LVDS

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.

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.

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.