Common mode filtering

AN4304

10/20

DocID024731 Rev 1

In Figure 10: "Input pin+ voltage with Ccm perfectly matched", the high frequency on

common mode has been removed with the filter shown.

Figure 10: Input pin+ voltage with Ccm perfectly matched

Differential mode can also be filtered as shown in Figure 11: "Differential voltage with Ccm

perfectly matched". This is the best case scenario where the two filtered capacitances

(Ccm1 and Ccm2) are perfectly matched.

Figure 11: Differential voltage with Ccm perfectly matched

Unfortunately, the capacitances used for filtering are already matched at about 20 %. It is

important to take this into consideration because due to the mismatch of the capacitances

and resistances, the filter cut-off frequency may be a bit different between the two inputs. In

differential mode, this could cause an AC error. An AC error may in turn be amplified by the

current sensing and cause additional error.

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