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Low-voltage differential signaling, or LVDS, also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard, but it is not a protocol. LVDS operates at low power and can run at very high speeds using inexpensive twisted-pair copper cables. LVDS is a physical layer specification only; many data communication standards and applications use it and add a data link layer as defined in the OSI model on top of it.

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LVDS was introduced in 1994, and has become popular in products such as LCD-TVs, automotive infotainment systems, industrial cameras and machine vision, notebook and tablet computers, and communications systems. The typical applications are high-speed video, graphics, video camera data transfers, and general purpose computer buses.

Early on, the notebook computer and LCD display vendors commonly used the term LVDS instead of FPD-Link when referring to their protocol, and the term LVDS has mistakenly become synonymous with Flat Panel Display Link in the video-display engineering vocabulary.

Differential vs. single-ended signaling[edit]

LVDS is a differential signaling system, meaning that it transmits information as the difference between the voltages on a pair of wires; the two wire voltages are compared at the receiver. In a typical implementation, the transmitter injects a constant current of 3.5 mA into the wires, with the direction of current determining the digital logic level. The current passes through a termination resistor of about 100 to 120 ohms (matched to the cable's characteristic impedance to reduce reflections) at the receiving end, and then returns in the opposite direction via the other wire. From Ohm's law, the voltage difference across the resistor is therefore about 350 mV. The receiver senses the polarity of this voltage to determine the logic level.

As long as there is tight electric- and magnetic-field coupling between the two wires, LVDS reduces the generation of electromagnetic noise. This noise reduction is due to the equal and opposite current flow in the two wires creating equal and opposite electromagnetic fields that tend to cancel each other. In addition, the tightly coupled transmission wires will reduce susceptibility to electromagnetic noise interference because the noise will equally affect each wire and appear as a common-mode noise. The LVDS receiver is unaffected by common mode noise because it senses the differential voltage, which is not affected by common mode voltage changes.

The fact that the LVDS transmitter consumes a constant current also places much less demand on the power supply decoupling and thus produces less interference in the power and ground lines of the transmitting circuit. This reduces or eliminates phenomena such as ground bounce which are typically seen in terminated single-ended transmission lines where high and low logic levels consume different currents, or in non-terminated transmission lines where a current appears abruptly during switching.

The low common-mode voltage (the average of the voltages on the two wires) of about 1.2 V allows using LVDS with a wide range of integrated circuits with power supply voltages down to 2.5 V or lower. In addition, there are variations of LVDS that use a lower common mode voltage. One example is sub-LVDS (introduced by Nokia in 2004) that uses 0.9 V typical common mode voltage. Another is Scalable Low Voltage Signaling for 400 mV (SLVS-400) specified in JEDEC JESD8-13 October 2001 where the power supply can be as low as 800 mV and common mode voltage is about 400 mV.

The low differential voltage, about 350 mV, causes LVDS to consume very little power compared to other signaling technologies. At 2.5 V supply voltage the power to drive 3.5 mA becomes 8.75 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal.

Logic levels:^[1]

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LVDS is not the only low-power differential signaling system in use, others include the Fairchild Current Transfer Logic serial I/O.

Applications[edit]

LVDS became popular in the mid 1990s. Before that, computer monitor resolutions were not large enough to need such fast data rates for graphics and video. However, in 1992 Apple Computer needed a method to transfer multiple streams of digital video without overloading the existing NuBus on the backplane. Apple and National Semiconductor (NSC) created QuickRing, which was the first integrated circuit using LVDS. QuickRing was a high speed auxiliary bus for video data to bypass the NuBus in Macintosh computers. The multimedia and supercomputer applications continued to expand because both needed to move large amounts of data over links several meters long (from a disk drive to a workstation for instance).

The first commercially successful application for LVDS was in notebook computers transmitting video data from graphics processing units to the flat panel displays using the Flat Panel Display Link by National Semiconductor. The first FPD-Link chipset reduced a 21-bit wide video interface plus the clock down to only 4 differential pairs (8 wires), which enabled it to easily fit through the hinge between the display and the notebook and take advantage of LVDS's low-noise characteristics and fast data rate. FPD-Link became the de facto open standard for this notebook application in the late 1990s and is still the dominant display interface today in notebook and tablet computers. This is the reason IC vendors such as Texas Instruments, Maxim, Fairchild, and Thine produce their versions of the FPD-Link chipset.

The applications for LVDS expanded to flat panel displays for consumer TVs as screen resolutions and color depths increased. To serve this application, FPD-Link chipsets continued to increase the>OutputInputCommon
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SCI-LVDS[edit]

The present form of LVDS was preceded by an earlier standard initiated in Scalable Coherent Interconnect (SCI). SCI-LVDS was a subset of the SCI family of standards and specified in the IEEE 1596.3 1995 standard. The SCI committee designed LVDS for interconnecting multiprocessing systems with a high-speed and low power interface to replace positive emitter-coupled logic (PECL).

Standards[edit]

The ANSI/TIA/EIA-644-A (published in 2001) standard defines LVDS. This standard originally recommended a maximum data rate of 655 Mbit/s over twisted-pair copper wire, but data rates from 1 to 3 Gbit/s are common today on high quality transmission mediums.^[3]Today, technologies for broadband digital video signal transmission such as LVDS are also used in vehicles, in which the signal transmitted as a differential signal helps for EMC reasons. However, high-quality shielded twisted pair cables must be used together with elaborate connector systems for cabling. An alternative is the use of coaxial cables. Studies have shown that it is possible in spite of the simplified transfer medium dominate both emission and immunity in the high frequency range. Future high-speed video connections can be smaller, lighter and cheaper to realize.

Serial video transmission technologies are widely used in the automobile for linking cameras, displays and control devices. The uncompressed video data has some advantages for certain applications. Serial communication protocols now allow the transfer of data rates in the range of 3 to 4 Gbit/s and thus the control of displays with up to full HD resolution. The integration of the serializer and deserializer components in the control unit due to low demands on additional hardware and software simple and inexpensive. In contrast, require bus solutions for video transmission connection to a corresponding network controller and, if necessary resources for data compression. Since for many applications a full function network is not required throughout the video architecture and for some compounds, data compression is not feasible due to image quality loss and additional latency, bus oriented video transmission technologies are currently only partially attractive.

See also[edit]

• Current-mode logic, another differential signaling standard
• FPD-Link, a similar but different LVDS
• Positive emitter-coupled logic (PECL and LVPECL)
• Display controller, an IC that produces the signal

References[edit]

1. ^Interfacing Between LVPECL, VML, CML, and LVDS Levels, SLLA120, Texas Instruments, December 2002.
2. ^Leading PC Companies Move to All Digital Display Technology, Phasing out Analog
3. ^'EIA-644 Bus Description, RS644 LVDS'. 080310 interfacebus.com

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External links[edit]

• LVDS Application and Data Book, SLLD009, Texas Instruments, November 2002.
• An Overview of LVDS Technology, AN-971, Texas Instruments, July 1998.
• LVDS Owner's Manual, 4th Edition, Texas Instruments, 2008.
• Introduction to M-LVDS (TIA/EIA-899), SLLA108, Texas Instruments, February 2002.
• Scalable Low-Voltage Signaling SLVS-400, JEDEC Standard, JESD8-13, October 2001.
• LVDS Compatibility with RS422 and RS485 Interface Standards, AN-5023, Fairchild Semiconductor, July 2002.

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