause signal reflections.
To mitigate these effects, ground vias should be placed near the signal vias to provide the shortest return path for the return current. Additionally, back-drilling techniques can be employed to remove the stub of the via, reducing capacitive loading and thereby improving impedance matching characteristics.
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