Technology / Technical guide
Common-Mode & Differential-Mode Electrical Noise
Electrical noise can change both conductors relative to a reference, or change the difference between them. Those are distinct modes. The distinction helps explain signal interference without assuming an actual communication interface, filter or converter grounding arrangement.
Define the reference before naming the noise mode.
Common and differential quantities describe a chosen conductor pair; they are not labels that can ignore the return paths.
01 / Two quantities
Describe shared voltage and difference separately.
For voltages v1 and v2 measured relative to the same chosen reference, common-mode voltage is vcm = (v1 + v2)/2.
For voltages v1 and v2 measured relative to the same chosen reference, common-mode voltage is vcm = (v1 + v2)/2. Differential-mode voltage is vdm = v1 − v2, with its sign set by the conductor order.
The decomposition can be reversed: v1 = vcm + vdm/2 and v2 = vcm − vdm/2. Analog Devices explains these two components for differential signals. A shared reference is necessary for the common-mode quantity to have a defined meaning.
02 / Illustrative change
An equal shift can leave the difference unchanged.
In a hypothetical arithmetic example, v1 = 3 V and v2 = 1 V give vcm = 2 V and vdm = 2 V.
In a hypothetical arithmetic example, v1 = 3 V and v2 = 1 V give vcm = 2 V and vdm = 2 V. Adding 0.5 V equally to both changes vcm to 2.5 V while vdm remains 2 V.
These numbers are only a mathematical illustration, not operating voltages for a MARSTON port. An unequal disturbance would generally change the difference as well. A real conductor pair can contain both modes at the same time.
03 / Coupling
A disturbance reaches a circuit through physical paths.
Electric and magnetic coupling can transfer disturbances between parts of an assembly.
Electric and magnetic coupling can transfer disturbances between parts of an assembly. Changing voltage can couple through capacitance, while changing current and magnetic flux can induce voltage. These mechanisms depend on geometry and frequency.
Common-mode current also needs a return path, which can include distributed parasitic capacitance in a physical assembly. Calling a voltage common mode does not make its associated current vanish or explain every path without a circuit boundary.
04 / Imbalance
A shared disturbance can become a differential effect.
Perfectly equal coupling is an idealisation.
Perfectly equal coupling is an idealisation. If the two conductors encounter different impedances or coupling, a shared disturbance can produce unequal effects. The resulting difference is one reason common-mode interference can affect a differential signal.
Balance concerns the relevant electrical paths, not simply whether two wires look alike. The useful explanation is the asymmetry and its frequency dependence. This page does not prescribe grounding changes, filter wiring or an installation modification.
05 / Receiver response
Rejection is finite and condition-dependent.
A differential receiver responds primarily to the difference, but common-mode rejection is finite.
A differential receiver responds primarily to the difference, but common-mode rejection is finite. Its response can depend on frequency and operating conditions. A rejection figure is therefore not a promise that every shared disturbance has no effect.
The permissible input common-mode range is a different concept from rejection. A device can only be interpreted within its applicable envelope. No receiver type, range, communication protocol or interface capability is assigned to MARSTON equipment here.
06 / Assembly scope
Keep mode analysis distinct from compliance.
The EMC boundary guide discusses emissions, immunity and complete assembly scope.
The EMC boundary guide discusses emissions, immunity and complete assembly scope. Noise-mode decomposition explains a mechanism within that broader subject; it does not establish a pass result.
A filter’s response also depends on the mode and connected paths, as discussed in the LC model guide. This article provides conceptual relationships without live measurements, component selection, wiring instructions or a claim of implemented noise suppression.
Continue the review with LC Filter Resonance: Natural Frequency and Damping.
Continue the review with PWM Switching Frequency, Carrier & Fundamental.
Continue the review with Wind Waveform Sampling: Nyquist Conditions & Aliasing.
General background: Analog Devices: common-mode decomposition, coupling and finite rejection. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Reference | Common origin for v1 and v2 | Required to define shared voltage. |
| Common-mode voltage | (v1 + v2)/2 | Conductor pair and reference are explicit. |
| Differential voltage | v1 − v2 | Sign follows the selected conductor order. |
| Mode conversion | Unequal response to a disturbance | Path imbalance can create a differential component. |
| Receiver behaviour | Rejection and allowed range | Distinct, conditional device properties. |
Engineering questions.
Does common mode mean no voltage difference anywhere?
No. It describes the shared component of a specified pair relative to a reference.
What happens when both voltages change equally?
Their common-mode voltage changes while their difference remains unchanged in the ideal decomposition.
Can both noise modes occur at once?
Yes. A real pair can contain shared and differential components together.
Can imbalance turn shared interference into a differential error?
Yes. Unequal coupling or impedance can produce unequal responses.
Does common-mode rejection mean unlimited immunity?
No. Rejection and the allowed input range are finite and condition-dependent.
Does this identify an actual protocol or compliant MARSTON filter?
No. It explains noise mechanisms without assigning interface hardware or compliance results.
Technical enquiry
Discuss the technical question.
Electrical noise can change both conductors relative to a reference, or change the difference between them. Share the model and applicable operating or design information for an engineering discussion.