Series Termination in PCB: What It Is and When to Use It

In PCB design, series termination is a technique where a resistor is placed in series with a signal line near the source to reduce signal reflections and ringing. It helps match the impedance of the driver to the transmission line, improving signal quality and reliability.

How It Works

Imagine sending a message down a long hallway where echoes can confuse the listener. In PCB signals, these echoes are called reflections, caused by sudden changes in impedance along the signal path. Series termination works like a shock absorber by placing a resistor in line with the signal near its source. This resistor smooths out the signal's transition, reducing the energy that bounces back.

When a signal travels through a PCB trace, it behaves like a wave on a string. If the string suddenly changes thickness, waves reflect back. The series resistor helps match the signal source's impedance to the trace's impedance, minimizing these reflections. This keeps the signal clean and stable, especially for fast digital signals.

Example

This example shows a simple series termination resistor placed near a microcontroller output pin driving a PCB trace.

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Microcontroller_Pin ---[100 Ω Resistor]--- PCB_Trace --- Load

Output

Signal reflections are reduced, resulting in cleaner signal edges and less ringing.

When to Use

Use series termination when you have fast digital signals traveling on PCB traces that are long enough to cause reflections. It is especially helpful in high-speed communication lines like SPI, I2C, or memory buses. If you notice signal ringing, overshoot, or undershoot on an oscilloscope, adding a series resistor near the driver can improve signal integrity.

It is also useful when the driver’s output impedance is much lower than the trace impedance, causing strong reflections. Series termination is a simple, low-cost way to fix these issues without redesigning the PCB.

Key Points

• Series termination uses a resistor placed near the signal source.
• It reduces signal reflections and ringing on PCB traces.
• Helps match driver impedance to trace impedance.
• Common in high-speed digital signal lines.
• Simple and cost-effective method to improve signal quality.

Key Takeaways

Series termination places a resistor near the signal source to reduce reflections.

It improves signal quality by matching driver and trace impedance.

Use it for fast signals on long PCB traces to prevent ringing.

It is a simple and effective way to enhance signal integrity.

Ideal for high-speed digital communication lines.