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.