Differential pair

A differential pair is a pair of conductors used for differential signaling. Differential pairs are usually found on a printed circuit board, in cables (twisted-pair cables, ribbon cables), and in connectors. The term can also refer to a pair of transistors used as the input stage of a differential amplifier.

Uses

The technique minimises crosstalk (electronics) and electromagnetic interference, both noise emission and noise acceptance, and can achieve a constant and/or known characteristic impedance, allowing impedance matching techniques important in a high-speed signal transmission line or high quality balanced line and balanced circuit audio signal path.

Differential pairs include:

• twisted-pair cables, shielded and unshielded
• microstrip and stripline differential pair routing techniques on printed circuit boards

The latter can be considered as a PCB implementation of the well-known twisted-pair cable, a common implementation of the differential pair. Differential pairs are generally used to carry differential or semi-differential signals, such as high-speed digital serial interfaces including LVDS, SATA, Hypertransport, Ethernet, Serial Digital Interface, etc. or else high quality and/or high frequency analog signals (e.g. video signals, professional audio signals, etc.)

Data rates of some interfaces implemented with differential pairs

• Serial ATA 1.2 Gbit/s
• Hypertransport 1.6 Gbit/s
• Infiniband 2.5 Gbit/s
• PCI Express 2.5 Gbit/s
• Serial ATA II 2.4 Gbit/s
• XZUI 3.125 Gbit/s
• Serial ATA III 4.8 Gbit/s
• PCI Express II 5.0 Gbit/s
• 10 GbE 10 Gbit/s

See also

• Differential TTL
• Low-voltage differential signaling (LVDS)
• Signal integrity
• DisplayPort
• PCI Express
• Backplanes
• DDR2 SDRAM

Links

• High-Speed Board Designs application note by Altera
• Interactive and Differential Pair Routing by Altium Limited
• Demystifying PCB Impedance Control, Practical PCB Formulas and Plots - Jose Alvarellos, August 30, 2001
• PCB Trace Impedance Calculator
• Differential Impedance Calculator
• PCB Impedance Control: Formulas and Resources - Douglas Brooks, President, UltraCAD Design, Inc.