High-Speed DAC Cable Structure Explained: Twinax, AWG, Shielding, and Signal Integrity｜Technology Year：2022

DAC, or Direct Attach Copper, is widely used for short-reach copper interconnects in data centers, servers, switches, NICs, and other high-speed networking equipment.

From the outside, a DAC may simply look like a thicker high-speed cable. In reality, its electrical performance depends heavily on the internal construction.

A typical high-speed DAC may include:

**Twinax Differential Pair

• Conductor / AWG
• Insulation / Dielectric
• Pair Shield
• Overall Shield
• Jacket
• Connector Termination**

Together, these structures affect:

Differential Impedance, Insertion Loss, Return Loss, Crosstalk, Skew, EMI, and Practical Cable Reach.

A DAC should therefore be treated as a complete controlled-impedance transmission channel rather than simply a connector attached to copper wire.

What Is Inside a High-Speed DAC Cable?

The exact construction varies according to SFP, QSFP, QSFP-DD, OSFP, data rate, cable length, and manufacturer design.

From a Signal Integrity perspective, however, the structure can be understood as follows:

The high-speed DAC is therefore a:

Controlled-Impedance High-Speed Transmission Channel

rather than a conventional copper cable.

Why Is Twinax Commonly Used in High-Speed DACs?

Twinax, or Twin-Axial Cable, uses two conductors with a tightly controlled geometric relationship to form a balanced differential transmission line.

Its main advantages include:

• Stable differential geometry
• Controlled characteristic impedance
• Reduced pair-to-pair crosstalk
• Improved high-frequency loss performance
• Better EMI control when combined with shielding
• Suitability for short-reach high-speed NRZ and PAM4 links

In a DAC, one high-speed lane is generally carried by a differential pair rather than by a single conductor.

For multi-lane interfaces such as QSFP-based systems, several high-speed Tx and Rx differential pairs operate together.

Each pair must maintain controlled:

**Geometry

• Impedance
• Skew
• Shielding**

to preserve overall channel performance.

Does Thicker AWG Always Mean Better High-Speed Performance?

Not necessarily, although AWG is an important design factor.

AWG stands for:

American Wire Gauge

A lower AWG number generally means a thicker conductor.

For high-speed copper cables, thicker conductors can reduce part of the conductor loss and may help extend passive copper reach.

However, they also create mechanical trade-offs.

Therefore:

A lower AWG number does not automatically mean a better DAC.

The correct design must balance:

**Loss

• Reach
• Cable Diameter
• Flexibility
• Connector Mechanical Load
• Cable Management**

A short DAC may use a thinner cable because flexibility and density are more important, while a longer passive cable may require a thicker conductor to maintain sufficient signal margin.

Why Do High-Speed DACs Need Shielding?

Differential signaling inherently provides some resistance to common-mode noise, but modern high-speed channels still face:

Pair-to-Pair Crosstalk
EMI
External Noise
Mode Conversion

As a result, high-speed Twinax cables commonly use multiple levels of shielding.

Pair-Level Shielding

Each differential pair may have its own shielding structure.

Its main purpose is to:

Reduce coupling between adjacent high-speed lanes.

Overall Shielding

An additional shield may surround the complete cable bundle.

Its main purposes are to:

Reduce radiated EMI and improve immunity against external electromagnetic interference.

Shielding is particularly important when many high-speed differential lanes are placed together inside a dense cable assembly.

However:

More shielding is not always automatically better.

Additional shielding can result in:

**Larger Diameter

• Higher Weight
• Lower Flexibility
• Higher Cost
• More Complex Termination**

High-speed DAC design is therefore a balance between:

Electrical Performance

and:

Mechanical Flexibility

Why Is Connector Termination One of the Most Critical DAC Areas?

The middle section of the cable can maintain relatively stable impedance through controlled Twinax geometry.

At the connector end, however, the physical structure changes rapidly:

Twinax Pair
→ Shield Termination
→ Contact / Paddle Card
→ Connector Interface
→ Host PCB

This transition can easily create:

Impedance Discontinuity

Important factors include:

• Wire strip length
• Pair separation
• Ground structure
• Shield termination
• Contact geometry
• PCB launch
• Solder, weld, or direct-attach structure

Any of these can affect:

Reflection
Return Loss
Crosstalk
Differential Impedance

Therefore, even if the bulk Twinax cable itself performs well, poor connector termination can still degrade the complete assembly.

This is why a high-speed DAC must be designed and evaluated as:

Cable + Termination + Connector + Host Interface

rather than as the cable alone.

How Does DAC Construction Affect Signal Integrity?

The relationship between physical cable design and Signal Integrity can be summarized as follows:

The key engineering principle is:

At high data rates, every physical dimension can become an electrical parameter.

