How to avoid resonance in a multi-stage filter for an electrical force measurement switch (PMS)?

When using multi-stage LC filtering in a PMS (e.g., input common-mode inductor + Y capacitor, followed by differential-mode inductor + X capacitor, and then board-level π-type filter), impedance mismatch in each stage of the filter may cause resonance at certain frequencies, amplifying interference, typically with peak values in the 1-10MHz band. To avoid resonance, damping design and impedance matching are necessary.

First, a small resistor (0.5-2Ω) should be connected in series in the LC filter network as damping, such as a damping network consisting of a 10Ω resistor in parallel across the common-mode inductor and a 100nF capacitor in series. Second, optimize inter-stage impedance: design the preceding stage (power supply side) with low output impedance and the following stage (load side) with high input impedance, which can be achieved by adjusting the inductor and capacitor values. For example, the inductance value of the second-stage differential-mode inductor (e.g., PBZ3216E120Z0T, 12μH) can be set to less than 1/10 of the value of the first-stage common-mode inductor (CMZ1211-501T, 500μH).

In addition, the frequency response was scanned using simulation tools (such as SPICE) to confirm the absence of resonance peaks. In actual testing, a sweep signal was injected into the output of the filter circuit to observe the input-output ratio. After optimization, the multi-stage filter exhibited smooth attenuation characteristics across the entire 0.15-30MHz frequency band, with no resonant amplification, ensuring the PMS conducted emission test passed on the first attempt.