High-Speed Design Tool

Differential Impedance Calculator

USB | HDMI | PCIe | LVDS | Ethernet

Calculate differential impedance for high-speed PCB differential pairs. Essential for USB, HDMI, PCIe, LVDS, and other high-speed interface designs requiring controlled impedance routing.

Impedance Parameters

MOD: IMP_CAL_V1

Calculation Mode

Trace Type

Configuration

Trace Width

Dielectric Height

Dielectric Constant (Er)

Copper Thickness

Impedance Analysis

Impedance Check

Characteristic Z0

Single-Ended Z₀

---Ω

Effective Er

0.000

Prop. Delay

0.000ps/mm

Capacitance

0.000pF/m

Inductance

0.000nH/m

Microstrip (Surface) Cross-Section

Common Differential Impedance Standards

Standard	Diff Impedance	Typical Spacing
USB 2.0/3.0	90Ω ±10%	5-8 mil typical
HDMI	100Ω ±15%	5-10 mil typical
PCIe	85Ω ±15%	5-8 mil typical
LVDS	100Ω ±10%	5-10 mil typical
SATA	100Ω ±10%	5-8 mil typical
Ethernet	100Ω ±10%	5-10 mil typical

Differential Pair Design Guidelines

Key Design Rules

• Keep trace spacing consistent throughout the route
• Match trace lengths within ±5 mils for high-speed signals
• Maintain constant impedance at bends and corners
• Avoid routing near board edges or cutouts
• Use length-matching serpentines if needed
• Reference to continuous ground plane

Impedance Formula

Zdiff ≈ 2 × Z0 × (1 - 0.48 × e^(-0.96×S/H))

Zdiff = Differential impedance (Ω)
Z0 = Single-ended impedance (Ω)
S = Trace spacing (mm)
H = Dielectric height (mm)

Differential Routing Best Practices

📏

Length Matching

For high-speed signals, match trace lengths within 5-10 mils. Use serpentine routing on the shorter trace to achieve matching. Place serpentines near the source.

🔄

Coupling

Tighter spacing increases coupling and reduces differential impedance. Looser spacing reduces coupling. Balance based on crosstalk and impedance requirements.

⚡

Via Transitions

When transitioning layers, place vias symmetrically. Add ground vias nearby to maintain return path continuity. Minimize via stub length for high-speed signals.

Need Single-Ended Impedance?

Use our impedance calculator for microstrip and stripline single-ended impedance calculations, or calculate trace width for current capacity requirements.

Impedance Calculator Trace Width Calculator

Differential Impedance FAQ

What is differential impedance?

Differential impedance is the impedance measured between two traces of a differential pair when driven with opposite polarity signals. It's typically lower than 2× single-ended impedance due to coupling.

How does spacing affect differential impedance?

Closer spacing increases coupling, reducing differential impedance. Wider spacing reduces coupling, making differential impedance approach 2× single-ended. Typical spacing is 1-3× trace width.

What's the difference between odd-mode and differential impedance?

Differential impedance = 2 × odd-mode impedance. Odd-mode is measured from one trace to ground with opposite signals, while differential is measured between the two traces directly.

Why is USB 90Ω but HDMI 100Ω?

Different standards evolved with different impedance choices based on connector design, cable characteristics, and signal integrity requirements. Always match the specification for your interface.