CM Filter
A common-mode filter uses coupled windings to present different impedances to common-mode and differential-mode currents. It can reduce interference on a differential link while preserving enough bandwidth for the wanted signal.
For the intended winding connection, equal and opposite differential currents produce largely cancelling magnetic effects. Common-mode currents reinforce them and encounter higher impedance. Real winding resistance, imperfect coupling and high-frequency parasitics create loss and limit performance.
In plain terms
Two linked lanes respond differently to traffic moving together and traffic moving in opposite directions. The filter is designed to impede common-mode noise more than the wanted differential signal, not to leave that signal completely untouched.
Why designers use it
- Reducing common-mode noise on compatible high-speed links.
- Addressing an identified interference problem with a filter selected for the actual signal bandwidth.
Best for
- USB links
- HDMI/MHL links
- MIPI links
Key specifications
- Common-mode rejection: Check the specified noise-frequency range
- Differential insertion loss: Check signal bandwidth and eye-diagram requirements
- Return loss: Check compatibility with the complete link
- Winding limits: Check resistance, current, insulation and connection drawing
When not to use it
- When its differential insertion loss or return loss violates the link's signal-integrity requirements.
- When the winding arrangement, current rating or insulation does not suit the application.
- As a guarantee that the finished product will pass an emissions or immunity test.
Common mistakes
- Assuming the windings have no DC resistance or voltage drop.
- Treating a common-mode impedance rating as the differential transmission-line impedance.
- Copying another product's filter without checking bandwidth, layout and measured performance.
Where you will find it
- ST AN4511 uses common-mode and differential insertion-loss curves to explain filter selection.
- The note demonstrates that filtering and the actual high-speed eye diagram must be evaluated together; rejection of noise alone is not enough.
A short history
A common-mode filter uses coupled windings. Opposing signal currents produce largely cancelling magnetic effects, while currents travelling in the same direction encounter greater impedance. This helps suppress common-mode interference on a signal pair. It is not perfectly invisible to the wanted signal: the filter's high-frequency characteristics still matter.