LVDS is a high-speed (hundreds of megabits per second) differential data transmission technology that operates at very low power-dissipation levels from common power-supply rails (5 or 3.3 V). Being differential, and supporting a ±1-V common-mode range, LVDS provides about twice the noise margin of GTL or BTL. Termination is greatly simplified, as no active pull-up voltages are required (as in the case of BTL and GTL technology). A single surface-mount resistor is all that is required. LVDS drivers swing from 250 to 450 mV, centered around 1.25 V, while the receivers support thresholds less than 100 mV. Standard LVDS drivers and receivers are commonly employed in point-to-point or multidrop (multiple receivers) applications, thus they can be used in switched-backplane applications, or on other special links across a backplane.
BLVDS also is a high-speed (hundreds of megabits per second) differential data transmission technology that extends the benefits of standard LVDS into multipoint bus configurations supporting bidirectional half-duplex bus communication. It differs from standard LVDS by providing a higher drive, which provides similar small-signal swings (about ±250 mV) while loaded with two terminations (one at both ends of the bus).
Since the signal swing is greatly reduced, fast transition times are possible, thus allowing the drivers to address high data rates ranging from hundreds of megabits/s to over 1 Gbps. The differential data transmission scheme provides a ±1-V common-mode range and live insertion (hot plug) of devices into an active bus. Additionally, the low voltage swing minimizes power dissipation and noise generation (crosstalk and EMI). BLVDS greatly simplifies the area of bus termination as it does not require special active termination devices, nor does it require a unique termination rail (such as 2.1 V for BTL) to be supplied. It simply requires a single surface-mount resistor across the pair at each end of the bus.
BLVDS also utilizes common power-supply rails (3.3 or 5 V), minimizes power dissipation in the interface devices, generates little noise, supports live insertion of cards, and drives heavily loaded multipoint busses at hundreds of Megabits/s. BLVDS addresses many of the challenges faced in a high-speed bus design and products are available as simple transceiver devices, optimized parallel bus transceivers with ultra low skew, and 10-bit serializer/deserializer devices.
Fitting It All Together
Having examined the benefits and features of each technology, and applying transmission line theory, a comparison of backplane performance with respect to loading can be produced (Fig. 4). This comparison is purely relative, as the capabilities of the common technologies incorporate a significant degree of overlap. Given enough design and debug time, each technology can be pushed beyond the limits described here. A comparison of levels and the resulting noise margins are shown in Figures 5, 6, and 7. Figures 5 and 6 are common, single-ended technologies, while Figure 7 is specific for LVDS and Bus LVDS.
Note that the common-mode range in differential data transmission technologies is what should be compared to the standard noise margins of single-ended technologies. Therefore BLVDS and LVDS, with their 250-mV swings, both provide about twice the noise margin of GTL- or BTL-based systems. For lower-speed systems, regardless of load conditions, a standard TTL family such as LCX or FACT may be used.
If a performance improvement is required, but backward compatibility is necessary, LVT or ABT or ETL may be considered. Very fast systems with a few boards (light loading) could find GTL a good design choice, provided live insertion is not required. For more heavily loaded systems running at high speed, BTL or even ECL/PECL may be required.
If ultra-high performance is required, and ultra low power dissipation is a must, then Bus LVDS is the driver technology of choice. In each case, regardless of the technology chosen, proper design rules should be followed to minimize reflections, crosstalk, and other transmission-line related issues. Upgrading driver technologies can help eliminate these problems, but no transceiver can mask a fundamentally poor design.