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What is LVDS (low-voltage differential signaling)

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Low-voltage differential signaling

Low-Voltage Differential Signaling (LVDS) is a fascinating topic in the realm of digital communication. Let’s dive into the technical details:

  1. Definition and Purpose:
    • LVDS, also known as TIA/EIA-644, is a differential, serial signaling standard.
    • Its primary purpose is to transmit data over long distances while maintaining high speeds and low power consumption.
    • LVDS is commonly used in applications such as LCD-TVs, in-car entertainment systems, industrial cameras, machine vision, notebook and tablet computers, and communication systems.
  2. Differential Signaling:
    • LVDS operates using differential signaling, which means it transmits information as the difference between voltages on a pair of wires.
    • Here’s how it works:
      • The transmitter injects a constant current (typically 3.5 mA) into the wires.
      • The direction of the current determines the digital logic level (0 or 1).
      • At the receiving end, a termination resistor (around 100 to 120 ohms) matches the cable’s characteristic impedance.
      • The current returns via the other wire, creating a voltage difference across the resistor (about 350 mV).
      • The receiver senses this polarity to determine the logic level.
  3. Noise Reduction and Electromagnetic Compatibility:
    • LVDS offers several benefits:
      • Reduced electromagnetic noise (EMI): The equal and opposite current flow in the two wires generates electromagnetic fields that tend to cancel each other out.
      • Tight coupling: The closely spaced transmission wires reduce susceptibility to common-mode noise interference.
      • Field coupling: The current loop back to the driver minimizes radiated EMI.
      • Equal impact: Common-mode noise affects both wire pairs equally, leaving the differential voltage unaffected.
  4. Data Rate and Cable Type:
    • LVDS can achieve speeds of up to 655 Mbit/s (rates up to 1-3 Gbit/s are possible).
    • It uses inexpensive twisted-pair copper cables for transmission.
  5. Layer in the OSI Model:
    • LVDS is a physical layer specification only.
    • Many data communication standards and applications use LVDS and add a data link layer (as defined in the OSI model) on top of it.

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