Differential Pair Impedance Calculator

This tool calculates the characteristic differential impedance (Z_diff) of tightly coupled microstrip or stripline PCB trace pairs, based on physical geometry and material properties. It also computes the corresponding single-ended impedance (Z_odd) and coupling coefficient (k_c). The results help ensure signal integrity in high-speed digital interfaces.

It’s tailored for common high-speed serial protocols including USB 2.0/3.x (90 Ω), HDMI (100 Ω), LVDS (100 Ω), PCIe, and MIPI D-PHY. The interface includes preset target impedances and practical parameter ranges aligned with industry layout guidelines for these standards.

W = trace width (center-to-center width of one conductor); S = edge-to-edge spacing between traces; H = distance from trace to reference plane (for microstrip) or between planes (for stripline); ε_r = relative permittivity of the PCB substrate; T = copper thickness (affects effective width via sidewall etching and current crowding).

For FR-4 boards: W and S commonly range from 0.1–0.5 mm; H is typically 0.15–0.25 mm for top-layer microstrip (e.g., 1oz copper over prepreg), and 0.4–0.8 mm for inner-layer striplines. Tighter coupling (S ≈ W) improves common-mode rejection but increases crosstalk sensitivity.

Copper thickness influences the effective trace cross-section and current distribution—especially at high frequencies due to skin effect and manufacturing taper (trapezoidal trace profile). Thicker copper slightly reduces resistance and increases effective width, lowering impedance by ~2–5% across typical T = 12–35 µm (½–1 oz) ranges.

It uses well-established closed-form approximations (e.g., Hammerstad & Jensen for microstrip, Schneider for stripline) with <5% error for typical PCB geometries. While less precise than 3D EM solvers, it provides fast, reliable first-pass design guidance—ideal for layout iteration before final sign-off simulation.

First verify units (mm, µm, dimensionless ε_r) and ensure S ≥ 2×T to avoid fabrication shorting. Adjust W and S iteratively—increasing W lowers Z_diff, while increasing S raises it. For fixed routing area, consider switching from microstrip to stripline (more stable Z_diff vs. process variation).

Yes — it models edge-coupled (same layer) microstrip/stripline configurations, which dominate PCB design. Broadside coupling (traces on adjacent layers) is not supported, as it requires different field solutions and is rarely used due to poor manufacturability and uncontrolled coupling.

FR-4 ε_r varies ±0.3–0.5 across frequency, temperature, and batch—causing up to ±7% Z_diff shift. For critical designs, use controlled dielectrics (e.g., Isola FR408HR, ε_r = 3.66 ±0.05) and validate with TDR measurement. This tool assumes nominal ε_r; adjust input conservatively for worst-case analysis.

Yes: Z_diff ≈ 2×Z_odd and Z_common ≈ Z_even/2, where Z_odd and Z_even are odd- and even-mode impedances. High coupling (low S/W) increases Z_diff–Z_single separation and improves common-mode noise rejection—critical for EMI compliance.