Choosing Impedance

Over the last few weeks, we have been looking at impedance. We began by talking about what impedance is and how we might visualize it. We then considered the physical characteristics of the signal chain, and how they govern impedance. In this final part, we will look at why impedance needs to be different for various applications. The first question to answer is straightforward: why are there different impedance values in the first place?

The answer lies in a combination of history, physics and the never-ending drive for higher transmission speeds. Standards such as Ethernet and PCI Express each landed on their own impedance targets, shaped by technical and practical reasons. As data rates continue to accelerate, designers now face the challenge of managing these differences while still achieving best-in-class signal integrity.

Why Does Impedance Vary?

The resistance of a direct current circuit simply needs to be as low as possible. When it comes to impedance, the need is for balance – ensuring that a signal can travel without distortion or reflection. But choosing the right impedance depends on the application:

• Ethernet (100 Ω): Ethernet evolved as a differential signaling standard designed for long-distance data transmission over twisted-pair cabling. A 100-ohm differential impedance provides strong noise immunity and compatibility with readily available copper cabling. This compatibility was a strong influence on Ethernet’s dominance in networking.
• PCI Express (85 Ω): PCIe’s shorter-reach, board-level environment allowed engineers to optimize around 85 ohms. This value balances bandwidth and insertion loss in printed circuit board (PCB) traces and connectors, which is especially important in compact, high-density computing systems.
• Printed Circuit Boards (approximately 90 Ω): Most PCBs exhibit a natural impedance value close to 90 ohms due to the material properties. It is a reliable target that designers can achieve consistently across many PCB designs.

What this list shows is the variety of impedance values in regular use. More important is not that one value is universally better, but that each reflects the technical and practical needs of its application.

Choosing the Middle Path

The variety of impedance values in use across different systems creates a complex choice for system architects: what impedance value should they use?

This is the reason why Samtec has targeted 92 ohms, a balanced solution that performs well across the range of industry standards. It works seamlessly across PCB applications while offering strong compatibility with Ethernet and PCIe signaling. In high-speed systems operating at 56 Gbps and beyond, small impedance mismatches can have large effects. Selecting 92 ohms is a practical compromise – it minimizes reflections while providing the greatest flexibility for the user.

Moving Signals Off the Board

Traditional PCB traces face physical limitations. As speeds climb, attenuation, crosstalk, and signal loss increase. Near-chip and board-to-board Flyover® cable assemblies bypass many of these challenges by moving critical signals into ultra-low-loss cables.

By controlling the impedance of the signal chain through the transition from the chip, across the PCB launch, and into the cable, Flyover® architecture delivers cleaner eyes, longer reach, and more robust performance at extreme data rates. It’s a direct response to the limitations of conventional PCB routing.

Samtec’s Eye Speed® cable offers ultra-low skew for precise timing and alignment, which is critical for maintaining signal integrity across multi-lane systems. For next-generation data rates up to 224 Gbps and beyond, Samtec’s Hyper Low Skew Eye Speed® cable provides the lowest skew performance in the industry. Combined with a 92-ohm design approach, it enables system architects to confidently scale to the fastest emerging standards while maintaining interoperability with existing ecosystems.

Choosing the Right Impedance

So, why does impedance matter? Because every application environment has its own requirements, and mismatches can mean the difference between flawless operation and costly design challenges. Ethernet prioritizes long reach and noise immunity, while PCIe is optimized for high bandwidth, and PCB stackups favor 90 ohms for manufacturability. Samtec’s 92-ohm cabling solutions unify these requirements for cutting-edge performance.

As speeds rise and standards continue to evolve, choosing the right impedance remains central to achieving signal integrity. Samtec’s 92-ohm Flyover® solutions and Eye Speed® cable technologies deliver best-in-class performance by bridging the gaps between standards and pushing past PCB limitations.

For a deeper dive into the principles behind impedance, interconnect design, and system optimization, check out the Samtec Signal Integrity Handbook. It’s an essential resource for engineers looking to navigate the challenges of high-speed system design.