diff pair impedance calculator

What this diff pair impedance calculator does

Differential routing is used for high-speed standards such as USB, Ethernet, LVDS, PCIe, and many RF/data interfaces. Instead of one trace carrying a signal, two tightly controlled traces carry equal and opposite signals. The key electrical target is differential impedance (Z_diff), commonly 85 Ω, 90 Ω, or 100 Ω depending on the protocol.

This calculator estimates:

• Z₀: single-ended characteristic impedance of one trace
• Z_odd: odd-mode impedance (dominant mode in differential signaling)
• Z_diff = 2 × Z_odd
• Z_even: even-mode impedance (useful for common-mode behavior intuition)
• Optional spacing recommendation to hit a target Z_diff

How to use it

1) Choose stackup type

Pick Microstrip if traces are on an outer layer above one reference plane. Pick Stripline if traces are embedded between two reference planes.

2) Enter geometry and material values

Use dimensions from your PCB stackup proposal:

• h: dielectric height from trace to reference plane
• w: trace width
• s: edge-to-edge spacing between the two traces
• t: copper thickness
• εr: dielectric constant at your signal frequency (frequency-dependent in real laminates)

3) Compare against your interface requirement

If your interface calls for 100 Ω differential and your result is low, increase spacing or reduce coupling. If the result is high, reduce spacing or adjust width/height according to manufacturable design rules.

Design guidance for practical PCB routing

• Ask your PCB fabricator for a controlled-impedance stackup before freezing trace geometry.
• Use the same reference plane under the entire pair path; avoid splits and discontinuities.
• Keep pair members tightly length-matched, but avoid excessive serpentine unless required.
• Minimize via stubs on very fast links; back-drill if needed.
• Maintain consistent spacing through bends and neck-down regions where possible.
• For robust results, validate final geometry in a field solver or with fab impedance coupons.

Microstrip vs stripline in one minute

Microstrip fields propagate partly in dielectric and partly in air, so effective dielectric constant is lower and more sensitive to environment and solder mask. Stripline fields are mostly confined in dielectric, generally offering better isolation and more stable impedance, at the cost of higher loss and less routing flexibility.

Limitations of fast calculators

Closed-form equations are useful early in design, but they are approximations. Real boards include roughness, glass weave effects, frequency dispersion, plating tolerances, solder mask influence, etch bias, and manufacturing variation. Treat these numbers as a strong first estimate, not final signoff.

For production-critical links, combine this calculator with your board house’s impedance model and a proper 2D/3D field solver.

Quick FAQ

Why is differential impedance not just 2 × single-ended impedance?

Because the two traces are electromagnetically coupled. Coupling changes odd/even mode impedances, so Z_diff depends strongly on spacing.

What unit should I use?

Use whichever is natural for your workflow (mm or mil). The calculator converts internally and reports spacing recommendations in your selected unit.

What is the best target for my design?

Follow your interface specification and component datasheets. Common values are 85 Ω (some PCIe channels), 90 Ω (USB), and 100 Ω (LVDS/Ethernet families).