Enter the characteristic impedance of one line in the pair and the odd mode impedance into the calculator to determine the differential pair impedance.

Differential Pair Impedance Formula

The following formula is used to calculate the differential pair impedance. 

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

Variables:

  • Zd is the differential pair impedance (Ohms)
  • Z0 is the characteristic impedance of one line in the pair (Ohms)
  • Zodd is the odd mode impedance (Ohms)

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

What is a Differential Pair Impedance?

Differential Pair Impedance is a measure of the opposition to current flow, seen by a signal, in a pair of differential signal traces. It is a key parameter in the design of high-speed digital and analog circuits, as it affects signal integrity, noise immunity, and power dissipation. The impedance is determined by the physical dimensions of the traces, the distance between them, and the properties of the dielectric material separating them. Matching the differential pair impedance to the source and load impedances minimizes signal reflections and maximizes power transfer.

How to Calculate Differential Pair Impedance?

The following steps outline how to calculate the Differential Pair Impedance.

  1. First, determine the value of Z0 (characteristic impedance of one line in the pair) in Ohms.
  2. Next, determine the value of Zodd (odd mode impedance) in Ohms.
  3. Next, use the formula Zd = 2 * Z0 / sqrt(1 + (2 * Z0 / Zodd)) to calculate the differential pair impedance (Zd) in Ohms.
  4. Finally, calculate the Differential Pair 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 one line in the pair) = 50 Ohms

Zodd (odd mode impedance) = 75 Ohms