Differential pair

Differential pair is a pair of conductors with special characteristics, used for differential signaling.

Examples of the differential pair include:

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

Differential pairs are usually found on a printed circuit board, in cables (twisted-pair cables, ribbon cables), and in connectors.

Differential pair can be considered as a PCB implementation of a well-known twisted-pair cable. However, from the modern point of view, twisted-pair is nothing but particular implementation of a 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.)
  • High quality and/or high frequency analog signals (e.g. video signals, professional audio signals, etc.)
• Minimize cross-talk and electromagnetic interference, both noise emission and noise acceptance by the signal carried by differential pair
• Achieve constant and/or known characteristic impedance, which is important in high-speed signal transmission, to use impedance matching techniques

Amplifiers

Differential pair can also refer to the pair of transistors used as the input stage of a differential amplifier.

See also

• Balanced line
• Balanced circuit
• Characteristic impedance
• Crosstalk (electronics)
• Electromagnetic interference
• Differential TTL
• Impedance matching
• Low-voltage differential signaling (LVDS)
• Printed circuit board
• Transmission line
• Signal integrity
• Twisted pair

Uses of Differential Pairs

• DisplayPort
• PCI Express
• LVDS
• SATA
• Backplanes
• DDR2 SDRAM

Data rates of some interfaces implemented with differential pairs

• Serial ATA 1.25 Gbit/s
• Hypertransport 1.6 Gbit/s
• Infiniband 2.5 Gbit/s
• PCI Express 2.5 Gbit/s
• Serial ATA II 2.5 Gbit/s
• XZUI 3.125 Gbit/s
• PCI Express II 5.0 Gbit/s
• 10 GbE 10 Gbit/s

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.

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