This calculator determines the differential impedance of a pair of microstrip traces based on their dimensions and substrate dielectric constant. It’s used to ensure proper signal integrity in high-speed PCB designs and differential transmission lines.
Formulas
Z₀ = (87 / √(εr + 1.41)) * ln((5.98 * h) / (0.8 * w + t))
Zd = (174 / √(εr + 1.41)) * ln((5.98 * h) / (0.8 * w + t)) * (1 – 0.48 * e−0.96(d/h))
Formula Explanation
- w = trace width
- d = spacing between traces
- t = trace thickness
- h = dielectric height
- εr = dielectric constant of substrate
- Z₀ = single-ended impedance, Zd = differential impedance
Example
Input: w = 8 mil, d = 10 mil, t = 1.5 mil, h = 5 mil, εr = 4.2
Result: Zd ≈ 98.6 Ω
Who uses it
Useful for PCB designers working on differential signal lines like USB, HDMI, or Ethernet to maintain proper impedance control.