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SIGNAL CHAIN BASICS (Part 37): LVDS–Low-Voltage Differential Signaling

LVDS is a low-voltage, differential signaling scheme, primarily used in high-speed data transmission. Defined in the ANSI/TIA/EIA-644 specification, it is the most common differential interface. The standard only specifies the electrical characteristics of drivers and receiver suitable for LVDS applications. It is, therefore, an electrical-only standard, commonly referred to by higher level protocol standards as their interface or physical layer.

Its use in high-speed analog-to-digital converters (ADC) allows for high-speed data outputs while maintaining high converter performance. Standalone ADCs must be capable of driving capacitive loads in the form of PCB traces and input logic of receiving circuitry. Here a single-ended driver in the ADC output stage causes large switching noise transients resulting from switching high-current transistors on and off. These transients can couple back to the analog front end of the ADC and adversely affect its performance.

An LVDS driver stage, however, operates with a 3.5 mA (typical) current source that is always on (Figure 1 ). The logic ones and zeros on the bus are created by simply rerouting the current in different directions through the conductors of the differential pair. It is this always-on characteristic that eliminates switching noise and EMI, the main causes for deteriorating ADC performance.



Figure 1: LVDS driver and receiver

Designed for point-to-point signal transmission, LVDS utilizes a simple termination scheme. A single 100-ohm resistor placed at the receiver input terminates the differential pair, thus eliminating reflections.

Because of the high-impedance receiver input, the entire current of the driver's current source flows through the termination resistor generating a low, differential bus voltage of 350 mV nominal. This voltage swings around a 1.2 V common-mode potential, which is the typical driver output offset voltage (Figure 2 ).



Figure 2: LVDS bus voltage levels

Differential signaling represents another benefit of LVDS over single-ended schemes, as it is less susceptible to common-mode noise and also generates less electromagnetic interference (EMI).

Because the receiver responds to differential voltages only, noise coupled into the adjacent signal traces is seen as common-mode modulations by the receiver and is, therefore, rejected. Also as the two conductors of the differential pair carry currents of equal amount, but opposite polarity, their magnetic fields largely cancel one another and EMI is minimized.

Depending on the data rate, standard LVDS ICs can drive distances of up to 10 meters. High-performance ADCs, however, shouldn't be forced to drive this distance. Instead, short output trace lengths of less than two inches are recommended to prevent noise from adjacent circuitry coupling onto the ADC outputs, which potentially could couple back to the ADC analog inputs.

While the low-power consumption, low EMI, and high noise immunity make LVDS the interface of choice for high-speed data converters, careful layout techniques must be applied to avoid impedance discontinuities and signal delay skews, that otherwise counteract the aforementioned benefits of LVDS.

This article concludes our series on data converter interfaces. Next month, we plan to cover full-wave rectifier circuits in single-supply systems.

About the Author


Thomas Kugelstadt is a Senior Applications Engineer at Texas Instruments, where he is responsible for defining new, high-performance analog products and developing complete system solutions that detect and condition low-level analog signals in industrial systems.
During his 20 years with TI, he has been assigned to various international application positions in Europe, Asia and the U.S. Thomas is a Graduate Engineer from the Frankfurt University of Applied Science. You can contact Thomas about this article at: scb@list.ti.com.

Previous installments of this series:

  • SIGNAL CHAIN BASICS (Part 36): Digital Audio Interfaces–Part 2, click here
  • SIGNAL CHAIN BASICS (Part 35): ANT–A unique option for wireless sensor networking, click here
  • SIGNAL CHAIN BASICS (Part 34): Designing the audio-signal chain for non-audio experts (Part 2), click here
  • SIGNAL CHAIN BASICS (Part 33): Use an op amp to drive a precision ADC, click here
  • SIGNAL CHAIN BASICS (Part 32): Digital interfaces (con't) — The I2 C Bus, click here
  • SIGNAL CHAIN BASICS (Part 31): Digital interfaces (con't) — The SPI Bus, click here
  • SIGNAL CHAIN BASICS (Part 30): Protocol selection over IEEE 802.15.4 silicon, click here
  • SIGNAL CHAIN BASICS (Part 29): Digital interfaces – Single-ended versus differential interfaces, click here
  • SIGNAL CHAIN BASICS (Part 28): Building (Electrical) Bridges, click here
  • SIGNAL CHAIN BASICS (Part 27): Control EMI resulting from board-level clock distribution, click here
  • SIGNAL CHAIN BASICS (Part 26): How to close timing on High-Speed ADCs, click here
  • SIGNAL CHAIN BASICS (Part 25): Designing the audio-signal chain for non-audio experts, Part 1, click here
  • SIGNAL CHAIN BASICS (Part 24): Basic networking using the IEEE 802.15.4 PHY/MAC protocol, click here
  • SIGNAL CHAIN BASICS (Part 23): EIA-485: Receiver equalization boosts networking performance, click here
  • SIGNAL CHAIN BASICS (Part 22): Phantom microphone power–the ghost in the machine, click here
  • SIGNAL CHAIN BASICS (Part 21): Understand and configure analog and digital grounds, click here
  • SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed, click here
  • SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators, click here
  • SIGNAL CHAIN BASICS (Part 18): The op amp as integrator, click here
  • SIGNAL CHAIN BASICS (Part 17): Hysteresis–Understanding more about the analog voltage comparator, click here
  • SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator, click here
  • SIGNAL CHAIN BASICS (Part 15): Analog/digital converter–dynamic parameters, click here
  • SIGNAL CHAIN BASICS (Part 14): Analog/digital converter–static parameters, click here
  • SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use, click here
  • SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool, click here
  • SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits, click here
  • SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter, click here
  • SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored, click here
  • SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored, click here
  • SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification–Bias Current, click here
  • SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset, click here
  • SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier, click here
  • SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types, click here
  • SIGNAL CHAIN BASICS (Part 3): Analog and the digital world, click here
  • SIGNAL CHAIN BASICS (Part 2): Op Amp–Basic operations, click here
  • SIGNAL CHAIN BASICS: Operational Amplifier–The Basic Building Block, click here

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