Magnetics and RJ45 Connector Selection and Trade-Offs for a GX3-Based Design - KBA00114

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 How do I decide whether to use an integrated RJ45 connector module with magnetics or a separate magnetics and a simple RJ45 connector for GX3 based solutions? 



Magnetics are most commonly used methods of meeting the requirements of 10/100/1000 Base-T Physical Medium Attachment (PMA) electrical interface. As a designer, you have a choice of either using a RJ45 connector module with integrated magnetics or using discrete magnetics and a simple RJ45 connector. However, there is no standard solution that meets all objectives of all designs at the lowest cost. Each solution has its own advantages and trade-offs.

Here’s a summary of the advantages and disadvantages of using an integrated RJ45 module over discrete magnetics and a RJ45 connector.


  • Less BOM and lower production (assembly) cost
  • Better electromagnetic compatibility (EMC) shielding of sensitive cable-side signals by the metal shield
  • Smaller footprint than separate magnetics and a connector
  • Smaller form-factor


  • Higher production cost
  • Due to space constraints in integrated design, magnetic cores are smaller and closer to each other, resulting in crosstalk, degrading EMC characteristics, increasing non-linear distortion, and magnetic losses

Integrated magnetic/connector modules have a metal shield placed around the whole part, which shields the part from noise that is present inside the system. However, due to the small size, there is a high risk that the noise is coupled between the cable side and the PHY side of the magnetics, resulting in reduced common-mode choke effect and degraded performance.

Separate magnetics and connector has less coupling between the different parts inside the magnetics, thereby improving commonmode filtering. However, after the signals are filtered by the magnetics, they are routed to the connector on the PCB. If the system contains noise, this can couple to the Ethernet lines and create EMC problems. Thus, you should balance these advantages and disadvantages while deciding what to use in your design.