Differential Pair Impedance Calculator

Calculate odd-mode and differential impedance of edge-coupled microstrip or stripline differential pairs (USB, HDMI, PCIe, DDR).

Calculator Electronics Updated Apr 23, 2026
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How to Use
  1. Pick topology: microstrip (outer layer) or stripline (inner layer).
  2. Enter trace width W, gap S (spacing between the two traces), dielectric height H, thickness T, εr.
  3. Result: single-ended Z₀, odd-mode Z_odd, differential Z_diff = 2 × Z_odd.
  4. Common targets: USB 2.0 = 90Ω, HDMI/PCIe = 100Ω, DDR4 = 80-100Ω.
Input
mm
mm
mm
mm
Presets
Diff Pair Cross Section
Z_diff
—Ω
Z_odd
—Ω
Z₀ (single)
—Ω
Coupling k
—

Show Work

Enter dimensions.

Formulas

Differential Z
Z_diff = 2 · Z_odd
Two conductors carrying opposite signals.
Odd Mode (microstrip)
Z_odd ≈ Z₀ · (1 − 0.48·e−0.96 S/H)
IPC-2141 approximation.
Odd Mode (stripline)
Z_odd ≈ Z₀ · (1 − 0.347·e−2.9 S/B)
Tighter coupling for same gap.
Coupling coefficient
k = (Z_even − Z_odd) / (Z_even + Z_odd)
0 = no coupling, 1 = full.
3W Rule
S ≥ 3·W (to neighbors)
Between pairs, not within.
Skew Budget
< 5 ps intra-pair
Modern high-speed specs.

History of Differential Signaling

Differential signaling dates to Alexander Graham Bell's 1881 twisted-pair telephone patent — using a balanced signal pair instead of single-ended-over-ground to cancel out electromagnetic interference. The same principle governs every modern high-speed digital interface: LVDS (1994), USB 2.0 (2000), HDMI (2002), PCIe (2003), DDR2+ (2003+), and USB-C/USB4 (2016+).

The move from single-ended to differential signaling in PCs accelerated in the 2000s as clock speeds exceeded a few hundred MHz and single-ended ground bounce became intolerable. A differential pair with tight coupling presents equal-and-opposite current to the return plane, cancelling the EMI that a single-ended signal would radiate. The receiver needs only to detect the voltage difference, making it immune to common-mode noise pickup.

The differential-impedance target depends on the interface spec: USB 2.0 is 90Ω nominal, HDMI and PCIe are 100Ω, LVDS is 100Ω, DDR4 is 80-100Ω. Matching this on the PCB requires controlling the trace width, spacing, and stackup — all three parameters interact, which is why modern PCB design relies on impedance-controlled stackups from the fab house.

About This Calculator

Pick microstrip or stripline topology and enter trace width W, intra-pair gap S, dielectric height H (or B for stripline plane-plane), copper thickness T, and εr. The tool computes single-ended Z₀, odd-mode impedance (one trace with the other driven opposite), and differential impedance Z_diff = 2 · Z_odd.

For tight-tolerance impedance (±5-10%), specify the stackup with your PCB fab and request impedance-controlled routing. Controlled-impedance fabs will run TDR verification coupons on each panel. For casual/hobbyist use, these calculated values are adequate. Everything runs client-side.

Frequently Asked Questions

What is differential impedance?

The impedance seen by the differential signal (voltage difference between the two traces). Z_diff = 2 × Z_odd, where Z_odd is one trace's impedance when the other carries an equal-and-opposite signal.

Why tightly couple?

Close trace spacing increases mutual coupling, which REDUCES common-mode susceptibility and EMI radiation — the two signals' fields cancel outside the pair. Loose coupling behaves like two independent 50Ω lines.

Gap tolerance?

±10% spacing variation shifts Z_diff by ~5%. For tight ±5% impedance, specify ±0.025mm on the fab drawing and include impedance-controlled stackup callout.

Common Use Cases

USB 2.0 (90Ω diff)

Typical FR4: W=0.5mm, S=0.125mm, H=0.25mm for 90Ω differential microstrip.

HDMI / PCIe (100Ω)

Inner-layer stripline: W=0.18mm, S=0.15mm on 0.3mm between planes.

Ethernet 1000BASE-T

100Ω diff, CAT6-equivalent PCB routing with tight length matching.

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