How to coordinate internal and external filtering of I/O modules

Effective coordination of internal and external filtering in I/O modules requires adherence to tiered processing and impedance matching principles. External filtering, located at the chassis inlet or module terminals, primarily addresses strong interference from the power grid or long cables, such as surges and EFTs. Components with high energy tolerance are essential, such as the 14D series varistors and gas discharge tubes. Internal filtering, located on the PCB, mainly handles noise generated by the module itself and high-frequency components remaining from external filtering. Components prioritizing high-frequency performance and size are crucial, such as the PBZ series ferrite beads and MLCC capacitors.

Impedance mismatch between internal and external filters is necessary; the external filter should present a high output impedance to the internal filter, while the internal filter should present a low input impedance, resulting in greater attenuation. During design, it's important to note that large external capacitors may affect the stability of the internal active filter. Conducted interference testing allows observation of the individual and combined effects of internal and external filters, enabling synergistic optimization. The clock spreading technique of the internal digital circuitry can be used in conjunction with external filtering to disperse narrowband spectrum energy, making it easier for the filter to attenuate.