Kevin Gingerich, Senior Member, Technical Staff, Applications Manager, Interface Products Group, Texas Instruments Inc., Dallas, John Goldie, LVDS & SDI Interface Applications, Manager, National Semiconductor Corp., Santa Clara, Calif.
Low-voltage differential signaling, or LVDS (TIA/EIA-644), has become extremely popular because of its high-speed and low-power characteristics for a wide range of applications. The technology's popularity will increase even further with the standardization of a new multipoint version: M-LVDS (TIA/EIA-899).
Essentially, M-LVDS transceivers are intended for use on a multipoint bus where up to 32 nodes may be connected on the shared line. They may also be used on a simpler bus with only two nodes or any combination in between. The interconnecting media may consist of a backplane, a cable or a mixture.
The signal levels have been chosen to tolerate reflections between loaded and unloaded segments on the bus. The standard specifies transmissions at up to 500 Mbits/second. That limit, however, may be further reduced by the interconnecting media bandwidth.
The M-LVDS standard defines the driver's output characteristics and the receiver's input characteristics. Protocol and connector details are left up to the referencing standard or application to specify. M-LVDS defines one line driver and two types of receivers.
M-LVDS supports a differential-noise margin of 430 mV for a Type-1 receiver or 330 mV for a Type-2 receiver. Typically, the differential-noise margin is of lesser concern than the common-mode noise rejection capability. In an LVDS system, the key is keeping the signal lines balanced and closely coupled. This reduces noise and electromagnetic interference but, equally important, helps to ensure that coupled noise is picked up as common-mode and then rejected by the receivers. M-LVDS has a typical common-mode voltage rejection of 2 V, which is budgeted against coupled noise and ground potential differences.
An interconnect that adheres to the guidelines of TIA/EIA-899 will improve signal quality, timing margins and noise margins. The standard calls for a 100-ohm differential impedance media with passive 100- ohms resistors terminating between the signal pair at the ends of the transmission line.
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