Microstrip PCB Differential Impedance Calculator

Differential Impedance Estimator for Microstrip Traces

Input Fields

Single-ended impedance

Zo

Ω

Impedance of a single microstrip trace

Trace spacing

s

mm

Spacing between the two differential traces

Substrate height

h

mm

Height from trace to reference ground plane

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Differential Impedance

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Microstrip Differential Impedance Formula

$$Z_{\text{diff}} = 2Z_0 \cdot \left(1 – 0.48 \cdot e^{-0.96 \cdot \frac{s}{h}}\right)$$

Where:

• $$Z_{ ext{diff}}$$ = differential impedance (Ω)
• $$Z_0$$ = single-ended impedance of one microstrip (Ω)
• $$s$$ = spacing between traces (mm)
• $$h$$ = height of dielectric substrate (mm)

This formula estimates the impedance of a differential pair assuming symmetrical, edge-coupled microstrips over a ground plane.

Microstrip PCB Differential Impedance – Calculation Example

Given:

• $$Z_0$$ = 50 Ω
• $$s$$ = 0.25 mm
• $$h$$ = 0.8 mm

Calculation:

1. $$Z_{\text{diff}} = 2 \cdot 50 \cdot \left(1 – 0.48 \cdot e^{-0.96 \cdot \frac{0.25}{0.8}}\right)$$
2. $$= 100 \cdot \left(1 – 0.48 \cdot e^{-0.3} \right)$$
3. $$= 100 \cdot \left(1 – 0.48 \cdot 0.741 \right)$$
4. $$= 100 \cdot \left(1 – 0.3557\right) = 100 \cdot 0.6443$$
5. $$Z_{\text{diff}} ≈ 64.43~\Omega$$

Differential impedance is critical for high-speed digital communication where noise immunity and signal integrity are key. This calculator helps designers quickly estimate the impedance of differential pairs on PCBs with microstrip geometry. It is commonly used for USB, PCIe, Ethernet, HDMI, and other fast serial interfaces. Precise impedance control ensures minimal signal degradation, reduced reflections, and better EMI performance.