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Differential Pair Calculator

Compute differential impedance (Zdiff), odd-mode impedance (Zodd), even-mode impedance (Zeven) and common-mode impedance for edge-coupled microstrip and broadside-coupled stripline differential pairs. Supports mm and mil units.

// Parameters

1 mil = 0.0254 mm

Trace Width (W)

Trace Gap (S) — edge to edge

Substrate Height (h)

Trace Thickness (t)

Relative Permittivity (εr)

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Quick Presets

⚠ Please check your inputs.

// Results

⚡

Enter parameters and click Calculate

Differential Mode

Zdiff (differential impedance)—Ω

Zodd (odd-mode)—Ω

Common Mode

Zcommon (common-mode)—Ω

Zeven (even-mode)—Ω

Single-Ended Reference

Z₀ (single-ended, isolated)—Ω

Coupling factor (Zodd/Z0)—

Differential Pair Relationships
Zdiff = 2 × Zodd · Zcommon = Zeven / 2
Zodd = Z0 × (1 − 0.347 × e^(−2.9×S/h)) [microstrip, IPC-2141]
Zeven = Z0 × (1 + 0.347 × e^(−2.9×S/h))
Target: Zdiff = 100 Ω → Zodd = 50 Ω → Z0 ≈ 55–65 Ω single-ended

Common Standards

Interface	Zdiff	Tolerance
USB 3.x	90 Ω	±15%
PCIe (all gen)	85 Ω	±15%
HDMI 2.x	100 Ω	±15%
LVDS	100 Ω	±10%
Ethernet 10G	100 Ω	±5%
DisplayPort	100 Ω	±10%
MIPI D-PHY	100 Ω	±10%

About the Differential Pair Calculator

Differential signalling uses two traces carrying equal and opposite signals. The receiver responds only to the difference between them, which cancels any noise or interference picked up equally by both traces. Differential pairs are used in virtually every modern high-speed interface â€” USB, PCIe, HDMI, Ethernet, LVDS, MIPI and more.

Differential vs Single-Ended Impedance

The differential impedance (Zdiff) is the impedance seen between the two traces of the pair when driven differentially. It equals twice the odd-mode impedance: Zdiff = 2 * Zodd. The even-mode impedance (Zeven) describes what happens when both traces are driven to the same potential â€” relevant for common-mode noise analysis. Most interfaces specify Zdiff â€” USB3 requires 90 ohm, PCIe requires 85 ohm, LVDS and HDMI require 100 ohm.

Coupling Between Traces

The coupling between the two traces reduces the odd-mode impedance below the single-ended impedance. Tighter spacing (smaller gap S) means stronger coupling and lower Zdiff. This is why differential pair routing rules specify both trace width AND trace spacing â€” you cannot hit a target Zdiff by adjusting width alone.

Edge-Coupled vs Broadside-Coupled

Edge-coupled pairs (side by side on the same layer) are by far the most common. Broadside-coupled pairs (one directly above the other on adjacent layers) provide very tight coupling but require careful layer stackup control and are more sensitive to layer registration tolerances.