Posted on: July 1, 2014

## What is Differential Mode Noise

The differential mode noise is an electrical signal which appears in one or two of the lines in a closed loop. The noise appears on two conductors of a closed loop, it appears in series with the desired signal while the current flows in opposite directions. In a typical circuit, the current will flow in opposite directions, to and from the load.

In most rectifier circuits, asymmetrical currents may flow to the earth due to asymmetrical impedances. In an electrical circuit, connecting an electrical load to the power source creates a current loop through which energy transfer occurs. The current from the source flows through the load and then return back to the source.

An electrical noise can enter the current loop and degrade the signal. The differential mode noise occurs whenever only one of the two conductors in the current loop encounters a noise source. The electrical noise will then circulate within the loop. This is common when there are conductors that are not coupled closely inside the equipment’s wiring or noise is generated in the power source or within the components in the load.

Figure 1: Differential mode noise – Image Credit

Non-intrinsic differential noise in common mode switched mode power supplies is usually caused by ground currents. This type of noise voltage generated and is usually due to some in some imbalances in the circuit. It is important to understand how this occurs when troubleshooting any electromagnetic interference issues and when designing a filter to eliminate the noise.

Ideally, the current flowing through the closed loop should be the same, with the current to the load and that in the return path from the load to the source being of equal magnitude. However, these may not happen if there is a source of noise either at the power supply, within the load or generated by wires. The noise may interfere with the efficiency and proper operation of the circuit and should therefore be minimized or reduced completely either by eliminating the source of the noise of filtering out the noise signal.

### How to reduce Differential Mode Noise

Figure 2: Differential mode noise filter – Image Credit

There are different approaches towards reducing the interference signal. One commonly used method is to use a filter. This is achieved by connecting an inductor in series with either the high side or low side, or both, and a shunt (parallel) capacitor across the two lines.

A proper circuit design and use of using reliable components is another way of minimizing the noise generation in a circuit.