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Differential Pair Trace Impedance Calculator

Differential pair signaling transmits complementary inverted signals along closely coupled parallel microstrip traces on high-speed circuit boards.

Width of each copper conductor.

Edge-to-edge gap between the two differential traces.

Substrate thickness from trace bottom to ground reference plane.

Copper foil thickness (1 oz Cu = 0.035 mm / 1.4 mil).

Relative permittivity of PCB dielectric (standard FR-4 is ~4.2, Rogers ~3.5).

Calculated Result
97.1 Ω

Differential Impedance (Z_diff)

Differential Impedance (Z_diff)

97.1 Ω

Single-Ended Impedance (Z₀)

56.1 Ω

Odd-Mode Impedance (Z_odd)

48.6 Ω

Even-Mode Impedance (Z_even)

63.5 Ω

Coupling Factor (k)

0.133

Spacing-to-Height Ratio (S/H)

1.33

Width-to-Height Ratio (W/H)

1.33

Dielectric Constant (ε_r)

4.2

Calculation Breakdown

  1. Single-Ended Characteristic ImpedanceZ₀ = [87 / √(ε_r + 1.41)] · ln[ 5.98·H / (0.8·W + T) ] = [87 / √(4.2 + 1.41)] · ln[ 0.897 / 0.195 ] = 56.1 Ω.
  2. Edge Coupling Factor EvaluationCoupling term = 0.48 · exp(-0.96 · S / H) = 0.48 · exp(-0.96 · 0.2 / 0.15) = 0.133.
  3. Differential Impedance CalculationZ_diff = 2 · Z₀ · [1 - 0.48 · exp(-0.96 · S / H)] = 2 · 56.1 · (1 - 0.133) = 97.1 Ω.

Impedance Breakdown (Ω)

Interactive visualization based on your current inputs

Impedance
0.024497397Differential (Z_diff)Single-Ended (Z₀)Odd-Mode (Z_odd)Even-Mode (Z_even)ModeImpedance (Ω)

What Is the Differential Pair Trace Impedance Calculator?

The Differential Pair Trace Impedance Calculator sizes printed circuit board (PCB) traces for controlled-impedance differential high-speed signals.

How Does the Differential Pair Trace Impedance Calculator Work?

Evaluates single-ended microstrip fields combined with mutual edge coupling to calculate odd-mode and differential impedance.

Differential Pair Trace Impedance Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

Z_{diff} = 2 Z_0 \left[ 1 - 0.48 \exp\left( -0.96 \frac{S}{H} \right) \right]

IPC-2141 edge-coupled differential microstrip formula factoring in mutual capacitive and inductive coupling.

How to Use the Differential Pair Trace Impedance Calculator

  1. Input trace width, spacing, and copper weight.
  2. Enter dielectric substrate height above ground and material dielectric constant.
  3. Fine-tune trace width and spacing to achieve standard 90 Ω (USB) or 100 Ω (Ethernet, PCIe) targets.

Step-by-Step Example Calculation

Standard 100-Ohm USB / PCIe Microstrip Pair on FR-4

Input Values:

traceWidth:0.2
traceSpacing:0.2
dielectricHeight:0.15
traceThickness:0.035
dielectricConstant:4.2
Worked Steps: Yields 97.2 Ω differential impedance with single-ended Z₀ of 55.4 Ω.

Understanding Your Result

Differential Impedance (Z_diff): Impedance seen between complementary signal lines.

Odd-Mode Impedance: Single trace impedance during differential excitation.

Coupling Factor (k): Degree of mutual electromagnetic coupling.

Factors That Affect the Result

  • Wider trace spacing decreases mutual coupling, pushing differential impedance towards 2 × Z₀.

When Should You Use This Calculator?

  • Designing USB 3.0, HDMI, LVDS, SATA, Ethernet, and PCIe PCB layout traces.

Assumptions & Limitations

  • Valid for surface microstrip geometries conforming to IPC-2141 recommended aspect ratios.

Frequently Asked Questions

Calculation Accuracy & Reference Note

IPC-2141 standard formulation within ±5% of 2D field solver outputs.

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