Calculator Inputs
Example Data Table
| Structure | Width | Spacing | Height | Er | Single Ended | Differential |
|---|---|---|---|---|---|---|
| Microstrip | 0.20 mm | 0.15 mm | 0.18 mm | 4.20 | 66.99 Ω | 105.09 Ω |
| Microstrip | 0.15 mm | 0.20 mm | 0.10 mm | 3.70 | 61.08 Ω | 113.57 Ω |
| Stripline | 0.12 mm | 0.18 mm | 0.20 mm | 4.00 | 34.94 Ω | 68.10 Ω |
Formula Used
Microstrip Single Ended Impedance
For narrow traces, the calculator uses:
Z0 = 60 / √Eeff × ln(8H / W + W / 4H)
For wider traces, the calculator uses:
Z0 = 120π / √Eeff × [W / H + 1.393 + 0.667 ln(W / H + 1.444)]
Differential Pair Estimate
The pair calculation uses an edge coupled approximation:
Zdiff = 2 × Z0 × (1 - A × e-B × S/H)
For microstrip, A is 0.48 and B is 0.96. For stripline, A is 0.347 and B is 2.90.
Delay Estimate
Velocity = c / √Eeff
Delay = Length / Velocity
How To Use This Calculator
- Select millimeters or mils.
- Choose microstrip or stripline structure.
- Enter trace width, spacing, height, and copper thickness.
- Enter dielectric constant and optional solder mask values.
- Add frequency, length, target impedance, and tolerance.
- Press Calculate to review impedance, delay, coupling, and target error.
- Use CSV or PDF buttons to save the current result.
Differential Pair Impedance Overview
Differential traces carry equal and opposite signals. Their impedance depends on trace width, spacing, dielectric height, copper thickness, and material constant. A controlled pair helps receivers reject noise and timing error. It also keeps high speed links predictable during routing.
Why Geometry Matters
Width sets the base single ended impedance. Spacing controls the magnetic and electric coupling between both traces. Tight spacing lowers differential impedance. Wide spacing moves the pair closer to two separate single ended lines. Dielectric thickness also matters. A taller dielectric usually raises impedance, while a wider trace lowers it.
Practical Board Inputs
This calculator accepts common board values in millimeters or mils. It includes microstrip and stripline selections. It also estimates delay, velocity, coupling strength, target error, and a suggested spacing for the chosen target. These results help during early stackup talks. They are still estimates, not a replacement for field solver verification.
Design Review Use
Use the output as a design review snapshot. Compare the calculated value with the target impedance. Then adjust width, spacing, or dielectric height. Export the table when you need a record for layout notes, fabrication questions, or revision history. The CSV file supports spreadsheets. The PDF file is useful for sharing.
Accuracy Notes
Closed form formulas are fast. They are also limited. Real boards include glass weave, solder mask, etching tolerance, copper roughness, plating variation, and reference plane openings. Connectors and vias can change impedance too. For final release, confirm critical interfaces with your fabricator and a two dimensional field solver.
Routing Guidance
Keep the pair length matched. Avoid sudden spacing changes. Maintain the same reference plane. Use gentle bends where possible. Place return vias near layer changes. Reduce stubs and unnecessary test pads. Match the stackup used by the board shop.
Final Thoughts
A differential pair calculator saves time before detailed simulation. It shows which variable matters most. It also reveals whether a target is realistic for a chosen stackup. Start with known fabrication limits. Then tune the numbers until the geometry is manufacturable. Good early estimates reduce layout rework and support cleaner high speed design decisions. When tolerances are tight, request impedance coupons and measure sample boards before approving any large production batch run.
FAQs
What is differential pair impedance?
It is the impedance seen between two coupled traces carrying opposite signals. Many high speed links use 90 ohm, 100 ohm, or similar controlled targets.
Does spacing affect impedance?
Yes. Smaller spacing increases coupling and usually lowers differential impedance. Wider spacing reduces coupling and moves the pair toward two separate traces.
What does dielectric height mean?
For microstrip, it is the distance from trace to reference plane. For stripline, use the related dielectric spacing used by your stackup model.
Is this a field solver?
No. This calculator uses closed form equations. It is useful for estimates, but final controlled impedance work should be checked with a field solver.
Why include copper thickness?
Copper thickness changes effective trace width. That can shift impedance, especially when traces are narrow or copper weight is high.
What is odd mode impedance?
Odd mode impedance is the impedance of one line when both traces carry equal and opposite signals. Differential impedance is about twice this value.
Can solder mask change the result?
Yes. Solder mask can slightly increase effective dielectric loading for outer layer traces. The effect depends on mask thickness, material, and coverage.
Why export CSV and PDF?
CSV is useful for spreadsheet review. PDF is better for quick sharing, layout notes, fabrication questions, and design history records.