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Differential Microstrip Impedance Calculator

Formula

To calculate the Differential Microstrip Impedance ( $Z_d$ ):

$Z_d = \frac{174}{\sqrt{\epsilon_r + 1.41}} \ln \left( \frac{5.98h}{0.8w + t} \right) \left(1 - 0.48 \exp \left( -0.96 \frac{d}{h} \right) \right)$

Where:

$Z_d$ is the Differential Microstrip Impedance (Ω)
$\epsilon_r$ is the Relative Permittivity
w is the Width
h is the Height
t is the Thickness
d is the Separation

What is Differential Microstrip Impedance?

Differential microstrip impedance is the impedance of a differential pair of traces on a PCB. It is an important parameter in high-speed digital and RF circuit design.

Example Calculation 1

Let's assume the following values:

Relative Permittivity ( $\epsilon_r$ ) = 4.5
Width (w) = 2 mm
Height (h) = 1.5 mm
Thickness (t) = 0.035 mm
Separation (d) = 0.5 mm

Using the formula:

$Z_d = \frac{174}{\sqrt{4.5 + 1.41}} \ln \left( \frac{5.98 \times 1.5}{0.8 \times 2 + 0.035} \right) \left(1 - 0.48 \exp \left( -0.96 \frac{0.5}{1.5} \right) \right)$

The differential microstrip impedance can be calculated accordingly.

Example Calculation 2

Let's assume the following values:

Relative Permittivity ( $\epsilon_r$ ) = 3.5
Width (w) = 3 mm
Height (h) = 1 mm
Thickness (t) = 0.05 mm
Separation (d) = 0.2 mm

Using the formula:

$Z_d = \frac{174}{\sqrt{3.5 + 1.41}} \ln \left( \frac{5.98 \times 1}{0.8 \times 3 + 0.05} \right) \left(1 - 0.48 \exp \left( -0.96 \frac{0.2}{1} \right) \right)$

The differential microstrip impedance can be calculated accordingly.