How to avoid resonance in multi-stage filtering of I/O modules

Avoiding resonance in multi-stage filter circuits within the I/O module is crucial; otherwise, it will generate gain instead of attenuation at specific frequencies. During design, the cutoff frequencies of each filter stage should be staggered. For example, the cutoff frequency of the first-stage LC filter should be set to 100kHz, and the second stage to 1MHz to avoid overlap. In LC filters, the parasitic capacitance of the inductor and the parasitic inductance of the capacitor can form high-frequency resonant points. Therefore, components with good high-frequency characteristics should be selected, such as ferrite beads instead of inductors and MLCCs instead of electrolytic capacitors.

A damping resistor can be connected in parallel across the inductor, or a small resistor can be connected in series with the capacitor to increase the damping at the resonant point and reduce the Q value. For active filters, phase compensation in the feedback network is critical, ensuring system stability at all frequencies. Frequency domain analysis using simulation software such as SPICE can identify potential resonant points in advance. In actual testing, a network analyzer can be used to measure the filter's transmission characteristics and observe for abnormal gain peaks. Proper layout to reduce parasitic parameters in the inter-stage connections can also effectively suppress resonance. The final design should have a smooth attenuation curve within the desired suppression frequency band.