Day 4 - Theory of High-Speed Design [e-blog] : Termination Techniques

Day 4: Termination Techniques for High-Speed Signals

Yesterday, we discussed different types of transmission lines. Today, we're going to tackle one of the most practical solutions for dealing with reflections on those lines: Termination.

As we learned on Day 2, reflections happen when there's an impedance mismatch. Termination is the act of adding a resistor (or a combination of components) at the end or sometimes the beginning of a transmission line to match the characteristic impedance (Z0​) of the trace. This effectively "absorbs" the signal energy, preventing it from reflecting back.

There are several types of termination, each with its own advantages and disadvantages:

1. Series Termination (Source Termination):

   • How it works: A resistor is placed in series with the trace, close to the driver (source) of the signal. The value of this resistor is chosen so that its resistance plus the output impedance of the driver matches the characteristic impedance of the trace ($R_s+Z_{driver}=Z_0$).

   • Pros: Reduces power consumption compared to parallel termination. Good for point-to-point connections.

   • Cons: Can increase signal rise time slightly.

2. Parallel Termination (Shunt Termination):

   • How it works: A resistor is placed in parallel with the trace, close to the receiver (load). The resistor's value typically matches the characteristic impedance ($R_t=Z_0$) and is connected to either ground or a termination voltage.

   • Pros: Can provide very clean signals, as it absorbs reflections at the receiver.

   • Cons: Consumes more power because the resistor is always drawing current.

3. AC Termination:

• How it works: Similar to parallel termination, but a capacitor is placed in series with the termination resistor to block DC current.

• Pros: Reduces static power consumption compared to standard parallel termination.

• Cons: Not suitable for signals with very low frequencies or DC components.

The choice of termination depends on various factors, including the type of signal, its speed, power consumption constraints, and the specific driver/receiver characteristics. Proper termination is crucial for maintaining signal integrity in high-speed designs.

Book Reference for Deeper Study

• Book: High-Speed Digital Design: A Handbook of Black Magic

• Authors: Howard Johnson and Martin Graham

• Why it's helpful: Chapter 6 of this book is specifically dedicated to Terminations. It provides practical, clear explanations of different termination types, how to calculate the resistor values, and when to apply each method. It will give you the hands-on knowledge you need to implement these techniques in your own designs.

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About Praveen S

Hello everyone, I am Praveen S, currently employed as "Technical Design Engineer" for Polycab Wires Pvt Ltd , I am a BE Graduate in EEE from NMAMIT Nitte (Karnataka, India). I am very passionate about learning and sharing my knowledge relating to the areas of "Electrical and Electronics". Here's my blog to share my ideas and to improve myself.