Common Mode Filter
Chokes for High Speed
Data Interfaces
Considerations for selecting a common mode choke
Introduction

Table 1. Typical data rates

High speed data interfaces like USB, HDBaseT™, HDMI,
DVI, and DisplayPort require careful consideration toensure reliable communication that is free of disruptive
EMI. Of the many tools at the designer’s disposal liketrace routing, termination and component placement,
the common mode filter choke remains one of the mostpowerful. For the variety of signal sizes, thermal variations
and spectral density in high speed communications, thecommon mode filter choke is an effective and widely usedinterface circuit component. Common mode chokes help
maintain the integrity of high speed communications andmay be necessary for FCC and international regulatory
standards conformance. FCC CFR 47 applies generally
to radio frequency devices (Part 15) and includes particular requirements for Industrial, Scientific and MedicalEquipment (Part 18). In addition to required standards
conformance, there may be other application-specificrequirements. For example, major auto makers maintaintheir own EMI requirements for vehicles.

Common Mode Choke Selection
The optimal common mode choke for a particular application depends on many factors. The first step inchoosing the best part is to select only those that will
not adversely attenuate the differential signal.
The communication standard (see Table 1) determines
the data rate and therefore the required bandwidth forthe differential mode performance.

Signal Interface

Data rates

USB2.0 (High Speed)

480 Mbit/s

USB3.1 Gen 1 (SuperSpeed)(former USB3.0)

5 Gbit/s

USB3.1 Gen 2 (SuperSpeed+)

10Gbit/s

HDMI®

1.65 Gbit/s to 6 Gbit/s;

HDBaseT

Upto 10.2 Gbit/sFireWire 800/S800T(IEEE1394b-2002/IEEE1394c-2006)

Up to 800 Mbit/s

FireWire S1600/S3200

1.57 Gbit/s and 3.15 Gbit/s

LVDS per ANSI/TIA/EIA-644-A

Up to 1.9 Gbit/s

PCI Express® 2.0

500 Mbit/s

SATA 1.0/2.0/3.0/3.1/3.2

1.5 Gbit/s;3 Gbit/s;6 Gbit/s;6Gbit/s;16 Gbit/s

DVI

Upto 3.96 Gbit/s

DisplayPort

Upto 8.1 Gbit/s

It is important, but relatively straightforward, to selecta part with low differential mode attenuation within thesignal frequency range. Figure 1 shows the -3dB differential mode cutoff frequency for Coilcraft’s 0805USBNSeries, which is clearly above at 1GHz and up to 6.5GHz.
Therefore distortion of the high frequency differential
mode signal and its harmonic can be kept at a very small
level, making our 0805USBN parts suitable for typicalUSB3.1 Gen 1 data rates, as are our 0805USBF and0603USB Series.

01Attenuation (dB)234-222-162-132-941-701-481
-271
-121

5678
1

10100

1000

10,000Frequency (MHz)Figure 1. Differential mode attenuation for 0805USBNFigure 2. Coilcraft 0805USBN common mode chokes

Document 1009-1 Revised 01/20/23

Even though the differential mode performance isstraightforward, and the requirement is predictable for agiven data rate, the real focus should be on determining
the required common mode performance. The amountof filtering required also depends on the data rate, but
is harder to predict because of all the possible physicalaspects of the application that may affect the amountof undesirable EMI generated. Design considerations
such as impedance matching, connector pin impedance,
trace widths and shielding may impact the final design.

How Common Mode Chokes Operate
As illustrated below, common mode chokes impede
noise common to both lines by flux addition when connected as shown. In differential mode the flux cancels
and the desired signal is not attenuated.

Since real world EMI challenges do not always fit nicely
into pre-arranged solutions, ready access to filter performance data is key. Selecting the appropriate common
mode choke has been simplified with Coilcraft’s online
Common Mode Choke Finder tool at: www.coilcraft.com/
en-us/tools/common-mode-choke-finder/#/search
To start, you specify the filtering requirements of yourapplication: the desired amount of impedance or theattenuation at a particular frequency (or frequencyrange). Alternatively, if a specific inductance value has
already been identified, the tool can search from that
input as well.
From this information the tool searches a wide databaseand presents the best solutions. All of the recommended
common mode chokes are presented in a sortable table(see Figure 3) that includes all the pertinent specifications

Common mode
Flux adds to impede common-mode current
1

2

4132Current

43

Flux

Differential mode
Flux cancels to pass differential-mode current
14142

32

3Current

Flux

When a challenge does arise, it is important toidentify the right solution. One easy choice is to identify
application-specific chokes; that is, filter chokes thathave been designed with specific applications in mind,
like the Coilcraft line of USB common mode chokes.

Figure 3. Coilcraft Common Mode Choke Finder and Analyzer search results page
Document 1009-2 Revised 01/20/23

Figure 4. Coilcraft Common Mode Choke Finder and Analyzer side-by-side comparison graphs
and actual size photos. The table is initially listed with thesmallest size (shortest length) parts at the top. You canthen sort the results by other specifications such as
impedance or attenuation, current rating, height, andmany other characteristics. Pricing information is also
included in the table.
One of the most powerful capabilities of this tool isthe ability to compare up to 6 different filters on thesame graph, making direct, side-by-side comparisons
between products much easier. Impedance and attenuation curves are shown for each selected part over thedesired frequency range (Figure 4). By moving the cursor over the curves, users can see the exact data pointat any frequency of interest via a small pop-up window.
Coilcraft’s Common Mode Choke Finder is the mostsophisticated and easy-to-use online tool available in
the industry. It is a great supplement to the developmentof your EMI filter design projects, making your common
mode chokes selection effective and efficient.
Of course you can also search the old fashioned way bybrowsing web pages. We’ve also made this easier bycollecting all our common mode chokes on one handy
page: www.coilcraft.com/en-us/products/emi/References
1. United States Code of Federal Regulations, Title
47, Telecommunication, Pt. 0-1.
2. Universal Serial Bus Specification Revision
2.0, Compaq, Hewlett-Packard, Intel, Lucent,Microsoft, NEC, Philips, April 27, 2000.
3. Universal Serial Bus Specification 3.0, HewlettPackard Company, Intel Corporation, Microsoft
Corporation, NEC Corporation, ST-Ericsson, Texas
Instruments, Revision 1.0 June 6, 2011.
4. High-Definition Multimedia Interface Specification Version 1.3a, Hitachi, Ltd., Matsushita ElectricIndustrial Co., Ltd., Philips Consumer Electronics,
International B.V., Silicon Image, Inc., Sony Corporation, Thomson Inc., Toshiba Corporation, November 10, 2006.
5. IEEE Std 1394.1-2004, E-ISBN: 0-7381-4648-X.
6. Electrical Characteristics of Low Voltage Differential Signaling (LVDS) Interface Circuits (ANSI/TIA/EIA-644-A-2001).
7. Creating a PCI Express™ Interconnect, White
Paper , Ajay V. Bhatt, Technology and ResearchLabs, Intel Corporation.

Document 1009-3 Revised 01/20/23