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AESTECHNO
IPC-2141

Differential-pair impedance calculator

This calculator returns the differential impedance Zdiff of an edge-coupled pair, in microstrip or stripline. It first computes single-ended Z0 from real geometry (trace width, spacing, copper thickness, dielectric height, er), then applies the coupling correction. Target 90, 100 or 85 ohms.

Inputs
Topology

Microstrip: surface pair over one plane. Stripline: pair buried between two planes.

Length unit

w, s, t, h and b are entered in this unit (the math normalises to mm).

Relative permittivity of the substrate. FR4 is about 4.2 to 4.6.

Width of one of the two traces in the pair.

Bare copper gap between the two traces. Smaller s means stronger coupling and lower Zdiff.

0.035 mm equals 1 oz, 0.0175 mm equals 0.5 oz.

Dielectric thickness between the trace and the reference plane.

Total distance between the two reference planes, with the pair centred.

Ground plane Dielectric · εr Differential pair W S T H

Cross-section (not to scale)

Result

108.8 Ω

Breakdown

Z0 = 66.6 Ω · target 100 Ω (Ethernet / LVDS / PCIe)

IPC-2141 first-order estimate, about plus or minus 10 percent. Confirm against the fab stack-up or a 2D field solver; copper roughness, solder mask and glass weave shift the result.

Frequently asked questions

FAQ

How does the calculator get Zdiff from the geometry?
In two steps. Step 1: single-ended Z0 of one trace comes from IPC-2141, using trace width w, copper thickness t, dielectric height h (or plane-to-plane spacing b for stripline) and dielectric constant er. Step 2: it applies the edge-coupled coupling correction, Zdiff = 2 x Z0 x (1 - k x exp(-a x s / h)), where s is the edge-to-edge spacing. You never have to supply Z0 yourself.
Why does spacing s lower Zdiff so much?
Edge-to-edge spacing s is the coupling lever. Smaller s means the two traces couple more strongly and Zdiff drops. As s grows, the term exp(-a x s / h) approaches zero, coupling vanishes and Zdiff approaches 2 x Z0 (two independent traces). That is why tightening the pair is the first move when your Zdiff is too high, and widening it when Zdiff is too low.
How accurate is this calculation?
It is an IPC-2141 estimate, roughly +/-10 percent. The closed-form formulas do not model trapezoidal etch, copper roughness, glass-weave effects or the real frequency dependence of er. For any critical pair (USB 3, PCIe, MIPI, Ethernet), always confirm against your fabricator's stackup or a 2D field solver before freezing the geometry.
What are the 90, 100 and 85 ohm targets for?
90 ohm differential: USB 2.0 and USB 3.x SuperSpeed lanes. 100 ohm differential: Ethernet, LVDS, PCI Express and most generic pairs. 85 ohm differential: SATA and some PCIe pairs depending on fabricator rules. Adjust w and s until Zdiff lands on the target, then lock the geometry into your routing constraints.
Microstrip or stripline: which topology should I pick?
Edge-coupled microstrip puts the pair on the surface over a single plane: simpler, lower loss, but more exposed to noise and etch variation. Stripline buries the pair between two planes: better shielding and reduced crosstalk, ideal at high speed, at the cost of extra vias and higher dielectric loss. The calculator handles both via the topology selector.
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AESTECHNO is an electronics design house based in Montpellier, France. Over 10 years of high-speed PCB design experience, with a 100 percent first-pass record on CE and FCC certification.