Understanding Output Buffer Types in Oscillators and Frequency Control Devices
Oscillators and frequency control devices come with a range of different output buffer types, each with its own advantages and disadvantages. The aim of this application note is to provide some background on each type and to offer advice on approaches to terminating devices with such outputs.
Common Output Buffer Types:
hcmos vs cmos (Complementary Metal-Oxide-Semiconductor):
Advantages: Low power consumption, high noise immunity.
Disadvantages: Limited speed compared to other types.
Applications: General-purpose digital circuits.
HCMOS (High-Speed CMOS):
Advantages: Higher speed variant of CMOS, often interchangeable with CMOS in the oscillator world.
Disadvantages: Slightly higher power consumption than standard CMOS.
Applications: High-speed digital circuits.
LVCMOS (Low Voltage CMOS):
Advantages: Operates at lower voltages, reducing power consumption.
Disadvantages: Lower noise margins due to reduced voltage levels.
Applications: Battery-powered and portable devices.
Sinewave:
Advantages: Smooth waveform, ideal for analog applications.
Disadvantages: Requires more complex circuitry to generate.
Applications: RF and analog signal processing.
Clipped Sinewave:
Advantages: Easier to generate than a pure sinewave, lower harmonic content than square waves.
Disadvantages: Not as clean as a pure sinewave.
Applications: Intermediate between digital and analog applications.
TTL (Transistor-Transistor Logic):
Advantages: Compatible with a wide range of digital logic circuits.
Disadvantages: Higher power consumption compared to CMOS.
Applications: Legacy digital systems.
PECL (Positive Emitter-Coupled Logic):
Advantages: High speed, low noise.
Disadvantages: Requires a positive power supply, higher power consumption.
Applications: High-speed data communication.
LVPECL (Low Voltage Positive Emitter-Coupled Logic):
Advantages: Lower power consumption than standard PECL, high speed.
Disadvantages: Requires careful power supply design.
Applications: High-speed data communication, clock distribution.
LVDS (Low Voltage Differential Signaling):
Advantages: Low power, high speed, excellent noise immunity.
Disadvantages: Requires differential signaling, more complex PCB layout.
Applications: High-speed data communication, video transmission.
CML (Current Mode Logic):
Advantages: Very high speed, low power.
Disadvantages: Requires careful termination, more complex design.
Applications: High-speed data communication, high-frequency applications.
Matching Output Buffers with Appropriate Supply Voltage
Different output buffer types require different supply voltages, which can significantly affect the overall design of the system. Here’s a brief overview:
CMOS/HCMOS: Typically operates at 3.3V or 5V.
LVCMOS: Operates at lower voltages, commonly 1.8V, 2.5V, or 3.3V.
TTL: Standard 5V.
PECL/LVPECL: Typically operates at 3.3V or 5V, with LVPECL often at 3.3V.
LVDS: Commonly operates at 3.3V.
CML: Typically operates at 3.3V or 2.5V.
Termination Approaches
Proper termination is crucial to ensure signal integrity and minimize reflections. Here are some general guidelines:
CMOS/HCMOS/LVCMOS: Typically, no termination is required for short traces. For longer traces, series termination (resistor in series with the signal line) can be used.
Sinewave/Clipped Sinewave: Requires impedance matching to the load, typically 50Ω.
TTL: Series termination is often used.
PECL/LVPECL: Requires parallel termination to Vcc-2V (e.g., 50Ω to 3.3V-2V for 3.3V PECL).
LVDS: Requires 100Ω differential termination resistor between the positive and negative lines.
CML: Requires 50Ω termination to ground or Vcc, depending on the design.
Conclusion
Understanding the different output buffer types and their appropriate supply voltages is essential for designing reliable and efficient electronic systems. Each type has its own set of advantages and disadvantages, making them suitable for specific applications. Proper termination techniques are also crucial to ensure signal integrity and optimal performance. By carefully selecting and implementing the right output buffer type, designers can achieve the desired performance and reliability in their systems.