Differential Pair Impedance Calculator
Differential and common-mode impedance for edge-coupled microstrip and stripline pairs (USB, Ethernet, LVDS).
- Differential Z
- 110.68 Ω
- Single-ended Z₀
- 70.56 Ω
- S/H ratio
- 0.833
Edge-coupled microstrip pair: Z_diff ≈ 2·Z₀ × (1 − 0.48·e−0.96·S/H).
S is edge-to-edge trace spacing. As S decreases, mutual coupling increases and Z_diff drops. Approximation valid for S/H in 0.1–3.0. For high-precision work (PCIe Gen4+, DDR5), use a field solver.
USB 2.0: 90 Ω differential
USB 2.0 spec requires 90 Ω ±15% differential impedance for the D+/D− pair. Typical 4-layer FR4 stackup: 0.15 mm traces, 0.15 mm gap, 0.18 mm dielectric to ground gets you close. Length-match the pair within 0.15 mm to avoid mode conversion.
Ethernet: 100 Ω differential
10/100/1000BASE-T uses 100 Ω differential. The MDI pair terminates into the magnetics, then to the connector. Route all 4 pairs (or 8 for gigabit) parallel with the same trace width and spacing.
PCIe/SATA: 85 Ω differential
PCIe and SATA both use 85 Ω ±5% differential. Tighter tolerance than USB — this is where a proper field-solver validation stops being optional.
Frequently asked
- What is differential impedance?
- The impedance seen by the differential signal (voltage between the two lines) as it propagates. Not the same as 2× the single-ended impedance because of mutual coupling between the traces.
- How to route a differential pair?
- Parallel traces with constant spacing. Length-match the pair. Route both traces on the same layer for the full length. Avoid stubs and 90° corners; use 45° or arcs. See routing guides in the editor for automated pair routing.
- USB 2.0 differential impedance?
- 90 Ω ±15%. This calculator gives you the trace geometry that lands there for a given stackup.
- PCIe differential impedance?
- 85 Ω ±5%. Tighter tolerance than USB. Verify with a field solver for anything faster than PCIe Gen3.
Describe a USB or Ethernet board and PCBEditor generates and routes it; compare the pair's width and gap with these numbers before you order.
Route a USB/Ethernet board →Related free tools
- Microstrip Impedance Calculator (IPC-2141)Characteristic impedance for a trace on an outer layer above a ground plane, per IPC-2141.
- Stripline Impedance Calculator (IPC-2141)Characteristic impedance for a trace buried between two ground planes, per IPC-2141.
- PCB Layer Stackup Planner (2/4/6/8-Layer)Plan a PCB layer stack with copper weights, dielectric heights, FR4 Dk values, and finished thickness — 2 to 8 layers.