How is common-mode impedance calculated from even-mode impedance?

Conditions

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

Reasoning, step by step

1. Identify the even-mode impedance $Z_{even}$ of a single trace.
2. Divide the even-mode impedance by two.
3. Express the result mathematically as $Z_{comm} = \frac{1}{2} Z_{even}$.
4. Understand that this converts the single-trace impedance into the equivalent impedance seen at the common-mode port.

Example

The video demonstrates this with SI9000 software results: if the even-mode impedance is approximately 70.08 ohms, the common-mode impedance is calculated as $70.08 / 2 \approx 35.04$ ohms.

Common misconceptions

• Assuming common-mode impedance is equal to even-mode impedance.
• Thinking that even-mode impedance is the total impedance of the common-mode port.

Watch the explanation

Explore next

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