The Definitive Guide to Twisted Pair Impedance: 100Ω Ethernet and 120Ω CAN Bus Compared

Executive Summary: Controlling Impedance in Differential Networks

Characteristic impedance in twisted pair cables governs signal integrity in high-speed differential networks. Industrial Ethernet architectures strictly require a 100Ω impedance, whereas CAN Bus and RS-485 networks require a 120Ω impedance. Using the wrong cable geometry alters mutual capacitance and inductance, causing signal reflections (return loss) that corrupt data frames and trigger systemic faults.

Key Engineering Rule of Thumb: For industrial automation and automotive networks, never substitute a 100Ω Ethernet cable in a 120Ω CAN Bus system. To prevent impedance shifts during physical routing and vibration, specify a solid PE dielectric with an extruded TPU jacket to rigidly lock the lay length (twist pitch) in place, guaranteeing consistent electrical performance per IPC/WHMA-A-620 Class 3 standards.

Engineering Deep Dive: The Mechanics of 100Ω vs. 120Ω

Unlike simple point-to-point power wires, the data cables built by a high-speed cable assembly and wire harness manufacturer act as transmission lines. The Characteristic Impedance ($Z_0$) is not a measure of DC resistance, but rather the ratio of voltage to current as a high-frequency wave travels down the cable.

Impedance is physically determined by three distinct manufacturing variables:

1. Conductor Outer Diameter (AWG)
2. Center-to-Center Conductor Spacing
3. The Dielectric Constant ($\epsilon_r$) of the insulation material.

100Ω Industrial Ethernet (Profinet, EtherCAT)

Industrial Ethernet relies on precisely constructed 100Ω twisted pairs inside every factory-grade industrial cable assembly to achieve gigabit speeds in real-world plants.

• The Technical Edge: Maintaining exactly 100Ω prevents Voltage Standing Wave Ratio (VSWR) spikes at the RJ45 modular jack connector or M12 connector junction. Variations in the twist rate (lay length) will cause impedance bumps.
• Manufacturing Constraint: To hit 100Ω, the conductors must be held slightly closer together than in a 120Ω cable, often utilizing a slightly higher dielectric constant material or a specific cross-web separator (in Cat6/Cat6a) to mitigate Near-End Crosstalk (NEXT).

120Ω CAN Bus (ISO 11898 / SAE J1939)

Originally designed for harsh automotive environments — the natural habitat of any ruggedized automotive cable assembly — Controller Area Network (CAN) bus systems operate on a 120Ω differential signaling standard.

• The Technical Edge: A CAN Bus network is physically terminated at both extreme ends with 120-ohm resistors. If the cable itself is not exactly 120Ω, the resulting impedance mismatch causes the signal to reflect off the ends of the bus, colliding with active CAN frames and causing the nodes to throw error flags.
• Manufacturing Constraint: Because 120Ω requires slightly lower capacitance between the conductors, the wire insulation must be slightly thicker, or the conductors must be spaced slightly further apart, than in 100Ω Ethernet cables.
• Impedance Matching Comparison Data

Frequently Asked Questions

Why can't I use a standard 100-ohm Cat5e cable for a 120-ohm CAN Bus system?

While they look similar, utilizing a 100Ω Cat5e cable in a 120Ω CAN network creates an immediate 20% impedance mismatch. This mismatch causes high-frequency signal reflections. In short cable runs, this might go unnoticed, but in long industrial runs, the reflected waves will distort the differential voltage threshold, leading to dropped frames, bus arbitration failures, and total system crashes.

How does the twist rate (lay length) affect twisted pair impedance?

The lay length directly impacts the mutual capacitance and inductance between the two wires. A tighter twist generally increases capacitance and lowers the impedance. More importantly, if the lay length is inconsistent due to poor manufacturing or aggressive physical bending in the field, the impedance will fluctuate wildly along the length of the cable.

How do you test and verify twisted pair impedance during manufacturing?

To guarantee compliance with IPC-620 Class 3 — the workmanship gate of any documented quality control program — custom cable assemblies are tested using Time-Domain Reflectometry (TDR) or a Vector Network Analyzer (VNA). A TDR sends a fast electrical pulse down the cable and measures the reflections. Any physical anomaly—such as crushed insulation, untwisted pairs at the connector, or incorrect dielectric thickness—will appear as a measurable spike or dip in the impedance plot.