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Differential Pair Calculator (Edge-Coupled Microstrip, 100 Ohm)

Edge-coupled microstrip differential impedance Zdiff for USB, Ethernet, PCIe, MIPI, and HDMI. Hammerstad-Jensen single line with Kirschning-Jansen even and odd modes.

Differential Pair Calculator

Edge-coupled microstrip differential pair. The standard for USB, Ethernet, PCIe, MIPI, and HDMI. Computes differential and common-mode impedance from W, S, and stackup.

Units
Length
Frequency

Inputs

Mode?
Ω
mm
mm
GHz
W = 0.319 mmWS = 0.2 mmh = 0.2 mmεr = 4.04ground plane
Edge-coupled differential microstrip
Differential Z
100.00Ω
Required width W
0.319mm
12.54 mil

More

Z0 single57.80 Ω
Even mode Z0e64.98 Ω
Odd mode Z0o50.00 Ω
Common mode Z32.49 Ω
Guided lambda_g36.03 mm

Analytical calculation

Solve trace width
Invert Z_diff(W) by bisection to hit the target differential impedance.
Single-ended impedance
Hammerstad-Jensen on one trace alone, ignoring the partner.
Even / odd permittivity
Kirschning-Jansen: the even mode sees a wider effective strip, the odd mode more of its field in air between the strips.
Even / odd modes
Q4 and Q10 are the fitted coupling terms in u and g. Far apart, both modes reach the single-ended value.
Differential impedance
Twice the odd-mode impedance.

Frequently asked

What W and S give 100 ohm diff on 0.2 mm prepreg?

For h = 0.2 mm, S = 0.18 mm and er ~ 4.0, Solve W gives about W = 0.31 mm to land Zdiff = 100 ohm. Tighter S means tighter coupling and a narrower W; loosen S to ~3 h and you can use roughly the same W as a 50 ohm single-ended trace. Use Solve W mode with your actual stackup for an exact answer.

Should I match length on diff pairs?

Yes. Aim well under 10 ps of intra-pair skew as a routing budget for USB 3 / PCIe Gen 4 / 10G Ethernet; the relevant specs (USB 3.2, PCIe base 4.0/5.0) state intra-pair skew limits in the 8 to 15 ps range at the receiver, so you want a comfortable margin. Length tuning is not impedance; both matter independently.

References

  • PrimaryKirschning, M. & Jansen, R. H. 1984, Accurate wide-range design equations for the frequency-dependent characteristic of parallel coupled microstrip lines, IEEE Trans. MTT-32(1), 83-90 (static even/odd impedances and effective permittivities), on Hammerstad, E. & Jensen, O. 1980 for the single line.
  • Cross-checkWadell, B. C. Transmission Line Design Handbook, Artech House 1991 (edge-coupled microstrip, even/odd-mode closed forms).
  • NoteStated for W/h and S/h from 0.1 to 10 and er up to 18, to about 1% in impedance. Outside that range the calculator warns, and a field solver is the check.

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