This is why high-speed cable manufacturing must tightly control:

• Conductor geometry
• Insulation extrusion
• Pair spacing
• Shield coverage
• Cable tension
• Termination length
• Connector assembly

Small mechanical variations that are insignificant in low-speed wiring may create measurable Signal Integrity differences in high-speed DACs.

Why Does a Passive DAC Not Need a Retimer?

A Passive DAC does not contain a Retimer, DSP, or other active signal-regeneration electronics.

Instead, it relies primarily on:

**Low-Loss Twinax

• Controlled Impedance
• Shielding
• Optimized Connector Transition**

to carry the host SerDes signal directly to the receiving equipment.

Typical advantages include:

• Low power
• Low latency
• Simple architecture
• Lower cost
• High reliability

The main limitation is:

Reach is constrained by the passive channel-loss budget.

As data rate or cable length increases, passive copper may no longer provide sufficient margin.

At that point, other architectures can be evaluated:

ACC
AEC
Retimer
AOC
Optical Transceiver

Therefore, Passive DAC is generally most attractive when the required reach is short enough that the complete copper channel can meet the electrical specification without active signal regeneration.

High-Speed DAC Selection Requires More Than 100G, 200G, or 400G

Two cables both labeled “100G DAC” or “200G DAC” may still perform differently because of variations in:

• Lane rate
• AWG
• Cable length
• Twinax construction
• Shielding
• Connector design
• Equipment equalization
• FEC
• Manufacturing consistency

A practical selection process should therefore include:

SyncTronics' current high-speed transmission portfolio includes SFP28, QSFP28, QSFP56, and other cable assemblies using controlled-impedance high-speed copper structures for networking, server, and data center applications.

High-Speed DAC Cable Structure: Key Takeaways

The four most important points are:

First, a DAC is not ordinary copper wire attached to a high-speed connector. It is a controlled-impedance high-speed transmission channel.

Second, Twinax geometry, dielectric materials, and shielding jointly determine impedance, loss, crosstalk, and skew.

Third, a lower AWG number generally means a thicker conductor that can reduce some loss, but it also increases cable diameter, stiffness, and cable-management difficulty.

Fourth, connector termination is one of the most sensitive locations for impedance discontinuity, so the cable and connector must be engineered as one complete assembly.

The real performance of a DAC is therefore determined not only by a label such as:

100G, 200G, or 400G

but by the combined behavior of:

**Twinax

• AWG
• Dielectric
• Shielding
• Connector Transition
• Manufacturing Tolerance
• Signal Integrity**

As Ethernet electrical interfaces continue moving toward higher lane rates, the importance of cable loss, crosstalk, impedance control, and connector transitions will continue to increase.

SyncTronics Solutions

High-speed DAC cable design and selection should consider interface type, lane rate, Twinax construction, AWG, cable length, characteristic impedance, shielding, connector transition, and the complete Signal Integrity environment. SyncTronics can support specification verification, cable-length evaluation, equipment compatibility review, and customized high-speed Cable Assembly solutions for SFP28, QSFP28, QSFP56, and other high-speed interconnect applications.

Need Wire Harnesses, Connectors, Electromechanical Components, or Custom Solutions? Contact Us Today

Whether you are looking for connectors, cables, wire harness assemblies, electromechanical components, or complete electronic interconnect solutions, SyncTronics provides professional technical support and engineering evaluation for product selection, specification verification, New Product Introduction (NPI), OEM/ODM customization, as well as high-speed transmission, high-reliability, and other specialized application requirements.

We welcome you to visit our Contact Us page and share your project requirements and contact information. Our team is committed to responding quickly and professionally, evaluating the most suitable products and electronic interconnect solutions based on your product specifications, application requirements, and project objectives. We also provide technical recommendations and customized engineering support to help shorten product selection and development time, reduce project risks, and accelerate your path to mass production. We look forward to the opportunity to work with you and provide reliable technical support and practical solutions for every stage of your project.

Related Articles

How Far Can High-Speed Copper Cables Reach? DAC & Twinax Guide | SyncTronics Technology

What Is Signal Integrity? High-Speed Cable Loss & Impedance Explained | SyncTronics Technology

What Is 100Ω Differential Impedance? High-Speed Cable & Connector Impedance Control | SyncTronics Technology

Retimer vs Redriver vs AEC | High-Speed Signal Extension Explained | SyncTronics Technology

Disclaimer and Technical Exchange Statement

This content is compiled based on publicly available information and industry experience, intended solely for reference during technical exchanges. It does not constitute any commercial, technical, or legal advice or basis. The specifications, materials, manufacturing processes, application methods, and related information described herein may vary due to differing design conditions, environmental factors, or usage scenarios. No guarantees are made regarding the completeness, correctness, or suitability of such content.

Readers should verify the suitability with the responsible design or technical personnel based on actual application scenarios, and independently assess all associated risks.