Designers need to limit the length of the input traces or cables connected to the AMC3301 family. However, some applications will require longer input traces or cables which will lead to excessive radiated emissions. This radiation can be attenuated by using ferrite beads or a common-mode choke in series with the input connections. When selecting a ferrite bead or common-mode choke, refer to the impedance over frequency plot in the components’ data sheet. A minimum of 1 kΩ ohm impedance (z) is recommended over the frequency range of interest, 150 MHz to 800 MHz for CISPR 11, with higher impedances attenuating radiated emissions more effectively. Table 3-1 lists recommended ferrite beads and a common-mode choke.

To demonstrate the benefits of adding the ferrite beads or a common-mode choke for the 1.5 m input and 30 cm input, refer to Figure 3-1 and Figure 3-2 respectively. The 74269244182 ferrite bead from Wurth Elektronik and DLW31SN222SQ2 common-mode choke from Murata were added in series to the input connections for these tests.

Figure 3-1 shows the radiated emissions of the 1.5 m input. Without ferrite beads or a common-mode choke is shown in blue and the CISPR 11 class B limit is violated. The attenuating benefit of the ferrite beads is shown in red and the common-mode choke in green. Both the ferrite beads and common-mode choke significantly attenuate the radiated emissions, allowing the AMC3301EVM to pass the CISPR 11 class B test.

Figure 3-2 shows the radiated emissions of the 30 cm input. All test cases pass the CISPR 11 class B test, including without ferrite beads or a common-mode choke as shown in blue. This indicates that additional components are not necessary to pass the test, but to demonstrate the attenuating benefits, the measurements with ferrite beads are shown in red and the common-mode choke in green.