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.