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LVDS (low-voltage differential signaling)

A technical standard for electrical characteristics of a low-power differential, serial communication protocol.


LVDS (low-voltage differential signaling) is a high-speed, long-distance digital interface for serial communication (sending one bit at time) over two copper wires (differential) that are placed at 180 degrees from each other. This configuration reduces noise emission by making the noise more findable and filterable. LVDS is often used in SerDes configurations.

LVDS outputs are capable of keeping up with the high data rates and keeping noise emission low, thus protecting the performance of the analog front end. Additionally, some manufacturers have reduced the number of wires necessary to transmit the signals from one point to another by serializing the output bits onto an LVDS pair, which further improves accuracy by avoiding skew-related errors that can occur when transmitting data in parallel.

LVDS has become the solution for many applications that demand low power consumption and high noise immunity for high data rates. Since its standardization under ANSI/TIA/EIA-644, LVDS has been implemented in a diverse set of applications and industries. The LVDS standard provides guidelines that define the electrical characteristics for the driver output and receiver input of an LVDS interface, but stop short of defining a specific communication protocol, required process technology, media, or voltage supply. The general, non-application-specific nature of the standard has been conducive to the adoption of LVDS across a wide variety of commercial and military applications. Moreover, growing demands for bandwidth have resulted in the emergence of high-performance technologies such as PCI Express and HyperTransport, which are based on high-speed LVDS connections. The low power and high noise immunity aspects of LVDS, along with the abundance of commercial off-the-shelf (COTS) LVDS components has led many military and aerospace applications to select LVDS as a robust, long-term solution for high-speed data transmission.

Electrical Characteristics of LVDS
The LVDS standard defines the electrical characteristics of the transmitter and receiver of an LVDS interface. LVDS uses differential signals with low voltage swings to transmit data at high rates. Differential signals contrast to traditional single-ended signals in that two complementary lines are used to transmit a signal instead of one line. That is, two signals are generated of opposite polarity, and then the data transmission references the two signals to one another. This transmission scheme provides the kind of large common-mode rejection and noise immunity to a data transmission system that a single-ended system referenced only to ground cannot provide.

Figure 1 illustrates a typical LVDS transmitter. This transmitter consists of a current-mode driver, which provides around 3.5 mA of current through the transmission lines of the differential pair. At the receiver, a 100 Ω termination resistor is used to match the impedance of the transmission line that connects the receiver to the driver. Closely matching the impedance of this termination resistor with the impedance of the transmission lines reduces harmful signal reflections that decrease signal quality. The termination resistor also provides a path between the complementary signal paths of the system. The high input impedance of the receiver causes the 3.5 mA current coming from the driver to flow through the 100 Ω termination resistor, resulting in a voltage difference of 350 mV between the receiver inputs. As the path for the current within the driver changes from one path to another, the direction of the current flowing through the termination resistor at the receiver changes as well. The direction of the current through the resistor determines whether a positive or negative differential voltage is read. A positive differential voltage represents logic-high level, and a negative differential voltage represents logic-low level. *

Figure 1. LVDS Driver and Receiver. Source: National Instruments article “Understanding LVDS for Digital Test Systems“*

*National Instruments contributed this definition of LVDS. Read more here.