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Beyond Design: Transmission Line Termination

In a previous column “Beyond Design: Interconnect Impedance,” we saw that the impedance of the interconnect is the most critical factor in high-speed PCB design. Unfortunately, the source impedance of a digital IC driver is typically lower than the impedance of the transmission line (10–35 ohms). This is far from the ideal situation for the perfect transfer of energy, and, in most cases, results in reflections and electromagnetic radiation if not addressed.

Whenever a signal meets an impedance variation along a transmission line, there will be a reflection, which can seriously impact signal integrity. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered with reliable performance. In this month’s column, I will look at how to effectively terminate transmission lines.

In an ideal world, the energy emanating from an IC output driver would travel through the PCB transmission line and be totally absorbed by the load. However, if energy is not completely absorbed, then the residual will be reflected back along the interconnect, reaching the original source of energy at the output driver. Reflected energy acts like a standing wave and adds or subtracts to the original signal, causing ringing. Resonance can develop at a signal’s fundamental frequency or harmonics resulting in multiple bounces and emission of radiation. This situation only occurs when the round-trip delay of the interconnect exceeds the signal rise time.

Even if the multilayer PCB has been designed with controlled impedance in mind, impedance discontinuities can still occur due to input gate capacitance, branches, stubs, or test pads; variations in dielectric materials; a neck-down in a via field; skew in a differential pair; or a gap in the return signal path. Generally, the reflected noise level should be kept below 10% of the voltage swing, but this depends on the noise budget. In more conservative designs, the noise budget may be as low as 5%.

There are numerous ways to terminate transmission lines, but the most common methods fall into three categories:

1. Series termination 
2. End termination 
3. Differential pair termination

To read this entire column, which appeared in the March 2020 issue of Design007 Magazine, click here.