The concept of an EMI filter
An EMI filter (Electromagnetic interference filter) is an electrical device or circuit used to suppress or filter out high-frequency noise current present on power and signal lines, i.e., it suppresses the conducted EMI on power and signal lines. By filtering out the noise current, the EMI filter protects sensitive electronic devices/systems connected on the line from harmful impacts of such noise current and ensures the reliable operation of the devices. This filter can be incorporated on a printed circuit board or utilized as a separate, standalone device.
Generally, the EMI filter is designed by using passive components such as inductors, capacitors, and resistors. But active filters are also available that use active components (e.g., transistor, Op-amp, etc.) with an external power supply. This filter works as a low-pass filter that filters out high-frequency currents present on power/signal lines and passes desirable currents to flow freely.
The EMI filter is also called an EMC filter (Electromagnetic Compatibility filter) or EMI suppression filter. When it is used to suppress noise in a power line, it is known as an EMI power line filter that allows power frequency (DC or 50/60/400 Hertz) current while blocking high-frequency noise.
