Example Data Table
| Use Case |
Width mm |
Gap mm |
Height mm |
Er |
Estimated Result |
| USB style review |
0.18 |
0.18 |
0.20 |
4.20 |
Near 100 Ω |
| Thin dielectric pair |
0.12 |
0.14 |
0.12 |
3.80 |
Lower geometry margin |
| Loose coupling check |
0.20 |
0.35 |
0.25 |
4.00 |
Higher differential value |
Formula Used
The calculator first estimates effective dielectric constant with the common microstrip approximation.
It then calculates single ended impedance from the width to height ratio. A small copper thickness
correction increases the effective width. Differential impedance is then estimated from coupled
microstrip behavior.
Effective width: Weff = W + ΔW. Effective permittivity:
Eeff = (Er + 1) / 2 + (Er - 1) / 2 × 1 / √(1 + 12H / Weff).
For Weff / H below 1, Z0 = 60 / √Eeff × ln(8H / Weff + 0.25Weff / H).
For wider traces, Z0 = 120π / [√Eeff × (u + 1.393 + 0.667ln(u + 1.444))].
Differential impedance uses this practical estimate:
Zdiff = 2 × Z0 × [1 - 0.48 × exp(-0.96 × S / H)].
It is a fast approximation, not a replacement for a field solver.
How to Use This Calculator
Enter the finished trace width, air gap between traces, dielectric height to the reference plane,
copper thickness, dielectric constant, route length, and target impedance. Press calculate.
The result appears above the form and below the header. Check the status line first. Then review
odd mode impedance, coupling, delay, and electrical length. Use CSV for spreadsheet records.
Use PDF for design notes and reviews.
Differential Microstrip Design Notes
A differential microstrip pair carries equal and opposite signals on two surface traces.
The return field uses the reference plane below the board. The pair is popular in USB,
Ethernet, LVDS, camera links, and many fast serial buses. Small geometry changes can move
impedance by several ohms, so a practical calculator should expose every common board variable.
Why impedance matters
A transmitter, line, and receiver behave best when impedances match. A poor match reflects
energy back toward the driver. That reflection can close an eye diagram, increase jitter,
and create unwanted radiation. Differential routing also relies on balance. The two traces
should share the same width, length, spacing, layer, and reference plane.
Key inputs
Trace width controls the single ended impedance. Wider copper lowers impedance. Dielectric
height also has strong influence. A thicker dielectric raises impedance because the field
spreads farther. Relative permittivity lowers impedance and slows the signal. Spacing controls
coupling. A tight gap reduces differential impedance because the odd mode fields attract each
other. Copper thickness has a smaller effect, but it still matters on heavy copper boards.
Practical design method
Start with the stackup from your fabricator. Enter the finished dielectric height, not only
the prepreg name. Use the finished copper thickness after plating when possible. Add solder
mask only as a conservative review, because mask thickness and dielectric value vary by
process. Compare the calculated value with the target, then adjust width and spacing in
small steps.
Layout guidance
Keep the pair together through the whole route. Avoid unnecessary stubs, sharp neck downs,
and split reference planes. When vias are needed, place returns nearby and keep both sides
symmetric. Length tuning should use gentle shapes and should not create large local spacing
changes. Connector launches, pads, and packages may need separate field solving.
Using this page
This calculator gives a quick engineering estimate. It is useful for early layout checks,
quote reviews, and documentation. Final controlled impedance should still be confirmed with
the board manufacturer or a field solver. Use the CSV and PDF exports to save chosen
assumptions beside each design review.
Record units carefully. Millimeters, microns, and inches can easily create wrong impedance
values when mixed during rushed reviews later.
FAQs
1. What is differential microstrip impedance?
It is the impedance seen between two coupled surface traces routed above a reference plane. It depends on width, spacing, dielectric height, copper thickness, and dielectric constant.
2. Why is spacing important?
Spacing controls coupling between the two traces. A smaller gap usually lowers differential impedance. A wider gap makes the pair behave closer to two separate single ended traces.
3. Is this calculator exact?
No. It uses practical closed form approximations. It is suitable for planning and review. Final production values should be checked with your board fabricator or a field solver.
4. Which dielectric height should I enter?
Use the finished dielectric height from the signal layer to its nearest reference plane. Do not use total board thickness unless that is the actual return plane distance.
5. Does copper thickness matter?
Yes, but usually less than width, spacing, height, and dielectric constant. Thick copper increases effective trace width and can reduce impedance in controlled impedance designs.
6. What target is common for differential pairs?
Many digital pairs use 90 Ω, 100 Ω, or 120 Ω targets. The correct value depends on the interface standard, connector, package, and board stackup.
7. Why does dielectric constant affect delay?
Signals slow down in dielectric material. A higher effective dielectric constant lowers propagation velocity. This increases delay per millimeter and changes wavelength at high frequencies.
8. Can I export the result?
Yes. Use the CSV button for spreadsheet data. Use the PDF button after calculation to save a simple report for documentation or design review.