This calculator determines the differential characteristic impedance of a pair of microstrip lines based on their geometry and dielectric material. It helps design high-frequency circuit boards with precise impedance control for differential signals.
Ideal for PCB design engineers, fast signal specialists and electronics students working on differential interconnects like USB, HDMI or Ethernet.
Formulas
Z₀ = (87 / √(εr + 1.41)) × ln((5.98 × h) / (0.8 × w + t))
Z d = (174 / √(εr + 1.41)) × ln((5.98 × h) / (0.8 × w + t)) × (1 − 0.48 × e −0.96 × (d/h) )
Explanation of formulas
- Z₀ represents the single impedance of an individual microstrip track.
- Z d is the differential impedance calculated from the coupling between two tracks.
- h is the thickness of the dielectric, w the width of the track, t its thickness, and d the distance between the tracks.
- εr is the dielectric constant of the substrate, which influences the speed and attenuation of the signal.
100 ohm differential microstrip impedance example
Input : w = 12 mils, d = 6 mils, t = 1.4 mils, h = 7 mils, εr = 4.2
Output : Z d ≈ 100 Ω
Benefits and uses
- Ensures precise adaptation for high-speed differential signals.
- Helps reduce crosstalk and signal losses on PCB.
- Optimizes designs for Ethernet, USB, LVDS and other fast interfaces.
- Facilitates the design of tracks with controlled impedance tolerance.