Termination Resistors in PCB Design

Signals in a trace are not unlike signals traveling through a transmission line—fail to terminate a line properly and you run the risk of your signal reflecting back to your receiver. Like a sound wave bouncing off a wall and producing an echo, signal reflection can lead to attenuation distortion, ringing, jitter, and other side-effects that result in reduced signal integrity.

Just as acoustic engineers can design walls that absorb sound and eliminate unwanted echos, so too can electrical engineers terminate signals to eliminate reflection. In this post, we’ll take a closer look at termination resistors in the context of PCB design.

What is a reflection?

While it’s easy to conceptualize how an echo can be created from a sound wave bouncing off the wall of a room, what’s actually going on when an RF (radio frequency) signal reflects off the terminal end of a trace or transmission line? The answer is an impedance mismatch.

Impedance is the combined effect of the resistance, inductance, and capacitance of the conducting medium, but it’s helpful to think of it as resistance to current flow when a certain voltage is applied. The key value to be aware of is the characteristic impedance of the transmission line.

Reflection occurs when a signal travels down the length of a conductive medium and encounters an impedance mismatch. This can occur due to natural imperfections within a trace, line, or cable. It’s easy to conceptualize how breaks in a fiber optic cable can lead to mirrors which readily reflect a portion of a light signal back to the source.

However, even if there were no imperfections throughout the length of a transmission line, the ends would still represent a huge impedance mismatch, because the physical line itself comes to an abrupt end. The amount of energy of your signal that gets reflected can be calculated from the impedance mismatch using a reflection coefficient.  

This is where the termination resistor comes in. It’s possible to create the appearance of an infinite line by matching the characteristic impedance of the transmission line at the terminal ends. The end result of terminating your traces with a perfectly matched resistor is 0% reflection.

How does RF termination work?

Much of routing in PCB design can be considered an exercise in impedance matching, where you try to make sure the input impedance of an electrical load matches the output impedance of its signal source. The easiest way to maintain these impedances is through the careful placement of termination resistors. Let’s take a closer look at the different types of RF termination.

Parallel Termination

If you place a single resistor from the trace to ground or Vcc you get what’s called a parallel termination. Parallel termination is easy to implement because the value of the resistor is easy to obtain, you only need one extra component, and it perfrorms well with distributed loads. The only major con with parallel termination is power dissipation via a continuous DC current path to ground. The power dissipation can add up across a circuit when you start terminating multiple nets.

Thevenin Termination

Another way to match the load and trace impedances is to use two load-end resistors whose parallel combination equals the trace impedance. One resistor connects to Vcc while the other connects to ground, providing a pullup and pulldown pair that balances the driver’s high and low logic levels. This variation of the parallel termination can also perform well in distributed loads at the expense of a constant current leakage from Vcc to ground. It can also be difficult to find optimum combination of resistor values for a given driver.

AC Termination

Yet another way to match impedances is to add a capacitor in series with the parallel terminating resistor. The addition of the capacitor mitigates the power dissipation problems of other parallel termination schemes, blocks low-frequency noise, and minimizes overshoot and undershoot. The added cost for this scheme is the added complexity of managing the RC time constant of the capacitor.

Series Termination

In series termination, you place the resistor near the driver to increase the impedance at the source and prevent reflections on the driver end of the trace. A resistor value is selected so that the combined sum of the termination resistor and the driver output are equal to the impedance of the trace. Series termination benefits from a lower power draw at the expense of reflection at the opposite end of the trace.

PCB design software makes impedance matching easier

Data errors due to reflections are most likely to occur when the round trip propagation time of a signal is equal to or greater than the transition  (rise or fall) time of the driver. In this post we covered the basics behind using termination to prevent reflections in your circuits