ECL vs LVDS vs CML: A Comparative Analysis

This article compares ECL, LVDS, and CML, outlining the key differences between them.

ECL (Emitter Coupled Logic)

ECL emerged as an alternative to the TTL logic family, primarily because of its high-speed data transmission capabilities. The transistors in ECL circuits remain in the active region, allowing for very fast state changes.

Characteristics of ECL:

• Propagation Rate: Approximately 1 to 2 ns
• Noise Immunity & Power Dissipation: Poorer than other logic families
• Voltage Levels: High Level: -0.8V, Low Level: -1.8V
• Architecture: Features a differential input amplifier, internal temperature and voltage compensated bias network, and emitter follower outputs.
• Output: ECL gates provide both true and complemented outputs.

Disadvantages of ECL:

• Requires relatively high currents.
• Relies on a negative power supply, making it challenging to interface with positive supply-based devices.

For more details, refer to Advantages and disadvantages of ECL. PECL and LVPECL are successors to the ECL logic family.

LVDS (Low Voltage Differential Signaling)

LVDS was developed by National Semiconductor and standardized as ANSI/TIA/EIA-644-A. IEEE 1596.3, another version of LVDS, specifies a lower swing and lower power alternative to ECL. The TIA version is the most commonly used.

Key Features of LVDS:

• High-speed and low-power differential interface.
• Supports both point-to-point and multi-drop bus configurations.
• The LVDS driver provides a 350mV differential output centered around +1.25V typically. It’s used with 100 Ohm interconnects terminated in 100 Ohms.
• Data rates range from DC to 2.5 Gbps.
• The latest version, M-LVDS (ANSI/TIA/EIA-899), supports multi-point buses with double terminations and data rates of 500 Mbps or less.
• LVDS and M-LVDS offer lower EMI (Electro-Magnetic Interference) due to small output current and equal/opposite current flow within the pair.
• LVDS is very versatile and supports different bus configurations.

CML (Current Mode Logic)

CML is favored for its simplicity and speed, making it a popular choice for high-speed point-to-point interfaces.

Characteristics of CML:

• High-speed point-to-point interface.
• Doesn’t require external terminating resistors, as these are handled internally by the driver and receiver.
• Supports data rates exceeding 10 Gbps.
• Can be DC or AC coupled, provided encoding is used.
• XAUI SerDes implementations commonly use CML on the high-speed side.
• CML interface is vendor-specific and not standardized, though it meets clause 47 of IEEE 802.3.
• Primarily employs a point-to-point bus configuration.

Comparison Table: ECL vs LVDS vs CML