E.05Impedance Calculator

Choose the line type; calculate impedance from trace width, or find the width needed for a target impedance.

Signal

In differential mode, the target impedance is Zdiff (e.g. USB 90 Ω, Ethernet 100 Ω).

Bottom ground plane
Calculate

For CPW without bottom ground, the substrate thickness.

From the top of the trace to the dielectric surface.

Total plane spacing B = 2H + T.

1 oz = 35 µm, 2 oz = 70 µm. Thickness is neglected for coplanar waveguide.

FR-4 ≈ 4.2–4.6 · Rogers RO4350B ≈ 3.66 · RO4003C ≈ 3.55

Results

Impedance Z₀
—
Differential Zdiff
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Odd mode Zodd
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Trace width
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εeff
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Delay
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Capacitance
—
Inductance
—

    Formulas

    Microstrip
    Hammerstad & Jensen (1980), with thickness correction
    Embedded microstrip
    εeff' = εeff · e^(−2C/H) + εr · (1 − e^(−2C/H))
    Stripline
    Z₀ = 30/√εr · ln{1 + 4/(πW') · [8/(πW') + √((8/(πW'))² + 6.27)]}
    Asymmetric stripline
    Z₀ = 2·Z₀(2H1+T)·Z₀(2H2+T) / [Z₀(2H1+T) + Z₀(2H2+T)]
    Dual stripline
    Asymmetric stripline, far plane = D + T + H2
    Coplanar waveguide
    Z₀ = 30π/√εeff · K(k₀')/K(k₀), k₀ = W/(W + 2G)
    Differential (microstrip)
    Zdiff = 2·Z₀·(1 − 0.48·e^(−0.96·S/H))
    Differential (stripline)
    Zdiff = 2·Z₀·(1 − 0.347·e^(−2.9·S/B)) Zodd = Zdiff / 2
    Line constants
    t_pd = √εeff / c C = t_pd / Z₀ L = t_pd · Z₀

    Line types and assumptions

    Microstrip is an uncoated outer-layer trace over a single reference plane. Embedded microstrip is the same trace covered by a dielectric (prepreg or coating). Stripline is an inner-layer trace centred between two planes; in asymmetric stripline the trace is closer to one plane. Dual stripline has two signal layers between two planes; traces on the other layer are assumed to run orthogonally and not overlap the calculated trace.

    In coplanar waveguide the trace is flanked by ground on the same layer; with a ground plane underneath it is called CPWG. Solder mask, dispersion and surface roughness are not included. Typical accuracy for single-ended lines is 1–3%; verify against your PCB manufacturer’s stack-up and controlled-impedance data before production.

    In a differential pair, two traces run side by side with spacing S (edge-coupled). Differential impedance Zdiff is twice the single-ended impedance reduced by coupling; it approaches 2·Z₀ as spacing grows. Odd-mode impedance is Zodd = Zdiff / 2.

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