A common mode choke is a passive device consisting of two wire coils wound on a magnetic core that block (choke) signals common to both lines in the device while being a low impedance to signals that are unique to one line or the other in the device. Common mode relates to both wires being wound as coils on the core. Due to the transformer-like coupling effects that occur in such a configuration, a common mode choke passes through differential signals or signals that are the same amplitude but opposite polarity, but choke common mode signals. Common mode signals are signals that appear on both inputs or outputs simultaneously of the same level and polarity, like electromagnetic interference (EMI). Such signals will cancel each other out in this configuration.

Common mode chokes can be used for power supplies. They tend to have laminated iron cores or ferrites and are used with capacitors to create low pass filter circuits for applications like removing the ripple on the output of bridge rectifiers. Toroidal ferrite core chokes are used in switch-mode power supply design to reduce the higher frequency output ripple. RF common mode chokes are also available and are different from the power or audio style chokes in that they are smaller, optimized for suppressing radio frequency interference (RFI).

Common mode chokes can be used in conjunction with transmission lines and high-speed serial interfaces like USB2.0 and HDMI for broadband noise suppression. They typically have very small size, can be surface mount (SMT), and can have low insertion loss up to several GHz for differential signals, whilst maintaining high impedance for common-mode noise over the same range.

A key characteristic of common mode chokes is that only noise power (common mode) is dissipated in the core material. This means that the only heating in a common mode choke is the wire losses in the copper relating to the differential current flow.  In reality, materials and physical characteristics of these devices are not perfect, and they operate at less-than-ideal performance.