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LC Noise Filter Effect Using Common Mode Choke Coils (Common Mode)

Make the following connections to check the attenuation effect of common mode noise in a filter circuit in a comparison of actual measurements and simulation for a noise filter using a common mode choke coil.

Here, for capacitor X and the load resistance (Load), in common mode, the phase relationship of the Hi side and Lo side corresponding to the signal line are equivalent, so no potential difference is generated between the Hi side and the Lo side.
Therefore, no current flows to capacitor X or load resistance, and it can be omitted as follows.

Also, the characteristic impedance Z0 of a filter composed of self-inductance capacitor Y is given by the following formula.

As such, it is necessary to match the output impedance of the signal sources. For LDFL002302LS-V0E, conduct SPICE simulation with the following circuit constants.
For capacitor Y, we use a 1μF ceramic capacitor to create a 7-element SPICE MODEL

【Circuit constants for LDFL002302LS-V0E】
Each component Circuit Numerical value of the equivalent circuit (input values)
stray capacitance C 2.1pF
iron loss(Loss of magnetic material) R 11.6kΩ
Self-inductance L 15.0mH
coupling coefficient k 0.9996333
copper loss(Loss of wire) DCR 30mΩ
Dummy resistance Rd 1GΩ
Output resistance R-Out 87Ω

For common mode, a low-pass filter is formed by the self-inductance of the common mode coil and capacitor Y. The cutoff frequency Fc can be expressed by the following formula.

From the above, we show the results of comparison between actual measurement values ​​and SPICE simulations when LDFL002302LS-V0E and a 1μF semi-capacitor are combined.

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