Common-mode (CM) and differential-mode (DM) noise describe two different ways unwanted voltage or current appears in a circuit. DM is the difference between the two conductors of a pair; CM is the noise shared by both conductors relative to a reference such as chassis or earth. Because they travel by different paths, the useful first step in conducted-emissions troubleshooting is to measure which mode is present—not to add a filter component at random.
What common-mode and differential-mode noise mean
Start with a two-wire circuit and ask what you are comparing. Differential-mode noise is the voltage between the two wires, or the component of current that travels out on one conductor and returns on the other. Common-mode noise is the component shared by both conductors relative to a reference such as chassis or earth; common-mode current on the pair flows in the same direction.
These are descriptions of a signal’s relationship to a pair and a reference, not names for a single root cause. CM voltage can arise from shared impedance—for example, another current creates a voltage drop that appears on both signal and return—or from parasitic coupling between a switching node and chassis. Those mechanisms can coexist in one design. Texas Instruments discusses the shared-impedance case and resulting ground bounce in its motor-driver layout guidance; its TPSM33620-Q1 EMI guidance also describes converter noise paths.
How the modes appear on a switching converter
A buck converter’s pulsed input current can create DM noise in the supply-and-return loop. Separately, a high-dV/dt switch node can couple through parasitic capacitance to chassis or earth, creating a CM return path. Which mode dominates depends on the circuit, board, cabling, and measurement setup; the mechanism alone does not identify the dominant emission.
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How to separate the modes in conducted-emissions measurements
In the method described by Analog Devices, a line impedance stabilization network (LISN) is placed between the supply and buck converter, and the two line-to-reference voltages are measured as V1 and V2. Both readings contain contributions from CM and DM noise. Their average estimates the common-mode voltage, while half their difference estimates the differential-mode voltage:
- Common-mode: VCM = (V1 + V2) / 2
- Differential-mode: VDM = (V1 − V2) / 2
The signs depend on probe polarity and measurement convention; use consistent connections when comparing the two lines. Analog Devices also describes using a T-type power combiner to separate the components. See its conducted-emissions separation method for the setup and derivation.
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For CM current on a power cord or harness, a current probe can measure the current flowing in the same direction on the conductors. One Analog Devices article describes a high-bandwidth probe and specific probe distances for its FM-band test setup; those distances belong to that setup and should not be treated as universal test instructions. Check the applicable standard and reproduce its defined arrangement. See the FM-band conducted-EMI discussion.
Frequency can guide a search, but cannot diagnose the mode
Analog Devices notes that, in the context of its discussion, lower-frequency conducted emissions are often DM and higher-frequency emissions in the FM band often CM. That is a troubleshooting heuristic, not a rule: board geometry, coupling paths, and test conditions can change the result. The same source says mitigation effectiveness varies by board.
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Choose a remedy for the measured path
Once the mode is identified, trace the current or coupling path that produces it. A filter aimed at the wrong mode may leave the dominant path largely untouched.
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If the measurement points to differential-mode noise
- Inspect the switching input-current loop and the supply-and-return path for excessive loop area or impedance.
- Consider a DM filter appropriate to the circuit and the measured frequency range; verify its effect in the actual setup rather than assuming a filter label guarantees a result.
- Keep return paths short, wide, and low impedance where appropriate, while preserving the intended current path and device requirements.
If the measurement points to common-mode noise
- Look for high-dV/dt nodes and parasitic capacitance to chassis or earth, then consider how cables and the reference connection complete the return path.
- Where circuit constraints permit, reducing switch-node copper area or slew rate can reduce coupling. These changes can affect switching behavior, so validate them against the design requirements.
- A common-mode choke is one possible filter component, not a universal cure. Select components for the circuit and measured problem, and confirm the change by remeasurement.
In one Analog Devices demo-board example, emissions exceeded CISPR 25 Class 5 limits from 30 MHz to 108 MHz; after changes focused on CM noise, the reported emissions fell enough for that demo board to comply. The result is specific to that board and test, not a performance guarantee for a choke, layout change, or other design. The article’s example and method are documented in the Analog Devices application article.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why common-mode noise can affect a differential signal
Common-mode noise is not automatically harmless, and differential signaling does not eliminate EMC concerns. A receiver’s common-mode rejection is finite; imbalance in a path or filter can convert some CM energy into a differential signal. That can corrupt a measurement or interfere with the intended signal. For sensitive sensing and filtering, balance and component matching matter. Analog Devices discusses CM-to-DM conversion in its EMI-filter article and measurement concerns in its TMCS11xx application article.
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A practical troubleshooting sequence
- Define the setup and reference. Identify the applicable emissions test, supply, LISN or other measurement network, cable arrangement, and reference used for voltage or current measurements.
- Measure both conductors or the relevant current path. Keep probe polarity and setup consistent so the readings can be compared.
- Separate CM and DM contributions. For the cited two-line LISN method, calculate the average and half-difference of V1 and V2; use the method’s specified connections and interpretation.
- Trace the likely source and return path. For DM, inspect the switching supply-and-return loop. For CM, examine high-dV/dt coupling, parasitic capacitance, chassis or earth references, and cable paths.
- Make a mode-specific change, then remeasure. Compare results in the same setup and check against the applicable product standard. A generic choke, ferrite, shield, or layout rule cannot guarantee compliance.
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