Common mode chokes are widely used in high speed serial data transmission, especially when the transmission medium is a cable connecting two sub-systems. They are used to reduce electromagnetic radiations from the cable and to help obtain compliance with regulatory requirements. Since the common mode chokes are directly in the data path, their electrical characteristics can impact the performance of the differential signal transmitted through the cable. This paper outlines the construction, the electrical properties and the common uses of common mode chokes, providing hardware engineers with information for selecting the correct components for a given application.
High Speed Serial Data Links
The FPD-Link II and III from National Semiconductor are serial interfaces commonly used in automotive Infotainment sub-systems. The DS90UB901Q/DS90UB902Q and DS90UB925Q/DS90UB926Q are two serializer/de-serializer chipsets offering FPD-Link III serial interface with a high speed forward channel and a bi-directional control channel for data transmission over a single differential pair. Common mode chokes are sometimes used in the shielded twisted pair cable connecting the serializer and the de-serializer sub-systems. Figure 1 shows a typical FPD-Link III interface implemented with the DS90UB901Q and DS90UB902Q.
Figure 1. Typical FPD-Link III Interface with the use of DS90UB901Q and DS90UB902Q
Construction of Common Mode Chokes
Common mode chokes appear in many forms. A ferrite clamp wrapped around a cable forms a common mode choke that becomes part of the cable. Many suppliers1 produce miniature common mode chokes with physical sizes as small as a 0805 or even a 0603 resistor designed for printed circuit board mounting.
A common mode choke is built with two identical wires symmetrically wound on a ferrite core. Figure 2 shows the construction of a 0805-size common mode choke. The structure is a pair of highly symmetrical mutual inductors. The equivalent circuit diagram is shown in Figure 3.
Figure 2. Typical Construction of a Surface Mount Common Mode Choke
Figure 3. Equivalent Circuit Diagram of a Common