How to Terminate High Speed Signals on PCB: Best Practices

To terminate high speed signals on a PCB, use termination resistors placed close to the signal source or load to match the transmission line impedance. Common methods include series termination, parallel termination, and Thevenin termination to reduce reflections and signal distortion.

Syntax

Signal termination involves placing resistors in specific configurations to match the impedance of the transmission line and prevent signal reflections.

• Series Termination: A resistor placed in series near the driver.
• Parallel Termination: A resistor placed from the signal line to ground or supply near the receiver.
• Thevenin Termination: Two resistors forming a voltage divider at the receiver end.

plaintext

Series Termination:
Driver ---[R_series]--- Signal Line --- Load

Parallel Termination:
Signal Line --- Load
           |
          [R_parallel]
           |
          GND or VCC

Thevenin Termination:
Signal Line --- Load
           |
         [R1]
           |
          VCC
           |
         [R2]
           |
          GND

Example

This example shows a series termination resistor used to match a 50-ohm transmission line on a PCB to reduce signal reflections.

plaintext

Signal Source ---[33 Ω resistor]--- 50 Ω Transmission Line --- Load

Output

Signal reflections are minimized, resulting in cleaner signal edges and less ringing on the PCB trace.

Common Pitfalls

Common mistakes when terminating high speed signals include:

• Placing termination resistors too far from the driver or receiver, which reduces effectiveness.
• Using incorrect resistor values that do not match the transmission line impedance.
• Omitting termination on high speed lines, causing signal reflections and data errors.
• Not considering PCB trace impedance and layout when selecting termination methods.

plaintext

Wrong:
Driver --- Signal Line --- Load (No termination resistor)

Right:
Driver ---[33 Ω resistor]--- Signal Line --- Load

Quick Reference

Termination Type	Placement	Purpose	Typical Resistor Value
Series Termination	Near driver	Match source impedance, reduce reflections	20-50 Ω
Parallel Termination	Near receiver	Match load impedance, absorb reflections	50-100 Ω
Thevenin Termination	Near receiver	Voltage divider to match impedance	Two resistors totaling ~50-100 Ω

Key Takeaways

Always place termination resistors close to the signal source or load for best effect.

Match resistor values to the characteristic impedance of the PCB trace (commonly 50 Ω).

Use series termination to reduce reflections from the driver side.

Use parallel or Thevenin termination at the receiver side to absorb reflections.

Poor or missing termination causes signal distortion and data errors in high speed circuits.