Differential pair
Encyclopedia
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 cable
s), and in connectors. The term can also refer to a pair of transistors used as the input stage of a differential amplifier
.
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:
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.)
Differential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
. Differential pairs are usually found on a printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
, in cables (twisted-pair cables, ribbon cable
Ribbon cable
A ribbon cable is a cable with many conducting wires running parallel to each other on the same flat plane. As a result the cable is wide and flat. Its name comes from the resemblance of the cable to a piece of ribbon.Ribbon cables are usually seen for internal peripherals in computers, such as...
s), and in connectors. The term can also refer to a pair of transistors used as the input stage of a differential amplifier
Differential amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.- Theory :Many electronic devices use differential amplifiers internally....
.
Uses
The technique minimises crosstalk (electronics)Crosstalk (electronics)
In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel...
and electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
, both noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
emission and noise acceptance, and can achieve a constant and/or known characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
, allowing impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
techniques important in a high-speed signal transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
or high quality balanced line
Balanced line
In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...
and balanced circuit
Balanced circuit
A balanced circuit is circuitry for use with a balanced line or the balanced line itself. Balanced lines are a common method of transmitting many types of electrical communication signals between two points on two wires...
audio signal path.
Differential pairs include:
- twisted-pair cables, shieldedShielded cableA shielded or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper , a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered...
and unshielded - microstripMicrostripMicrostrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components...
and striplineStriplineStripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...
differential pair routing techniques on printed circuit boardPrinted circuit boardA printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
s
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
Sata
Sata is a traditional dish from the Malaysian state of Terengganu, consisting of spiced fish meat wrapped in banana leaves and cooked on a grill.It is a type of Malaysian fish cake, or otak-otak...
, Hypertransport
HyperTransport
HyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...
, Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, Serial Digital Interface
Serial Digital Interface
Serial digital interface is a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video...
, etc. or else high quality and/or high frequency analog signals (e.g. video signals, professional audio
Professional audio
Professional audio, also 'pro audio', refers to both an activity and a type of audio equipment. Typically it encompasses the production or reproduction of sound for an audience, by individuals who do such work as an occupation like live event support, using sound reinforcement systems designed for...
signals, etc.)
Data rates of some interfaces implemented with differential pairs
- Serial ATASerial ATASerial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
1.2 Gbit/s - HypertransportHyperTransportHyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...
1.6 Gbit/s - InfinibandInfiniBandInfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...
2.5 Gbit/s - PCI ExpressPCI ExpressPCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
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 TTLDifferential TTLDifferential TTL is a type of binary electrical signaling based on the TTL standard. Normal TTL signals are single-ended, which means that each signal consists of a voltage on one wire, referenced to a system ground. The "low" voltage level is zero to 0.8 volts, and the "high" voltage level is 2...
- Low-voltage differential signaling (LVDS)
- Signal integritySignal integritySignal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise,...
- DisplayPortDisplayPortDisplayPort is a digital display interface standard produced by the Video Electronics Standards Association . The specification defines a royalty-free digital interconnect for audio and video. The interface is primarily used to connect a video source to a display device such as a computer monitor...
- PCI ExpressPCI ExpressPCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
- BackplaneBackplaneA backplane is a group of connectors connected in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors forming a computer bus. It is used as a backbone to connect several printed circuit boards together to make up a complete...
s - DDR2 SDRAMDDR2 SDRAMDDR2 SDRAM is a double data rate synchronous dynamic random-access memory interface. It supersedes the original DDR SDRAM specification and has itself been superseded by DDR3 SDRAM...
Links
- High-Speed Board Designs application note by AlteraAlteraAltera Corporation is a Silicon Valley manufacturer of PLDs . The company offered its first programmable logic device in 1984. PLDs can be reprogrammed during the design cycle as well as in the field to perform multiple functions, and they support a fairly fast design process...
- 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.