Enter the characteristic impedance of each line and the common mode impedance into the calculator to determine the differential impedance. This calculator can also evaluate any of the variables given the others are known.

Differential Impedance Formula

The following formula is used to calculate the differential impedance. 

Zd = 2 * Z0 / sqrt(1 + (2 * Z0 / Zc))

Variables:

  • Zd is the differential impedance (Ohms)
  • Z0 is the characteristic impedance of each line (Ohms)
  • Zc is the common mode impedance (Ohms)

To calculate the differential impedance, multiply the characteristic impedance of each line by 2. Then, divide this result by the square root of 1 plus the result of twice the characteristic impedance divided by the common mode impedance.

What is a Differential Impedance?

Differential Impedance is a key parameter in the design of high-speed digital and analog circuits. It refers to the electrical resistance encountered by a signal when it travels along a pair of conductors in a differential transmission line, such as a twisted pair cable or a printed circuit board (PCB) trace pair. The differential impedance is the impedance that one line would have when driven in relation to the other line, which is ideally grounded. It is crucial for maintaining signal integrity, reducing electromagnetic interference, and achieving optimal performance in high-speed data transmission systems.

How to Calculate Differential Impedance?

The following steps outline how to calculate the Differential Impedance.

  1. First, determine the value of Z0 (characteristic impedance of each line) in Ohms.
  2. Next, determine the value of Zc (common mode impedance) in Ohms.
  3. Next, use the formula Zd = 2 * Z0 / sqrt(1 + (2 * Z0 / Zc)) to calculate the differential impedance (Zd) in Ohms.
  4. Finally, calculate the Differential Impedance.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Z0 (characteristic impedance of each line) = 50 Ohms

Zc (common mode impedance) = 100 Ohms