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Since Low Voltage Differential Signaling (LVDS) became a data transmission standard in 1995, its eye-popping improvements in data transmission speed and power consumption have allowed it to make inroads into applications using more venerable serial transmission methods, such as RS-422 and RS-485. LVDS brings significantly higher speeds and lower power to the interface -- an essential step in meeting the ever-increasing bandwidth requirements of networking, telecommunications and multimedia applications. By using LVDS, engineers can build systems that have signaling rates 40 times faster (400 Mbit/s) and consume 10 times less power than the older data transmission schemes. Recent developments in LVDS technology promise to not only continue this trend, but to accelerate it.

LVDS is Cable-Ready

In the past, LVDS has been mainly used to send data over relatively short distances, usually between ICs on a board, between boards inside a system enclosure, or between enclosures in fairly benign electromagnetic environments. Now companies are developing LVDS circuits that are more tolerant to noise and can transmit signals over longer distances and in harsher electromagnetic environments. These factors, and the inherent advantages of LVDS, make it more applicable for data cable connections in instrumentation, process control, telecommunications systems, point-of-sales (POS) terminals, automotive systems and others. One of the technical advancements that have enabled these new applications involves extending the common-mode range beyond that defined in the LVDS standard (TIA/EIA-644).

The standard specifies a common-mode voltage centered at 1.2 V and going from zero to 2.4 V. This common-mode range and the LVDS signal levels allow for 1 V of ground noise. While this is sufficient for many applications, others will have greater noise pick up simply due to longer cables or more noise from outside the interface. Since noise is random, in most instances, it is very difficult to determine how much noise tolerance is enough for a given application. In general, more noise tolerance is always better.

Input common-mode voltage range (VICR) extending outside of the supply rails of a differential line receiver or any comparator is accomplished by attenuating the input signals such that the signals internal to the line receiver stay within an acceptable range. The trick is to have the largest attenuation possible and still provide a differential input threshold of +/- 100 mV for LVDS standard compliance and noise margin on the interface. Transistor matching and other factors on an IC prevent internal input voltage thresholds less than 20 mV without special processing and cost. With this constraint, the inputs can be attenuated by a factor of five and still maintain the +/- 100 mV threshold at the input and allow for processing variances. This extends the VICR of the new LVDS receivers from -2 V to 4.4 V and increases the allowable ground noise to +/- 3 V.

While still not as noise tolerant as 422 or 485 with their +/- 7 V ground noise capability, this is still a significant improvement over standard LVDS and compares with the noise tolerance of the ubiquitous RS-232 interface.

Besides an extended common-mode range, a new LVDS innovation is starting to incorporate functionality that makes cable-connected LVDS applications more reliable and easier for the engineer to use.

Fail-Safe Functionality

When using balanced signaling, a number of events can remove the input signal to a differential receiver. For example, the signal pair may be short-circuited by a crushed cable, the cable may be disconnected, or, during normal operation, the data transmitter may simply be turned off. Behavior of the receiving circuit can be unpredictable since there may be insufficient voltage difference across the balanced signal pair to define the logic state. Having a predictable receiver response is termed fail-safe.

Most existing differential receivers (including LVDS, 422 and 485) now have provisions for open-circuit fail-safe. This is the condition where the inputs of the differential receiver are not connected to anything. Often, the addition of a line termination resistor across the inputs defeats this fail-safe and produces a need for a terminated fail-safe feature.

A common remedy is to add external circuitry to keep a differential offset voltage across the signal pair at all times. This provides a known bus state as long as the external circuit is powered and remains connected to the bus. This external method adds components, uses power and reduces differential noise margin. All things considered, the best place to handle the terminated fail-safe feature is in the differential receiver itself.

By precisely controlling the differential voltage thresholds that the receiver detects, a terminated-fail-safe scheme can be implemented in the receiver itself, thereby eliminating the drawbacks of external circuitry. This is the method used in a LVDS receiver announced by Texas Instruments, which actually has three differential receivers for each data pair.

Once the absence of an input signal can be detected, other possible functions besides fault conditions can be implemented. The most common is creation of a logical OR function on a multiplexed bus. In this operating mode multiple drivers are allowed to drive the bus in one polarity or state only and must turn off for the other state. The condition when all drivers are off can now be detected by fail-safe receivers and interpreted as a change in the bus state. This is often used in bus arbitration, but requires external bias networks with the same drawbacks mentioned earlier.

Integrated Terminators Simplify Design

Because of the high switching speeds of LVDS, developers must almost always add transmission line terminating resistors into their designs to reduce reflections and assure the best quality signals. New LVDS transceivers are eliminating the need for these external resistors because they are integrated within the receivers themselves. With integrated termination, signal quality can be improved as a result of terminating closer to the end of the transmission line than would be possible externally. Since four, eight and sixteen receivers in one device are now being introduced; integrated termination becomes a big benefit just in component and space savings.

There's LVDS in Your Future

Some of the older data transmission standards have been in use for 20 years or more. On the basis of speed and power savings alone, LVDS will be a likely candidate to replace some of these standards in the future. As the functionality and capability of LVDS continues to be extended in the years ahead, the momentum will continue to grow. LVDS technology, in some shape or form, will certainly be an important part of many next-generation systems of the future.

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