How is differential impedance calculated from odd-mode impedance?

Conditions

• The differential pair is driven in odd mode (opposite polarity voltages).
• $Z_{odd}$ denotes the characteristic impedance of a single trace under odd-mode excitation.

Reasoning, step by step

1. Identify the odd-mode impedance $Z_{odd}$ of a single trace.
2. Multiply the odd-mode impedance by two.
3. Express the result mathematically as $Z_{diff} = 2 Z_{odd}$.
4. Understand that this converts the single-trace impedance into the equivalent impedance seen at the differential port.

Example

The video provides a numerical example: if the differential impedance is approximately 99.66 ohms, the odd-mode impedance is calculated as $99.66 / 2 \approx 49.83$ ohms.

Common misconceptions

• Assuming differential impedance is equal to odd-mode impedance.
• Thinking that odd-mode impedance is the total impedance of the differential port.

Watch the explanation

Explore next

• How is common-mode impedance calculated from even-mode impedance?
• What is the definition of odd mode?
• Why is odd-mode impedance defined per single trace?