Why PCB Boards Are Controlled to 50 Ohms Impedance
- Historical Roots of the 50-Ohm Standard
- Convenience and Production Considerations
- Loss and Physics Behind 50 Ohm Impedance
The 50-ohm standard for PCB boards has historical roots related to standard cables, originating from military applications during World War II. The U.S. established 50 ohms as the standard, which later became the industry norm.
From a production standpoint, achieving 50 ohms impedance on PCB circuit boards is more convenient. Too low or too high impedance can pose challenges in terms of line width, dielectric thickness, EMI suppression, and crosstalk, making 50 ohms the optimal choice for high-density boards.
Basic physics principles show that 50 ohm impedance results in the smallest skin effect loss. By considering the skin effect resistance of the shielding layer and inner conductor, a balance is struck to minimize loss and optimize signal integrity on PCB boards.
Advantages of 50 Ohm Impedance in PCB Design
When it comes to PCB design, the choice of impedance plays a crucial role in determining the overall performance of the circuit. In the PCB industry, 50 ohms is often considered the default value due to its various advantages.
Electrical Performance Perspective
- Lower impedance leads to better PCB trace performance.
- Reducing the distance from the plane decreases EMI, crosstalk, and susceptibility to capacitive loads.
- Drive capability of the chip is crucial for the full path consideration.
Custom Matching Needs
While 50 ohms is a well-considered compromise, it is not mandatory. The choice of impedance depends on specific matching needs. For example, 75 ohms remains the standard for remote communication, cables, and antennas.
Specialized Chips
Some specialized chips achieve better EMI and crosstalk suppression by reducing the transmission line impedance. For instance, most Intel chips require impedance control at 37 ohms, 42 ohms, or even lower.
