What does “LVDS” mean?
Low Voltage Differential Signaling (VDS) is an electric transmission technique for digital signals which are characterized by low voltage levels and differential signal transmission. This technology is used in many applications such as for connecting screens and computers, as well as in the automotive sector.
Advantages of differential signal transmission with LVDS
With differential signal transmission, two signal conductors with the same voltage but opposing signals are used and the differential voltage is analyzed in the receiver. If the distance between the conductors is small, common-mode interference has the same effect and can therefore be removed by calculating the difference. In practice, with LVDS, a current source is used to generate a current of 3.5 milliamps, for example, which causes a voltage of 350 millivolts to be applied to a 100 Ohm terminating resistor on the receiver. The inverted level of -350 millivolts is generated as a result of switching the current direction. Twisted signal conductors minimize the effect and radiation of magnetic fields.
Low voltage levels with LVDS, and their impacts
LVDS works with differential voltage levels of between 247 and 454 millivolts, in contrast to other technologies that work with signal levels of several volts. The advantage of low voltage levels and thus low currents lies in the power consumption of the transmission path. At a current strength of 3.5 milliamps and a voltage of 350 millivolts, the terminating resistor at the receiver has a power consumption of just slightly more than 1.2 millwatts. At the same time, the interference radiation emitted is reduced by the low level.
Use of LVDS
LVDS was originally published in 1994 by National Semiconductor, and due to the wide distribution, it was standardized in 1995 as ANSI/TIA/EIA-644 by National Semiconductor as the editor for this standard. The standard defines a data rate of 655 Mbit/s, however, data rates in the Gbit/s range were already achieved with modern chips. LVDS was originally used for connecting screens in entertainment electronics and computers, due to the constantly increasing resolution, and also found its way into the automotive sector due to its high level of reliability and low cost implementation. The LVDS standard is used as a technology in other different protocols such as FPD link because ANSI/TIA/EIA-644 only defines the electric levels such as the transmitter and receiver properties, and does not provide any guidelines for higher protocol levels. The highest data rate is achieved in simplex mode, such as when transmitting data in one direction from a data source to a data sink, e.g. from a camera to a display. With corresponding chips, half-duplex transmission is also possible. Here, the transmission path can be operated alternately in both directions, however this reduces the maximum data rate. Due to increased demand, a multi-drop variant of LVDS was also developed. Here, further receivers are connected via stubs to the transmission path between the transmitter and receiver of the point-to-point connection. The maximum transmission rate depends on the cable type, the length of the transmission path and the type of transmission.
Jonas Rumold
Jonas Rumold is a Junior Development Engineer in high-frequency development at MD ELEKTRONIK. His responsibilities include high-frequency measuring devices, measurement methods, evaluation of shielded and unshielded high-frequency data cables, and the current standards for data transmission in the automotive sector.