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Differential Microstrip Impedance Calculator | NextPCB
 

Differential Microstrip Impedance Calculator

Estimate the differential impedance of an edge-coupled trace pair on an external layer — for USB, HDMI, PCIe, and other high-speed serial interfaces. For full stackup design, see our complete impedance calculator.

Differential Pair Parameters

Uses the IPC-2141 microstrip formula plus the standard coupling correction. See our Microstrip Calculator for single-ended traces, or the full stackup impedance tool for a manufacturable stackup match.

Estimated Differential Impedance

air (bare copper) S W T H ground plane edge-coupled differential pair

Differential Impedance

0.00 Ω

Zdiff, edge-coupled microstrip pair

Single-Ended Z0

0.00 Ω

Odd-Mode Zodd

0.00 Ω

= Zdiff/2; may show ±0.01Ω from display rounding

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Calculation Reference

This calculator first computes each trace's single-ended microstrip impedance, then applies the IPC-2141 coupling correction for an edge-coupled pair on the same external layer, referenced to a continuous ground plane below.

Z₀ = 87/√(Er+1.41) × ln( 5.98H / (0.8W+T) )
Zdiff = 2×Z₀ × ( 1 − 0.48·e−0.96·S/H )
Source: IPC-2141, "Controlled Impedance Circuit Boards and High Speed Logic Design."

Frequently Asked Questions

What is differential impedance, and why does it differ from single-ended impedance?

Differential impedance is the impedance seen by a signal traveling across a coupled pair of traces carrying complementary (inverted) signals, rather than one trace referenced to ground alone. Bringing the pair closer together increases coupling between them, which lowers the differential impedance below twice the single-ended value, so spacing is just as important as trace width for hitting a differential target like 90Ω for USB or 100Ω for HDMI and Ethernet.

How is differential microstrip impedance calculated?

This calculator first computes each trace's single-ended impedance with the IPC-2141 microstrip formula, then applies the IPC-2141 coupling correction, Zdiff = 2 × Z₀ × (1 − 0.48 × e−0.96×S/H), where S is the edge-to-edge spacing between the pair and H is the dielectric height. As spacing S grows large relative to H, the coupling term shrinks toward zero and Zdiff approaches twice the single-ended impedance, which is the uncoupled limit. Like the single-ended microstrip formula, this assumes bare copper exposed to air rather than covered by solder mask, so real solder-mask-covered pairs will run somewhat lower than this estimate.

What differential impedance target should I use?

The target comes from your interface specification, not from this calculator: USB 2.0 and USB 3.x typically target 90Ω differential, while HDMI, SATA, PCIe, and 1000BASE-T Ethernet commonly target 100Ω differential. Always confirm the exact figure in your specific chipset's datasheet or the relevant interface standard, since tolerances and exact targets can vary by revision.

Why does tighter spacing reduce differential impedance?

As the two traces move closer together, more of each trace's electric field couples directly to its neighbor instead of to the reference plane, which increases the effective capacitance between the pair and lowers the impedance the differential signal sees. This is why simply widening two traces without also adjusting their spacing won't reliably hit a differential target — width and spacing have to be solved together.

What routing practices matter beyond just hitting the impedance number?

Keep the spacing between the pair constant along the full route, since local necking or widening creates impedance discontinuities even if the average value looks correct, and match the electrical length of the two traces (commonly within a few mils for fast serial links) to minimize skew between the two signals. Avoid routing the pair near a plane split or gap, since that removes the return-path reference the differential impedance calculation assumes is present underneath.

Should I use this or the full NextPCB stackup impedance tool?

Use this calculator for a quick single-geometry estimate when you already know roughly what stackup you're working with. If you're still deciding on layer count, board thickness, or copper weights, or you want trace width and spacing solved directly against one of NextPCB's real, manufacturable stackups, use the full PCB Controlled Impedance Calculator instead, which also returns a recommended stackup rather than just a geometry.

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