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AN4511

Benefits of the common mode filter

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5

Benefits of the common mode filter

The improvement given by the common mode filter is shown by two examples:

1.

The skew brought by two different trace lengths

2.

Radiated noise on a USB-MHL cable

Another example is described in AN4356.

5.1

Effect on skew

Figure 15 shows the used simulation schematic. MHL link is built at 2.25 Gbps.

Figure 15. Skew simulation schematic

The common mode noise levels are compared when the two lines have the same length

and when there is a difference of two millimeters, 10% difference. The common mode noise

is not equal to zero (Figure 16).

Figure 16. Common mode noise with and without skew

The spectral content of the common mode signal is shown in Figure 17. This figure shows

the more polluting frequencies. In this case filtering is necessary.