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Common Mode Filter Chokes for High Speed Data Interfaces

Parent Category: 2014 HFE

By Coilcraft

Introduction

High speed data interfaces like USB, HDMI, Wi-Fi, IEEE1394/FireWire and LVDS require careful consideration to ensure reliable communication free of disruptive EMI. Of the many tools at the designer’s disposal like trace routing, termination and component placement, the common mode filter choke remains one of the most powerful. For the variety of signal sizes, thermal variations and spectral density in high speed communications, the common mode filter choke is an effective and widely used interface circuit component. Common mode chokes help maintain the integrity of high speed communications and may be necessary for conformance with FCC and international regulatory standards. FCC CFR 47 applies generally to radio frequency devices (Part 15) and includes particular requirements for Industrial, Scientific and Medical Equipment (Part 18). In addition to required standards conformance there may be other application-specific requirements. For example, major auto makers maintain their 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 in selecting a common mode choke is to select only those that will not adversely attenuate the differential signal.  The communication standard determines the data rate and therefore the required bandwidth for the differential mode performance. 

Interface

Data rates

USB 2.0

480 Mbps 

USB 3.0 (Superspeed)

Up to 4.8 Gbps

HDMI®

Clock rates 165 MHz to 340 MHz

IEEE1394 / Firewire®

Up to 800 Mb per second

LVDS per ANSI/TIA/EIA-644-A

Up to 1.9 Gb/s

PCI Express® 2.0

500 Mb/s

Table 1 • Typical data rates.

It is relatively straightforward to select a part with low differential mode attenuation up to the appropriate data rate. As seen in Figure 1, the differential mode attenuation does not rolloff until above 1 GHz, making the 0603USB parts suitable for typical USB 3.0 data rates. 

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

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

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 by Coilcraft with the Common Mode Filter Finder web-based tool at:

http://www.coilcraft.com/apps/filter_finder/filterfinder.cfm

The tool starts with requesting the most pertinent information: The desired amount of attenuation frequency range of interest. From this information the tool searches a wide database and presents the best solutions. The user then can select the choke most appropriate for the application. The data provided includes pertinent specifications like current rating and DC resistance as well as overall size. Then common mode and differential mode curves are presented for the frequency range specified, allowing a quick and meaningful comparison of the options.

Alternatively, if a specific inductance value has already been identified, the database can be searched from that input. The result is then displayed as a sortable list, handy to find a part with all the needed features.

Of course, there’s always the old fashioned way – browse the web pages. Coilcraft has collected the common mode chokes into one handy place: http://www.coilcraft.com/prod_emi.cfm.

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, Hewlett-Packard 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 Electric Industrial 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 Research Labs, Intel Corporation.

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