Free Engineering Tool
Differential Pair Impedance Calculator
Calculate differential, odd-mode, even-mode, and common-mode impedance for edge-coupled microstrip and stripline pairs. Based on IPC-2141A / Wadell formulas with live target compliance for USB, PCIe, DDR, HDMI and Ethernet.
Differential Pair Calculator
Choose microstrip or stripline, enter trace width, gap and dielectric. Results include all mode impedances and interface compliance.
Design Inputs
Edge-to-edge spacing
Trace bottom to reference plane
Results
Differential Impedance (Z_diff)
Z_odd (Odd Mode)
46.9 Ω
Z_even (Even Mode)
62.7 Ω
Z_common
31.4 Ω
Z0 (Single-Ended)
54.8 Ω
Coupling Coefficient (k)
Interface Targets (±10%)
IPC-2141A / Wadell closed-form approximation for edge-coupled pairs. Accuracy is typically within a few percent of a 2D field solver for w/h ratios between 0.1 and 3.0. Trapezoidal etch profile, solder mask, and glass weave skew shift the real value — let AtlasPCB model your exact stackup and TDR-verify.
Have your differential targets? We design the stackup and TDR-verify every differential pair.
Get Instant QuoteThe Math
How differential impedance is calculated
The differential impedance of a coupled pair depends on the single-ended impedance Z0 and the coupling between the two traces. The coupling factor decays exponentially with the gap-to-height ratio.
Edge-Coupled Microstrip
Z0 = (87/√(εr+1.41)) × ln(5.98h/(0.8w+t))
Z_diff = 2×Z0 × (1 - 0.48×e^(-0.96×s/h))Outer layer, single reference plane below. Coupling depends on s/h.
Edge-Coupled Stripline
Z0 = (60/√εr) × ln(4b/(0.67π(0.8w+t)))
Z_diff = 2×Z0 × (1 - 0.347×e^(-2.9×s/b))Inner layer between two planes. Coupling depends on s/b.
Source: IPC-2141A "Design Guide for High-Speed Controlled Impedance Circuit Boards" and Wadell "Transmission Line Design Handbook". Accuracy within a few percent of 2D field solvers for typical edge-coupled geometries (w/h between 0.1 and 3.0, s/h > 0.2).
FAQ
Differential pairs, answered
What is differential impedance?
Z_diff is the impedance a differential signal sees on a coupled pair. High-speed interfaces (USB 90Ω, PCIe 85Ω, DDR 100Ω) specify a target Z_diff to minimize reflections.
Z_diff vs Z_odd vs Z_even?
Z_odd = impedance per trace in differential mode. Z_diff = 2×Z_odd. Z_even = impedance per trace in common mode. Z_common = Z_even/2. Coupling coefficient k = (Z_even-Z_odd)/(Z_even+Z_odd).
How does gap affect Z_diff?
Wider gap → less coupling → Z_diff approaches 2×Z0. Narrower gap → more coupling → Z_diff decreases. Sensitivity depends on s/h (microstrip) or s/b (stripline).
Microstrip or stripline for diff pairs?
Stripline gives better noise immunity and tighter coupling — preferred for PCIe, DDR, SerDes. Microstrip is simpler and cheaper. Most designs route critical pairs on internal stripline layers.
Differential pairs, TDR-verified
We design the stackup for your target impedance and TDR-verify every differential pair. ±8% tolerance, measured report with every order.