AN4304

Common mode filtering

DocID024731 Rev 1

11/20

Equation 3 below expresses the transfer function using a common mode filter. To simplify

the calculation, only the mismatch of the capacitance is taken into account (the mismatch

of the resistances is generally low at about 1 %). Vin is the voltage source.

Equation 3 shows that if Ccm1 and Ccm2 are not perfectly matched a differential voltage is

created on the input of the current sensing. Equation 3 also shows that two poles appear

(see below).

The transfer function is illustrated in Figure 12: "Bode diagram".

Figure 12: Bode diagram

Equation 3 is true for a sinusoidal signal. Normally, the input signal is decomposed in

Fourier series (although this depends on the type of input signal) and a first approximate

calculation is made taking only the fundamentals into account. From Equation 3 we can

extrapolate that at frequencies much higher than ω2, the differential voltage error, due to

the mismatch of the Ccm1 and Ccm2 capacitances, is given by Equation 4 below: