Differential-Mode Noise
Interference that flows in opposite directions between two conductors, that is, along the outgoing and return lines. It rides directly on top of the signal or supply voltage and is visible in measurement. To suppress it, a capacitor is placed between the lines and an inductance in series.
Differential-mode current follows the same path as the normal load current; the difference is that it carries an unwanted frequency component. The switching pulses of a brushed motor, the current ripple of a PWM driver and relay switching are typical sources.
- X capacitor: a capacitor connected between the positive and negative lines offers the high-frequency component a short-circuit path close to the source.
- LC filter: when a series inductance in the line is used together with a capacitor between the lines, suppression improves markedly over a capacitor on its own.
- Telling them apart: if the noise appears when you measure between positive and negative with an oscilloscope it is differential; if it appears simultaneously between both lines and chassis it is common mode.
This is one of the functions of the capacitors at the supply input of DC motor drivers; as the cable gets longer the line inductance rises, so the ripple at the driver input grows. Placing the driver close to the battery or shortening the input cable is the step to try before adding a filter.
The context in which this term is used: Installation, redundancy and field continuity