Whitepaper

Effectively protecting super-speed
interfaces against EMI
January 24, 2019 by Jan PreibischSuper-speed interfaces such as HDMI and USB have become the norm forquickly transferring lots of data, but they need dedicated and effective
ESD and EMI protection.

High data speeds, up to 20 Gbit/s, require a carefully
designed PCB layout to maintain signal integrity. This
challenge requires proper impedance matching to avoid
unacceptable losses and reflections.

Diﬀerential signal

Common mode chokeCommon mode signal

Common mode choke

Figure 1: Common mode choke signal with differential and common
mode input

In consumer and computing based products, super-speed
interfaces have become the norm where large quantities ofdata need to be transferred or copied in a short timeframe,such as high-definition video and audio. All super-speed
interfaces, such as HDMI 2.0 and USB 3.2, use differential
data signals and operate with reduced signal levels.

Various electromagnetic interference (EMI) aspects need
to be considered especially when sensitive Wi-Fi, cellular or
GPS receivers are located close to potentially radiating highspeed interfaces. As both high-speed wired lines and popularwireless standards frequently interfere in the Gigahertz
range, EMI issues shouldn’t be overlooked because these cansignificantly affect the user experience.
To maintain data integrity in super-speed interfaces,
designers must have effective EMI filtering in place. One
useful device to include is the common-mode filter (CMF) orcommon-mode choke (CMC), which comprises two coupledcoils. When current flows in the same direction in both coils,
the filter has a high impedance. Conversely, when current
flows in opposite directions, the impedance is low.

When common mode filters are applied to differential datalines, the differential part of the data carrying the information
can pass. At the same time, the overlaid common mode signal
content is blocked (Figure 1). Common mode noise can begenerated from non-perfect transmitters, coupling of othersignals, and non-symmetrical signal lines. It is created easily in
the PCB layout if a signal pair changes direction and the lengthof the signal lines is not equal (Figure 2).

Blue + Red
ReceiverTransmitterBlue + Red

Figure 2: Common mode noise from routing

Magnetic and electrical fields are cancelled out for adifferential signal in the far field, if the traces are routed
within a small distance. In contrast, common mode
signals can radiate into the environment and thus reduce
the sensitivity of receiver blocks, especially in compact
designs. For example, an activated USB 3.x interface canhinder proper reception of Wi-Fi data by a smartphone orcomputing device.

The challenge of guarding USB and HDMI interfaces against
EMI is accompanied by the need to protect them (and thesystems they are connected to) against potential ESD strikes.
Power delivery capability and reversible plugs in USB Type-C
connectors are additional obstacles for USB interfaces. Most
solutions currently employ separate devices to address eachindividual need, but Nexperia offers an alternative integrated
approach combining CMF+ESD protection in one chip, such as
the new PCMF series.
Integrating CMF and ESD protection into a single device
reduces overall PCB mounting space requirements. It also
prevents additional impedance mismatches in the signaltraces, which can occur when using separate parts.
PCMF solutions from Nexperia deliver optimal robustness
and low clamping while also filtering EMI that could interferewith both 2.4 GHz and 5 GHz channels.
These effective two-in-one solutions are helpful for
designers who prefer to choose whether an EMI filter isneeded at a later stage of the design process. Our PCMF
types are footprint-compatible with several PESD types for
ESD-only protection, so the ESD-only and PCMF solution canbe realized on the same PCB layout. This way any unused CMF
footprints don’t need to be bridged with resistors as requiredin conventional designs, further simplifying overall design
and assembly.
To get a deeper understanding of EMI and ESD protection,
check out our ESD Application Handbook which provides anin-depth exploration of all the facets of ESD characteristics,
design, testing and further information about interfaces &applications.

About Nexperia
Nexperia is a leading expert in the high-volume
production of essential semiconductors that arerequired by every electronic design in the world.
The company’s portfolio includes diodes, bipolartransistors, ESD protection devices, MOSFETs,
GaN FETs and analog & logic ICs. Headquartered in
Nijmegen, the Netherlands, Nexperia annually ships
more than 90 billion products, meeting automotivestandards. These products are recognized as
benchmarks in efficiency - in process, size, power andperformance - with industry-leading small packages
that save valuable energy and space. Nexperia has over
12,000 employees across Asia, Europe and the US.
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Date of release:
January 2020