The Differential Microstrip Impedance conversion calculator (using Zo) computes the differential impedance (Zd) of an edge-coupled microstrip pair based on the known single-ended impedance, the spacing between traces, and the dielectric height. It provides a simple way to estimate the impedance of differential pairs when the single-ended impedance is already determined.
Formula
Zd = 2 * Z0 * ( 1 – 0.48 * exp( -0.96 * (d/h) ) )
Formula Explanation
- Z0: single-ended impedance in Ohms.
- d: spacing between the two differential traces in mils.
- h: dielectric thickness (distance between the traces and the ground plane) in mils.
- Zd: calculated differential impedance in Ohms.
- The exponential term accounts for electromagnetic coupling between the traces. Smaller spacing (d) increases coupling and lowers Zd.
Uses of this calculator
- Designing and tuning differential pairs on PCBs for consistent impedance.
- Converting from known single-ended impedance to differential impedance quickly.
- Maintaining signal integrity in high-speed serial communication interfaces.
- Used for impedance control in signals such as USB, PCIe, HDMI, and Ethernet.
What is the differential impedance for Zo = 50 Ω, d = 6 mil, and h = 3 mil?
Input: Z0 = 50 Ω, d = 6 mil, h = 3 mil
Output:
- Zd = 2 * 50 * ( 1 – 0.48 * exp( -0.96 * (6/3) ) ) = 100 * ( 1 – 0.48 * exp(-1.92) ) = 100 * ( 1 – 0.48 * 0.147 ) = 100 * ( 1 – 0.071 ) = 100 * 0.929 = 92.9 Ω
- Differential Impedance (Zd) = 92.9 Ω