Appendix E: Filter Design
The most effective filtering is placed on the PC board near the connectors. This applies to both signal lines and power input. Filters work by combining blocking action with diversion of noise currents. We block currents with high series impedance (resistors, inductors, or ferrites) and divert currents with low shunt impedance (capacitors). Generally, you can expect about 30 to 40 dB reduction in noise signal; however, you must be careful not to affect the desired signal too much. Points to keep in mind: It's always best to avoid creating the noise currents in the first place. Common-mode chokes on cables (such as clip on ferrites) are limited to about 10 dB reduction at the most. A filter is like a fence; know where the boundary should be. For I/O or power supply inputs, locate the filter at the connector. For noisy ICs, locate the filter as close to the noise source as practical. When designing filters and using filtering components, always remember the return path is part of the filter. If a filter (e.g., a capacitor) is placed close to the noise source but it then has a return path which is rather long to get back to the source, then it may not work very well even if the cap is physically close to the source of noise.
