Damping and Reflection Transfer with a Series Termination Resistor
Even these transmission lines might require a series termination resistor
Trace, source, and load impedance matching are important in boards that contain transmission lines. Whenever the rise time of a signal is much shorter than the round trip propagation time for signals, you should make sure these impedances match in your interconnects. Impedance controlled design and routing can help ensure your traces have consistent impedance, but some critical components and highspeed signalling standards may require implementing a series termination resistor.
Termination vs. Damping with a Series Termination Resistor
Using impedance controlled routing throughout your board will help ensure that your traces have consistent impedance, although the parasitic conductance of the substrate and DC resistance of the copper itself will create some deviation in the output impedance at low frequencies. However, impedance controlled routing does not affect the output impedance from a source or load, thus an impedance matching network should be used at the source, load, or both.
There are several different termination networks, each of which is suitable for different applications. In high speed signalling standards like LVDS, the impedance at the load is normally highZ with the singleended impedance of the transmission line set to a specific value (normally 50 Ohms). Series termination is then used at the source as the output impedance of the driver is normally less than the singleended impedance of the transmission line. The simplest way to size the value of the termination resistor is to subtract the output impedance from the transmission line impedance:
Formula for the series termination impedance value
There are two issues to consider when sizing a series termination resistor for use with a particular driver in your PCB:

Suppressing reflection: the series resistor at the source can suppress reflection at the source back into the driver. Similarly, termination at the receiver end will suppress reflection back along the transmission line.

Damping: a series termination resistor will increase the damping constant in the equivalent RLC circuit model for a transmission line. If the series termination resistor takes just the right value, you can critically dampen the transient oscillation that occurs due to switching and suppress overshoot, albeit at the cost of increasing the rise time.
Newer ICs tend to have welldefined and controlled impedance at the driver output, so you can size your series termination resistor very close to a specific value (if you even need one at all), and you only need to worry about the variation in the quoted value for your series termination resistor. Some ICs have large variations in the output impedance between the ON and OFF states. This variation also exists between manufacturers, the operating temperature range, and power supply voltage range.
For example, the output impedance from a MOSFET driver varies quadratically with the difference between the gatesource voltage and the switching threshold. If the output impedance varies over a range that is much smaller than the transmission line impedance, then series termination is still appropriate.
Power MOSFET transistors
For example, if the output impedance from your driver varies from 20 to 30 Ohms in the ON and OFF states, respectively, then the best series termination resistor to use is 25 Ohms. This will define an impedance of 45 to 55 Ohms at the source, which places you well within a +/ 10% variation, assuming there are no other factors that cause output impedance variations. Thank you to Dr. Howard Johnson for pointing this out.
The other option to set a defined impedance at the output is to run the IC at a higher power supply voltage, although this may break your thermal budget and will increase power consumption. However, your MOSFET output will saturate and you will have a welldefined impedance at the output. Ignoring these options and using a series termination resistor will create a tradeoff between the three points listed above.
The Tradeoff Between Damping and Matching
If you analyze the equivalent RLC model that defines a transmission line with a series termination resistor, you can quickly determine the level of damping provided by the presence of a series termination resistor. In highspeed design involving transmission lines, you can safely ignore the parasitic inductance in the substrate and the DC resistance of the copper trace in the transmission line.
As a digital driver switches, the sudden change in signal level will induce a transient oscillation along the transmission line that is superimposed on top of the ON or OFF signal level. This transient is responsible for overshoot at the receiver, thus overshoot should be suppressed where possible. When the transmission line is critically damped, a transient oscillation will be completely suppressed and while still having the fastest rise time.
If you calculate the transient oscillation frequency and damping in this standard model, you can determine the value of the series termination resistor required to produce critical damping:
Series termination resistor required for critical damping
One can immediately see that the source output impedance and series termination resistor needs to be double the characteristic transmission line impedance in order to reach critical damping. This illustrates the tradeoff between damping and impedance matching: one cannot critically damp the response and perfectly match impedance simultaneously. If you match the source impedance exactly to the transmission line impedance, then you will produce an underdamped oscillation when the driver switches.
Finding Compromise
Determining the right compromise between damping and impedance matching really requires considering the noise margin at the receiver. If the receiver has a large noise margin, then most likely you can perfectly match to the characteristic impedance without worrying about overshoot; you won’t induce involuntary switching or enter the undefined region for your logic family. If the noise margin is narrow, then you may need to allow a slight mismatch and power transfer reduction from the source and use a larger resistor, which will bring the response closer to critical damping. Although this reduces the amplitude of the transient oscillation, it also increases the rise time somewhat, which will limit the maximum data rate.
Because of the issue mentioned earlier, where the output impedance of a driver can be different in the ON and OFF states, you might be able to critically damp one edge of the pulse, while the other edge exhibits some ringing during switching. If the load is a highZ receiver that does not require termination, then you can even have a reflection that produces a stairstep response on one or both edges of the pulse.
Always test transmission lines in your prototype for signal integrity
The signalling standard your components use may carry their own termination requirements. Instead of using series termination, you could use a signalling standard with a defined constant current output and output impedance much greater than 50 Ohms. You would then only need to worry about properly terminating the receiver end with parallel termination or some other recommended termination network.
With the different options available for termination in different applications, it helps to use a PCB design package that includes the integrates your layout and simulation tools in a single platform. With Altium Designer, you’ll have full control over your design, and your simulation tools will take data directly from your layout. This helps you determine the right series termination resistor value or any other components you need for a termination network.
Download a free trial of Altium Designer to see how the accurate layout features and powerful signal integrity tools can help you. You’ll have access to the best design features the industry demands in a single program. Talk to an Altium expert today to learn more.