I was baffled when my fast-growing child asked about where the rainbow ends. The natural response was the ocean because that’s what I’ve been told. But to be frank, I’ve never seen a full rainbow nor knew the exact point of its origin and end.
While you could choke up an answer to a curious child, there are no quick solutions if you assume that the path of a high-speed signal ends at the receiver. Such an assumption leads to numerous problems caused by reflections and distortions due to the unterminated trace on the PCB.
Why PCB Trace Termination Is Important.
Electronics has evolved rapidly much to the delight of consumers. Gadgets are competing for speed while the form factor is shrinking at each iteration. For PCB designers, it means dealing with high-speed signals more often than ever.
High-speed signals are prone to reflection when a mismatch of impedance occurs between the driver and the PCB trace. You could imagine waves traveling furiously to the shore and crashing on the rocks. The reflection subsequently distorts the upcoming data and creates an issue with signal integrity.
It’s never wise to assume that the path of a high-speed signal ends at the receiver as there is a chance for impedance mismatch. Rather than taking chances, it is better to terminate the PCB trace to ensure the impedance of the driver matches the impedance of the PCB trace. When both impedance is matched, reflection does not occur.
What Is PCB Trace Termination?
In simple terms, PCB trace termination is the process of matching the PCB trace impedance to those of the driver. Usually, this is done by placing resistors in specific configurations along the PCB trace. The value of the resistor is chosen according to how the resistor is placed along the PCB trace.
High speed signals require proper PCB trace termination.
Common PCB Trace Termination Techniques
There are a few termination techniques that you can use to ensure high-speed signals on your PCB suffer from no reflection or distortion on the trace.
1. Series Termination
The series termination is an often-used technique. It is performed by placing a terminating resistor in between the driver and the receiver. The resistor is placed near to the driver, and its value is chosen so that the combined impedance of the resistor and driver matches those of the PCB trace.
The advantage of series termination is that there is no DC value present, which means there’s no unnecessary power waste on the resistor. However, the drawback of this technique is that it doesn’t eliminate the first reflection on the far end. Also, it can be hard to estimate the value of the resistor so that subsequent reflection does not affect the integrity of the signal.
2. Parallel Termination
Parallel termination is the most commonly used termination technique for high-speed signals. It involves placing a shunt resistor in parallel to the receiver. In this technique, you’ll need to place the termination resistor as close to the receiver. The value of the resistor must match the impedance of the line for the termination to be effective.
This is a simple technique, where the resistor is connected to either Vcc or GND. With parallel termination, all reflections at the far end are absorbed by the matching resistor. Of course, the disadvantage of this technique is that a small amount of current will flow through the resistor, increasing the power dissipation.
Ensure the right value of terminating resistor is chosen.
3. Thevenin Termination
Thevenin termination has a very similar concept to parallel termination, except it uses two resistors instead of one. In Thevenin termination, two resistors where the parallel resistance matches the impedance of the trace.
While both the resistors could function as a pull-up or pull-down for the signal, Thevenin termination results in constant draining of current on regardless of the state of the driver.
There’s no better way to ensure that you have the right termination resistor value than modeling and simulating it out on a PCB design software. The OrCAD PCB Designer is particularly useful in working through high-speed signals and identifying potential signal integrity issues.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.