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PCB Differential Pair Impedance Calculator

Estimate single-ended and differential impedance for coupled microstrip or stripline routing from PCB geometry and dielectric constant. Use the result for early stackup exploration before manufacturer field-solver verification.

Engineering tool

PCB Differential Pair Impedance Calculator

Estimate single-ended and differential impedance for coupled microstrip or stripline geometry.

Trace structure

Result console

Single-ended impedance Z0
71.184Ω
Differential impedance Zdiff
111.662Ω
Coupling factor
21.568%
Structure
Microstrip

Moderate coupling: spacing materially affects differential impedance.

Use this approximation for early routing only. Confirm the final stackup, solder mask, copper thickness, roughness, and fabrication tolerances with the PCB manufacturer's field solver.

Differential pair PCB cross-sectionTwo copper traces above a reference plane labeled with width, spacing, dielectric height, thickness, dielectric constant, and differential impedance.Width WSpacing SHeight HThickness TDielectric ErReference plane • Zdiff
Coupled traces above or between reference planes form the differential transmission line.

Formula reference

Differential Pair Impedance Formulas

These closed-form equations are simplified approximations for early geometry selection.

Microstrip Z0 ≈ 87 / √(Er + 1.41) × ln[5.98H / (0.8W + T)]Microstrip Zdiff ≈ 2Z0 × [1 - 0.48e^(-0.96S/H)]Stripline Z0 ≈ 60 / √Er × ln[4H / (0.67π(W + T))]Stripline Zdiff ≈ 2Z0 × [1 - 0.347e^(-2.9S/H)]

Variable definitions

W
trace width
S
edge-to-edge spacing
H
dielectric height to reference plane
T
finished copper thickness
Er
dielectric constant

Worked Example

For W = 0.15 mm, S = 0.15 mm, H = 0.18 mm, T = 35 µm, and Er = 4.2 in microstrip, the calculator estimates the single-ended and coupled differential impedance using the ratios S/H and conductor geometry.

Engineering Notes

  • Differential impedance depends strongly on width, spacing, dielectric height, and Er.
  • PCB manufacturers use field solvers for final impedance control.
  • Solder mask, copper roughness, and fabrication tolerance affect real impedance.
  • Tighter spacing increases coupling and lowers differential impedance.
  • Always confirm the production stackup with the PCB manufacturer.

Support reference

FAQ

What is differential impedance?

Differential impedance is the impedance seen by equal and opposite signals traveling on a coupled pair of traces.

What is the difference between Z0 and Zdiff?

Z0 is the estimated impedance of one trace relative to the reference plane. Zdiff includes electromagnetic coupling between both traces and is usually less than twice Z0.

How does trace spacing affect differential impedance?

Reducing spacing strengthens coupling and generally lowers differential impedance. Increasing spacing makes Zdiff approach twice the single-ended impedance.

What is a typical USB differential impedance?

USB differential pairs are commonly designed around 90 Ω differential impedance, but the exact tolerance and test conditions depend on the USB specification and product implementation.

Why should controlled impedance be verified by the PCB manufacturer?

The manufacturer knows the finished stackup, dielectric materials, etching compensation, copper thickness, and process tolerances and can use a field solver for production geometry.

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