Differential Pair Impedance

Zdiff ≈ 2 × Z0 × (1 − 0.48 × e^(−0.96s/h))

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Result
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Formula

Zdiff ≈ 2 × Z0 × (1 − 0.48 × exp(−0.96 × s/h))

Description

Differential impedance of a coupled pair is less than twice the single-ended impedance due to electromagnetic coupling between the traces. When the traces are far apart (s >> h), coupling is negligible and Zdiff approaches 2×Z0. As traces are brought closer, mutual capacitance and inductance increase, reducing Zdiff. Typical differential impedance targets are 90Ω (USB 2.0), 85Ω (PCIe), or 100Ω (USB 3.0, Ethernet). Maintaining consistent spacing throughout the route is essential for impedance control.

Variables

  • Zdiff — Differential impedance (Ω)
  • Z0 — Single-ended impedance of each trace (Ω)
  • s — Edge-to-edge spacing between traces (mm)
  • h — Height above ground plane (mm)

Practical Notes

This empirical formula is for microstrip differential pairs. Stripline differential pairs use a different coupling correction factor. Tight coupling (small s/h) improves common-mode noise rejection but makes Zdiff more sensitive to spacing variations. USB 2.0 requires 90Ω ±10%, USB 3.x requires 85Ω ±15%. Always length-match differential pairs to within the specified skew tolerance.

Related Formulas

Trace Current Capacity (IPC-2221) Trace Resistance Trace Voltage Drop Via Current Capacity (IPC-2221) Microstrip Impedance Stripline Impedance

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