Differential Microstrip Calculator

Differential impedance is the impedance between two coupled traces when driven with equal but opposite signals. It's typically twice the odd-mode impedance and is crucial for high-speed differential signaling like USB, HDMI, and Ethernet.

Odd-mode impedance occurs when the traces carry equal but opposite signals (differential signaling). Even-mode impedance occurs when both traces carry identical signals relative to ground (common-mode signaling).

Closer trace spacing increases coupling between traces, which reduces the odd-mode impedance and therefore the differential impedance. Wider spacing reduces coupling and increases impedance values.

Standard FR4 has a dielectric constant (εr) of approximately 4.3-4.5 at 1 GHz. However, this value can vary with frequency, temperature, and specific FR4 formulation, so check your PCB manufacturer's specifications.

Analytical calculators provide good estimates (±10-15%) for typical geometries but may be less accurate for extreme aspect ratios or complex stackups. For critical applications, use field solver simulation tools for verification.

For standard FR4 (εr=4.3) with 6.7 mil dielectric thickness, traces around 5 mil wide with 5 mil spacing typically achieve close to 100Ω differential impedance. Always verify with calculations for your specific stackup.

Controlled impedance ensures proper signal integrity by minimizing reflections, reducing crosstalk, and maintaining signal timing. Impedance mismatches can cause signal degradation, EMI issues, and data transmission errors.

Thicker copper reduces the effective trace width-to-height ratio, slightly decreasing impedance. While the effect is often small, it becomes more significant for thin dielectrics or wide traces and should be included in precise calculations.