Low-Voltage Differential Signaling (LVDS)

1 Low-Voltage Differential Signaling ( lvds ). Application Note 1382-6. by Stephen Kempainen, National Semiconductor Who Should Read This Wherever you need high-speed solutions move information on a Application Note? data transfer (100 Mb/s and board, between boards, modules, higher), lvds offers a solution. shelves, and racks, or box-to-box. Digital designers utilizing low- There are many applications in The transmission media can be voltage Differential Signaling many market segments that use copper cables or printed circuit ( lvds ) for high-speed data lvds for data transmission. board (PCB) traces. In the future, transmissiom. These include: lvds will also carry protocols for inter-system communication.

2 lvds Provides Higher Bit Rates, stackable hubs for data Lower Power, and Improved communications Table of Contents Noise Performance wireless base stations lvds Provides Higher Bit Rates, and ATM switches in Lower Power, and Improved Due to the Internet's tremendous telecommunications Noise Performance .. 1. growth, data transfers are flat-panel displays and servers Generic lvds .. 2. increasing dramatically in all in the computer market Multiple Technologies and areas of communications. In addi- peripherals like printers and Supply Voltages .. 3. tion, data streams for digital digital copy machines Gigabits at Milliwatts .. 4. video, HDTV, and color graphics high-resolution displays in Flat Supply Current vs.

3 Are requiring higher and higher industrial applications Operating Frequency .. 6. bandwidth. The digital communi- flat-panel displays in the Low Electromagnetic Interference .. 7. cations deluge is the driving force automotive market Cost Benefits .. 8. for high-speed interconnects Many Channels per Chip .. 8. between chips, functional boards, In these applications, high-speed DC Balance for Longer Cables .. 9. and systems. The data may be data moves within and between Bus lvds .. 10. digital, but it is analog Low- systems. Moving data within a The Complexities of voltage Differential Signaling system (intrasystem data trans- Signal Integrity .. 11. ( lvds ) that designers are choos- fer) is the main use for lvds Serializer/Deserializer Example.

4 11. ing to drive these high-speed solutions today. Moving informa- lvds in Low-Power Applications.. 12. transmission lines. lvds 's proven tion between systems (intersys- Test and Evaluation Considerations .. 13. speed, low power, noise control, tem data transfer) requires stan- and cost advantages are popular dard communication protocols BER Eye Diagram .. 15. in point-to-point applications for such as IEEE 1394, Fibre Recovered Clock Jitter .. 16. telecommunications, data Channel, and Gigabit Ethernet. ParBERT 81250 Simplifies communications, and displays. Since the hardware and software the Characterization and overhead for intersystem proto- Testing of lvds Devices .. 17. lvds uses high-speed analog circuit techniques to provide cols is too expensive to use for 81200 Provides the Tools to Test lvds .

5 18. multi-gigabit data transfers on intrasystem data transfers, a sim- copper interconnects. ple and low-cost lvds link is an Related Literatute .. 19. attractive alternative. Thus, lvds Support, Services, and Assistance.. 20. Generic lvds . Low-Voltage Differential Signaling The Differential receiver is a high- is a generic interface standard for impedance device that detects high-speed data transmission. Differential signals as low as The ANSI/TIA/EIA-644-1995 20 mV and then amplifies them standard specifies the physical into standard logic levels. The sig- layer as an electronic interface. nal has a typical driver offset of This standard defines driver and V, and the receiver accepts an receiver electrical characteristics input range of ground to V.

6 Only. It does not define protocol, This allows rejection of common- interconnect, or connector details mode noise picked up along the because these details are applica- interconnect of up to 1 V. tion specific. The lvds . Standard's Working Group chose In addition, hot plugging of lvds . to define only the electrical char- drivers and receivers is possible acteristics to ensure that lvds because the constant current becomes a multipurpose interface drive eliminates damage poten- standard. Therefore, each appli- tial. Another feature is the re- cation that uses lvds should also ceiver's failsafe function, which reference the appropriate proto- prevents output oscillations when col and interconnect standard.

7 The input pins are floating. The equivalent circuit structure of the lvds physical layer is shown in figure 1. In the driver, a Power supply tolerance greater than 10%. current source limits output to about 3 mA, and a switch box Tx Rx steers the current through the termination resistor. This differ- ential driver produces odd-mode Current steering driver Differential receiver transmission: equal and opposite 3 mA. currents flowing in the transmis- Supports 1. VCommon Mode sion lines. The current returns within the wire pair, so the cur- rent loop area is small, and there- or fore generates the lowest amount of EMI (electro-magnetic interfer- Current returns Single resistor ence). The current source limits within the pair termination (small loop area any spike current that could for lowest EMI).

8 Occur during transitions. Because there are no spike currents, data Figure 1. The equivalent circuit structure rates as high as Gb/s are of the lvds physical layer. possible without a substantial increase in power dissipation. In addition, the constant current driver output can tolerate trans- mission lines shorted together, or to ground, without creating thermal problems. 2. Multiple Technologies and Supply Voltages When choosing the signal-level between logic states is only voltages for drivers and receivers, 300 mV, lvds can change states The diagram in figure 2 empha- the standards committee consid- very fast. An lvds signal also sizes the advantage of a low volt- ered lvds implementation in changes voltage levels without a age swing for higher perform- technologies such as Bipolar, fast slew rate.

9 Slowing the transi- ance. For example, when the sig- BiCMOS, CMOS, and even GaAs. tion rate decreases the radiated nal level changes 300 mV in In addition, the working group field strength. Slower transitions 333 ps, the slew rate is only targeted a wide range of power reduce the problem of reflections V/ns, which is less than the supplies (such as 5 V, V, and from transmission-path imped- 1 V/ns benchmark slew rate com- V) for implementing lvds , to ance discontinuities, decreasing monly acceptable for minimizing ensure that lvds would be the emissions and crosstalk prob- signal distortion and crosstalk. If interface of choice for future lems. Low voltage swing reduces you use the old benchmark that generations of products.

10 Power consumption because it the rise and fall times should be lowers the voltage across the ter- no more than two thirds of the bit Low-Voltage signals have many mination resistors and lowers the width, then signals with 333-ps advantages, including fast bit overall power dissipation. transitions can operate as high as rates, lower power, and better 1 Gb/s with plenty of margin. noise performance. Design engi- neers have previously used full- tRISE = tFALL. swing CMOS and LVTTL (low volt- Single Ended age transistor-transistor logic), VoB V. V. but as bit rates increase, these VoA V. solutions become unattractive. More recently, designers have +300 mV. Differential single + 100 mV. turned to reduced-swing tech- Vod = VoA - VoB.