Protection
Common-Mode Filter + ESD (CMF+ESD)
Two-in-one parts: EMI filter for differential pairs plus integrated ESD protection.
What is it?
A CMF+ESD device combines a common-mode choke (which suppresses common-mode noise on a differential pair like USB or HDMI) with low-capacitance ESD diodes in a single small package. This saves board space and reduces parts count for high-speed differential interfaces. The common-mode choke rejects noise that appears on both lines equally (often radiated EMI), while the ESD diodes clamp static discharge events.
When do you need it?
- USB 2.0 / 3.x differential pairs in mobile devices where EMI must pass radiated emissions testing.
- HDMI and DisplayPort lines near antennas (e.g., smartphones, laptops with mmWave radios).
- Automotive infotainment differential buses (CAN-FD, FlexRay) that must pass CISPR 25 emissions.
- Any design where board space for separate CMF + ESD diodes is tight.
How to pick the right one
- CMR (Common-Mode Rejection) at target frequency
- Check the impedance curve at the noise frequency you need to reject, typically 100 MHz to 6 GHz.
- DC resistance (RDC)
- Lower RDC reduces insertion loss and heat. <0.5 Ω per line typical.
- Differential-mode capacitance
- Lowest for >5 Gbps signals, ESD diode portion should be <0.5 pF.
- ESD rating
- IEC 61000-4-2 contact discharge rating, ±8 kV minimum for system-level.
- Package
- Typically SOT-23-6 or compact 6-pin DFN.
What Magnias offers
Magnias offers CMF+ESD integrated parts for USB 2.0, USB 3.x, and HDMI differential pairs. Selection depends on data rate and target rejection frequency, talk to your FAE for the right variant.
Common questions
Can I just use a separate CMF and ESD diode?
Yes, and many designs do. Integrated CMF+ESD wins when board space is tight or when you want a tuned response that's hard to achieve with discrete parts.
Does the CMF affect signal integrity on USB 3.x?
Slightly, there is always some differential-mode loss. Pick a CMF specified for your data rate; USB 3.x CMFs are designed to roll off below the signal band.
How is CMF different from a differential filter?
A CMF only attenuates common-mode noise; differential signals pass through. A differential filter would attenuate the wanted signal, wrong tool for the job.