What is the difference between common mode impedance, differential mode impedance and characteristic impedance?
| Common mode impedance | Differential mode impedance | Characteristic impedance |
|---|---|---|
| refers to a characteristic in response to common mode signals (noise) | refers to a characteristic in response to leakage inductance | refers to the line impedance when two lines of a common mode/filter choke are considered as a differential transmission line - it has no correlation with the common mode impedance |
What is the difference between common mode and differential mode?
The common mode refers to signals or noise that flow in the same direction in a pair of lines.
The magnitudes of the signals are equal. These signals will add together instead of cancelling each other out. The result will be high impedance to the common mode signal, which will become heavily attenuated. This potential will vary with respect to ground. As this varies from ground, radiation or noise will become present.
These signals are unwanted, as they do not carry any data. Using independent grounds can also create a difference in potential resulting in common mode noise. This is very common as these can create ground loops.
Common mode chokes, or common mode inductors, consist of two or more coils of insulated wire on a single magnetic core. Each winding is put in series with one of the conductors. This means that the magnetic fields of the wires combine to present high impedance to the noise signal. The desired signal can pass through the coil easily, but the noise component is attenuated, or completely blocked, by the resulting high reactance of the inductor. Overall, the choke has a low electrical resistance, allowing for minimal power loss in the process.
Here’s a link to the common mode filters Digi-Key stocks.
The differential (normal) mode refers to signals or noise that flow in opposite directions in a pair of lines. For differential mode signals, the voltage on V1 minus V2 equals the Differential Voltage. Through a differential choke or differential inductor, this signal will create opposing magnetic fields which will cancel each other out. The signal will pass through the choke unattenuated as it sees zero impedance created.
Here’s a link to the differential mode parts Digi-Key stocks.
As an example, consider a configuration where two copper wires are wound around a ring-shaped ferrite core (Fig. 1). In this structure, two patterns may exist: one where the currents in the wires flow in the same direction, and another where they flow in opposite directions. These two patterns are “common mode” and “differential mode”.
Fig. 1: Common Mode and Differential Mode Visualized
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