Differential Pair

A differential pair is two PCB traces routed side by side, carrying signals of equal magnitude and opposite polarity. The receiver looks only at the voltage difference between the two lines, so any noise that couples equally onto both traces (common-mode noise) cancels out. This makes differential signaling the standard for high-speed interfaces such as USB, PCIe, HDMI, and Ethernet, where single-ended traces would be too noise-prone.

Routing requirements

The two traces must stay tightly and symmetrically coupled to work. Key rules are matched length (so both edges arrive together, avoiding skew), constant spacing (which sets the differential impedance), and a continuous reference plane underneath. Common target differential impedances are roughly 90 ohms for USB, 100 ohms for Ethernet, and 85 ohms for PCIe. Breaking symmetry, crossing a plane split, or letting one trace wander converts useful differential signal into common-mode noise that the cancellation can no longer remove.

Relevance to mining boards

Control boards in ASIC miners use differential pairs for clocks and high-speed buses between the SoC, memory, and the ASIC chains. When a board is reworked, a lifted pad or a hand-soldered jumper on one half of a pair introduces length mismatch and impedance discontinuity, which can surface as flaky communication or chains that drop offline under load. Treat both traces of a pair as a single unit during any repair.

Differential pairs are a specific case of controlled impedance routing, and they rely on a solid ground plane for their return path.