The Differential Pair Impedance Calculator helps engineers and PCB designers determine the impedance of differential transmission lines used in high-speed signal applications. It calculates the impedance between two signal traces, ensuring signal integrity and minimizing reflections in circuits such as USB, HDMI, and Ethernet. This tool is crucial for maintaining proper signal quality in high-frequency electronics.
Formula of Differential Pair Impedance Calculator
Basic Formula for Differential Impedance:
Differential Impedance (Zdiff) = 2 × Odd Mode Impedance
For a more detailed calculation:
Zdiff = 2 × Zodd
where:
- Zdiff is the differential impedance of the transmission line pair.
- Zodd is the odd-mode impedance of each trace in the differential pair.
For PCB Design, a More Detailed Formula:
Zdiff = 2 × Z0 × √(1 - (C12 / C11))
where:
- Z0 is the characteristic impedance of a single-ended trace.
- C12 is the mutual capacitance between the two traces.
- C11 is the self-capacitance of a single trace to the ground plane.
Differential Impedance Approximation for Microstrip Lines:
Zdiff ≈ 2 × Z0 × (1 - 0.48 × e^(-0.96 × S / H))
where:
- S is the spacing between the two differential traces.
- H is the height of the traces above the reference ground plane.
Differential Impedance Reference Table
This table provides general estimates for differential impedance values based on typical PCB configurations.
| Trace Width (mm) | Spacing (mm) | Height (mm) | Differential Impedance (Ohms) |
|---|---|---|---|
| 0.2 | 0.15 | 0.5 | 90 |
| 0.3 | 0.2 | 0.6 | 85 |
| 0.4 | 0.25 | 0.8 | 100 |
| 0.5 | 0.3 | 1.0 | 110 |
These values help PCB designers set appropriate trace dimensions to meet impedance requirements.
Example of Differential Pair Impedance Calculator
A PCB designer needs to calculate the differential impedance of a transmission line with trace width of 0.3 mm, spacing of 0.2 mm, and height above the reference plane of 0.6 mm.
Using the microstrip approximation formula:
Zdiff ≈ 2 × 85 × (1 - 0.48 × e^(-0.96 × 0.2 / 0.6))
By solving, the estimated differential impedance is 85 Ohms.
This calculation ensures that the transmission lines are correctly designed for high-speed signals.
Most Common FAQs
Differential impedance ensures that signal pairs maintain proper integrity, reducing noise and reflections in high-speed circuits such as USB and Ethernet.
Wider spacing increases impedance, while closer spacing decreases it. Proper spacing is critical for achieving the desired impedance value.
Yes, but material properties and layer stackup variations must be considered for accurate impedance calculations.