Measured effectiveness of a toroid choke in reducing common-mode current

After a brief discussion of the origin of common-mode currents in interconnect cables, the author discusses the theoretical basis for the effectiveness of common-mode chokes for reducing the common-mode current while leaving the desired differential-mode current undiminished. A method for measuring their actual effectiveness is presented along with measured values for a typical commercial unit. For effective suppression of common-mode currents, the return conductor for a given signal should always be connected to a coil which is closely coupled to the coil carrying the output signal current. For this reason, it is doubtful that effective common-mode current reduction can be achieved in a cable carrying a very large number of signals on parallel conductors unless the signal return paths are relatively well-isolated from one another. This means that great care should be used in laying out the input circuitry of peripheral devices to keep the return currents decoupled.<>