Formula Used
The calculator uses a quasi-static coplanar waveguide approximation with practical pair coupling correction.
Coplanar impedance: Z0 = 30π / √εeff × K(k') / K(k)
Geometry ratio: k = Weff / (Weff + 2G)
Differential impedance: Zdiff = 2 × Z0 × (1 - C)
Coupling estimate: C = 0.48 × e ^ (-0.96 × S / (Weff + G))
Propagation delay: delay = √εeff × 3.33564095 ps/mm
Here, W is trace width, S is pair spacing, G is coplanar ground gap, and K is the complete elliptic integral.
How to Use This Calculator
Enter the board unit first. Use mm for metric stackups, or mil for imperial drawings.
Add trace width, spacing between pair traces, coplanar ground gap, dielectric height, and copper thickness.
Enter dielectric constant, solder mask data, loss tangent, conductivity, route length, and frequency.
Press Calculate. The result appears above the form and below the header section.
Use CSV for spreadsheets. Use PDF for a simple report file.
Example Data Table
| Use Case |
Width mm |
Spacing mm |
Ground Gap mm |
Height mm |
Er |
Target Ω |
| USB style pair |
0.18 |
0.20 |
0.18 |
0.18 |
4.20 |
90 |
| Generic serial pair |
0.15 |
0.18 |
0.22 |
0.20 |
3.80 |
100 |
| Dense routing study |
0.10 |
0.12 |
0.15 |
0.12 |
4.10 |
85 |
Design Notes for Coplanar Differential Routing
A coplanar differential pair places two signal traces on one layer, with ground copper beside them and a return plane below. This shape is useful when fast serial links need controlled impedance, tight routing, and compact layer use. The calculator estimates the odd mode result from width, spacing, dielectric height, copper thickness, and material data.
Why Geometry Matters
Trace width controls the single ended impedance first. A wider trace lowers impedance. A larger coplanar ground gap raises impedance because the side fields spread farther before returning. Pair spacing controls coupling between the two traces. Tight spacing lowers differential impedance because each trace sees the opposite conductor strongly. Dielectric height also matters. A thin dielectric pulls more field into the board and lowers impedance.
Material Effects
Dielectric constant changes wave speed and impedance. Higher values reduce both impedance and delay velocity. Loss tangent predicts dielectric heating at high frequency. Copper thickness and conductivity affect conductor loss. The model uses practical corrections, so it is best for early stackup planning. Final values should be confirmed with a field solver or board vendor rules.
Using Results Wisely
The result block shows single ended impedance, differential impedance, odd mode impedance, coupling factor, delay, electrical length, and estimated losses. Compare the differential value with your target, such as 90 ohms or 100 ohms. Then adjust trace width, pair spacing, or ground gap. Change one variable at a time. This makes the trend easier to understand.
Manufacturing Margin
Fabrication tolerance can shift impedance. The tolerance range estimates how width changes may move the result. Use it to judge whether a stackup is too sensitive. A robust design keeps impedance near target after common etching variation. Leave enough copper clearance for the fabricator. Avoid abrupt neck downs, uneven ground gaps, and missing return paths. Add stitching vias near ground pours when layer transitions or reference changes occur.
Practical Limits
Very narrow gaps may be hard to etch consistently. Very large gaps reduce coplanar benefit. Keep solder mask data realistic, because mask only touches part of the field. Enter values from the real stackup, not a catalog guess. Review impedance after selecting copper weight, laminate, finish, and solder mask for better releases.
FAQs
What is coplanar differential impedance?
It is the impedance seen by a differential signal pair routed beside coplanar ground copper. It depends on trace width, pair spacing, ground gap, dielectric height, and material properties.
What does pair spacing mean?
Pair spacing is the edge to edge gap between the two signal traces. Smaller spacing increases coupling and usually lowers differential impedance.
What is the coplanar ground gap?
It is the gap from a signal trace edge to the nearby ground pour on the same layer. Larger gaps usually raise impedance.
Why is dielectric height important?
Dielectric height controls the distance to the reference plane. A smaller height concentrates fields in the dielectric and often lowers impedance.
Does solder mask affect impedance?
Yes. Solder mask adds dielectric material above the traces. Its effect is usually smaller than core geometry, but it can matter on tight high speed designs.
Can this replace a field solver?
No. This calculator is best for planning and comparison. Use a field solver or board fabricator data for release-critical impedance values.
What target should I use?
Use the interface requirement. Common targets include 85, 90, and 100 ohms differential. Always check the relevant standard or device datasheet.
Why export CSV or PDF?
CSV helps spreadsheet review. PDF gives a quick design record. Both are useful when comparing stackup options or sharing routing notes.