Common-Mode Impedance
Understanding Common-Mode Impedance
A two-conductor differential interconnect can carry two independent signal modes. In the differential, or odd, mode the conductors carry equal and opposite voltages and the signal energy is confined to the field between them. In the common, or even, mode the conductors carry equal and in-phase voltages, and the return current flows in the surrounding ground reference. Common-mode impedance is the ratio of the common-mode voltage to the common-mode current for this second case. It is a distinct parameter from differential impedance, and both must be controlled to build a clean, low-emission link.
Common-Mode Versus Differential-Mode Excitation
When a perfectly balanced driver sends a true differential signal, the two conductors are equal and opposite and there is ideally no common-mode component. In practice, skew between the two lines, mismatched drivers, asymmetric routing, and connector imperfections convert a fraction of the differential signal into a common-mode signal. That converted energy sees the common-mode impedance, not the differential impedance, so the two impedances together determine how the unwanted mode propagates, reflects, and ultimately radiates. The figure of merit that quantifies how well a balanced device rejects this in-phase signal is the common-mode rejection ratio.
Relationship to Even-Mode and Odd-Mode Impedance
For a symmetric pair of coupled lines, the common-mode and differential-mode impedances follow directly from the even-mode and odd-mode impedances of the structure. In the common mode the two conductors sit at the same potential, so they behave as two equal impedances to ground placed in parallel. The result is that the common-mode impedance is approximately half the even-mode impedance of a single line. By contrast, the differential impedance is twice the odd-mode impedance. Keeping these relationships straight is essential when reading coupled-line solver output or matching a balun to a pair, since solvers usually report the even and odd values while datasheets quote the common and differential values.
Common-Mode Impedance and the Formula Set
Zcm = Zeven / 2
Zdiff = 2 × Zodd
From per-line values (general):
Zcm = ½ × (Zself + Zmutual)
Where Zcm = common-mode impedance seen by an in-phase signal on the pair; Zeven = even-mode characteristic impedance of one conductor when both are driven together; Zdiff = differential impedance; Zodd = odd-mode characteristic impedance of one conductor when the pair is driven with opposite polarity; Zself = single-line impedance to ground; Zmutual = coupling impedance between the lines. Example: a 100 Ω differential pair with Zodd ≈ 50 Ω and Zeven ≈ 65 Ω gives Zcm ≈ 32 Ω.
Why Common-Mode Impedance Matters for EMI
Common-mode current is one of the most efficient unintentional radiators in an electronic system. Because both conductors carry the noise in phase, their fields add rather than cancel, and a cable or trace pair behaves like a small antenna driven against ground. The level of that current depends on the common-mode impedance of the path. Designers manage electromagnetic interference by deliberately shaping this impedance: a common-mode choke raises the series common-mode impedance to block the unwanted current while leaving the differential signal almost untouched, and common-mode termination resistors to ground absorb the energy instead of letting it reflect and radiate.
Terminating the Common Mode
A differential receiver only terminates the differential mode well if it also accounts for the common mode. A bare differential termination resistor across the pair presents the correct differential impedance but leaves the common mode open, so any converted common-mode energy reflects. A split, or center-tapped, termination places two resistors in series across the pair with their midpoint taken to ground through a small capacitor. This presents the differential impedance to the differential signal and a defined common-mode impedance to the in-phase signal, controlling both modes at once. Signal-integrity tools report common-mode return loss, the reflection of the in-phase signal off a port, and a well-matched common mode keeps that return loss low across the band of interest.
Typical Mode Impedance Values
| Mode / Parameter | Typical Value | Notes |
|---|---|---|
| Differential impedance (Zdiff) | 90 to 100 Ω | Common board and cable target |
| Common-mode impedance (Zcm) | 25 to 38 Ω | Roughly half the even-mode value |
| Even-mode impedance (Zeven) | 60 to 75 Ω | Both lines driven in phase |
| Odd-mode impedance (Zodd) | 45 to 50 Ω | Lines driven out of phase |
| Common-mode choke impedance | 100 Ω to several kΩ | Frequency dependent, peaks in MHz to GHz range |
Common-mode impedance appears in the specifications for high-speed serial links, Ethernet and USB cabling, automotive and industrial differential buses, and the input ports of differential RF amplifiers and mixers. Cable and connector vendors publish a common-mode impedance alongside the differential value so that system designers can budget both modes and keep radiated emissions within compliance limits.
Frequently Asked Questions
What is common-mode impedance?
Common-mode impedance is the impedance a differential pair presents to a common-mode signal, the in-phase component that appears equally on both conductors with respect to a shared ground return. It is set by each line's impedance to ground and the coupling between the two conductors, and it is distinct from the differential impedance that governs the wanted signal.
How is common-mode impedance related to even-mode impedance?
For a symmetric coupled pair the common-mode impedance is approximately half the even-mode impedance, because the two conductors carry the same voltage and behave as two equal impedances to ground placed in parallel. By contrast the differential impedance is twice the odd-mode impedance. So a 100 Ω differential pair with an even-mode impedance near 65 Ω presents a common-mode impedance of roughly 32 Ω.
Why does common-mode impedance matter for EMI?
Common-mode currents flow in phase on both conductors and return through ground, so their fields add instead of cancel and the pair radiates like a small antenna. Shaping the common-mode impedance controls those currents: a common-mode choke adds high series impedance to block them while leaving the differential signal untouched, and split termination to ground absorbs the in-phase energy instead of letting it reflect and radiate.