Non-Transparent Interconnects: Signal Integrity and Interconnects PCB Design Tutorial

In the intricate world of electronics engineering, understanding when interconnects cease to be transparent is a crucial aspect of ensuring optimal signal integrity. The answer to this question, as often is the case in engineering, is a nuanced "it depends." However, the essence of engineering lies not just in acknowledging dependencies but in articulating them and providing metrics for under what conditions these dependencies come into play. In this tutorial, we delve into the complexities of interconnects, focusing on their transparency and the impact on signal integrity.

The Nature of Interconnects

While the details of circuits and the nature of interconnects play a significant role in determining transparency, simplifying assumptions and rules of thumb become invaluable for quickly estimating when an effect might become important. In this discussion, we explore two critical sources of noise – switching noise on power rails and signal paths.

Power Rail Collapse

Switching noise on power rails, known as power rail collapse, stems from changing power rail currents passing through the power lead inductance between IC power pins and the voltage regulator module (VRM) or the nearest decoupling capacitor. The VRM regulates the voltage supplied to the power rail on the board. An illustration depicting ground bounce noise affecting a target line is shown below.

 This noise occurs when one, two, three, and finally four input/output (I/O) signals switch simultaneously by traversing traces on a circuit board. The board in question has a common return trace for these signals.

Ground Bounce

On signal paths, the second type of switching noise, ground bounce, arises from changing return currents passing through a higher inductance return path that is not a wide, continuous plane. This noise intensifies with multiple signals sharing the same return conductor and switching simultaneously. Ground bounce occurs in various scenarios, such as IC packages, connectors, multiconductor cables, solderless breadboard interconnects, and poorly designed circuit boards.

Estimating Ground Bounce Noise

To estimate when ground bounce noise becomes a concern, we use a few simplifying assumptions. The voltage noise across the inductance of an interconnect is given by the formula:

Where:

• $\mathrm{\Delta }V$ is the voltage noise generated (mV),
• $L_{\text{len}}$ is the inductance per length of the interconnect (nH/in),
• $\mathrm{\Delta }I$ is the change in current or transient current (mA),
• $R_{\text{T}}$ is the rise time of the changing current (ns).

Considering a transparent interconnect when the induced voltage noise is below a maximum acceptable level, the relationship between the longest length that remains transparent is given by:

 

Applying the Rule of Thumb

Using typical values for simple interconnect wires, where inductance per length is about 20 nH/inch, and assuming voltage noise less than 50 mV is insignificant with a 50 mA current change, we arrive at a rule of thumb:

 This rule suggests that for a signal rise time of 100 nsec, interconnects shorter than 5 inches appear transparent. Beyond this threshold, interconnect design becomes critical to circuit performance.

Practical Applications

Most solderless breadboards have wires less than about 10 inches long. This implies that, as long as signal rise times are longer than approximately 200 ns, the interconnects in solderless breadboard circuits will likely remain transparent. However, it's essential to note that this estimate is conservative.

If current changes are more substantial, the length condition for transparent interconnects is reduced accordingly. For instance, with a current change of 500 mA, interconnects shorter than 2 inches may need to be considered for transparency.

Beyond the Basics

In real-world scenarios, even if switching noise is present, its impact may be negligible if it occurs at a time when the circuit is not sensitive. This explains why some circuits can function well on solderless breadboards or circuit boards without strictly adhering to signal integrity design guidelines. The interconnects in these cases are transparent, or the signals are insensitive to the generated noise levels.

By following best design practices, especially with solderless breadboard interconnects, it might be possible to extend their useful range into rise times as short as 2 ns, provided interconnect lengths can be kept short. The measured signal from a small Teensy microcontroller module plugged into a solderless breadboard, with a rise time as short as 1.95 ns using a short interconnect, exemplifies the practical application of these principles.

 

Conclusion

In the complex realm of electronics, understanding the conditions under which interconnects cease to be transparent is paramount. By employing simplifying assumptions and rules of thumb, engineers can make quick estimates of when certain effects, such as ground bounce noise, become significant. This tutorial has provided insights into the factors affecting interconnect transparency, offering practical guidelines for design considerations and illustrating the delicate balance between transparency and the impact of interconnect design on signal integrity. As engineers navigate the intricate landscape of electronic circuits, the ability to assess when interconnects are not transparent is a valuable skill that contributes to the success of their designs.