How to calculate the differential mode filter parameters of the motion controller MC?

Calculating the parameters of the differential-mode filter for a motion controller (MC) requires specifying the noise frequency, impedance, and attenuation target. For differential-mode filtering of power lines, LC or π-type structures are commonly used. The inductance value L is calculated based on the lowest noise frequency f to be attenuated and the load impedance Z, using the formula L = Z/(2πf). A differential-mode inductor with sufficient impedance at f should be selected. The capacitance value C, together with the inductor, forms a cutoff frequency fc = 1/(2π√LC), which should be lower than the noise frequency. In practice, the ESR of the capacitor and the parasitic capacitance of the inductor must be considered.

For differential-mode filtering of signal lines, such as RC circuits, the resistor R must be selected to balance signal attenuation and driving capability. The capacitor C is calculated based on the cutoff frequency fc = 1/(2πRC). After calculating all parameters, the filtering effect and signal integrity must be verified through simulation. On an actual PCB, parasitic parameters can affect performance; therefore, the final parameters may need to be adjusted based on actual measurements. Calculation example: To suppress 100kHz differential mode noise, with a load impedance of 50Ω, the inductor should be at least 80μH. With a 0.1μF capacitor, the corner frequency is about 18kHz, which provides sufficient attenuation for 100kHz.