What is the mathematical definition of the common-mode signal voltage in a differential pair?

Conditions

• There are two transmission lines and their corresponding reference planes.
• The driver outputs a positive-polarity signal and a negative-polarity signal.

Reasoning, step by step

1. Identify the positive-polarity signal voltage, denoted as $V_1$.
2. Identify the negative-polarity signal voltage, denoted as $V_2$.
3. Add $V_1$ and $V_2$ together.
4. Divide the sum by two to obtain the common-mode signal voltage.
5. Express the result mathematically as $V_{comm} = \frac{1}{2}(V_1 + V_2)$.

Example

The video shows that in an ideal differential pair where $V_1$ and $V_2$ are equal in magnitude but opposite in polarity, the common-mode signal $V_{comm}$ fluctuates around zero.

Common misconceptions

• Believing the common-mode signal is the difference between the two voltages.
• Assuming the common-mode signal is always exactly zero in practical circuits.

Watch the explanation

Explore next

• What is the definition of the differential signal?
• How does the common-mode signal behave when there is impedance mismatch?
• Why is minimizing the common-mode signal important in signal integrity?