Noise is undesired fluctuation in the electrical signals or currents/voltages in electrical and electronic systems that can potentially affect the performance or even cause damage to devices and systems. According to the conduction mode, the noise is classified into two types: Differential mode noise and common mode noise.

Common mode noise: Common mode noise is the noise currents that flow in the same direction on both the signal and return line and complete the circuit through the parasitic capacitances and ground path (Figure 1). For example, in a signal cable, this noise current is conducted in the same direction on all the lines in the cable. The common mode noise is often referred to as common mode voltage (CMV).

Figure 1: Common mode noise (current) flow understanding

The common mode currents are equal in magnitude and same polarity (i.e., 0^o phase difference), which generate magnetic fields with equal magnitude and same polarity, which do not cancel each other out. This leads to Radiated EMI emissions from the conductors, which will interfere with the other nearby conductors/electronic devices in the surrounding environment. For example, the common mode noise in the Unshielded Twisted Pair (UTP) cable plays an important role in the generation of RFI (Radio Frequency Interference) in communications systems. The common mode noise currents can also be the source of conducted EMI, where it can create spurious conducted signals within a system that cause communications errors and malfunctions due to signal disturbances. 

Hence, the common mode noise currents can become the source of conducted and radiated EMI, potentially causing malfunctions or degradation of performance in electronic devices. For example, in the case of computer equipment, it can cause intermittent reboots, lockups, bad data transfer, component failure, and other failures/disturbances.

Differential mode noise: 

Figure 2: Differential mode noise (current) flow Differential mode noise is the noise currents that flow on the signal and return paths with equal magnitude and opposite directions (a 180° phase shift) to each other. This current does not flow through the ground path. The differential mode noise (also known as Normal mode noise) is the result of the normal operation of a circuit. 

Differential mode current means the current that flows on the signal and return paths (while circuit normal operation) with equal magnitude and opposite directions (a 180° phase shift) to each other, carrying a useful signal or desired information (Figure 2). In an ideal system, this opposite direction of current flow with the same magnitude on signal and return conductor; creates the magnetic field around each conductor with a 180° phase shift, which cancels out each other, resulting in NO radiated emission. 

But in a practical system, a noise source can be present either at the power supply or within the load or generated by wires. For example, SMPS output may have spikes (noises) due to fast switching of the high currents. These noises/currents could disturb the proper functionality of the electronic circuit. Also, these noise currents could emit unwanted radiation (predominantly in the form of magnetic field interference) by forming a loop antenna with the signal and return line. This unwanted radiated emission can interfere with other nearby lines/devices. These noises are called differential mode noises.   Since the signal loops are necessary for circuit operation, the loop size and area must be controlled during the stage of designing to minimize radiation/EMI (see Figure 3).

Both the differential and common mode noises can be caused by the high-speed switching operation of power semiconductor switches in power electronics converters, power switching from motor controls, circuit breakers or relays actuating, noise from electrical distribution systems, and AC line transients, such as line surges due to lightning strikes. 

Electronic equipment is 10 to 100 times more sensitive to common mode noise than the differential mode noise (normal mode noise). Also, when considering Microelectronic circuits, common-mode noise is more potentially harmful than normal-mode noise. A proper design and use of noise mitigation techniques will reduce the EMI and help to meet the EMC (Electromagnetic Compatibility) requirements of EMC standards.  An electronic product that meets the EMC standards requirements will operate properly in its intended electromagnetic environment and not cause any interference with other nearby electronic devices in the surrounding environment.

Radiated emission level (V/m) calculation for Common mode noise and differential mode noise:

Figure 3: Radiation due to common mode and differential mode currents, Blue dot represents the observation point The radiated emission or electric field intensity Ec of radiation or due to common mode noise current is expressed by using the following formula. Here, Ic -common mode noise current, r - is the distance to the observation point from the conductor, and f is the noise signal frequency (Figure 3). The formula shows that the radiated emissions caused by common mode noise currents are proportional to the frequency.  

For example, when a 100 MHz common mode noise of 1 µA flowing in a 20 m cable, the value of radiated emission level, or electric field intensity (Ec) at a distance of 1m (90^o) is: 

Figure 4: Observation point location, cable length and other details. Similarly, the radiated emission or the electric field intensity Ed of radiation due to differential mode noise current is expressed by using the following formula. Here, Id - differential mode noise current, r - is the distance to the observation point, f is the noise signal frequency, and S is the loop area (Figure 3). The formula shows that the radiated emissions caused by differential mode noise currents are proportional to the square of the frequency.  

For example, when a 100 MHz differential mode noise current of 1 µA flows in a loop area of (0.2 x 0.01 m²) cable (Figure 4), the value of  radiated emission level, or electric field intensity (Ed) at a distance of 1m (90^o) is: 

The above calculation results of Ec and Ed show that for the same noise current values, the radiation due to common mode noise current is far greater (in this example, approximately 100 times greater). Hence, common-mode noise is more potentially harmful than differential-mode noise (or normal-mode noise).

The following table provides the radiation emission limit specified by regulatory standards like FCC and MIL –STD 461 for the maximum allowable common mode current in a 1-m long cable at 50 MHz. 

Common causes of common mode noise and mitigation methods:

Electromagnetic Interference (EMI): When Radiated EMI is coupled equally on all lines from external sources, such as nearby electronic devices, power lines, or other electronic equipment, can induce common mode noise in a circuit. Shielding sensitive electronic devices/circuits from noise sources will help to minimize Radiated EMI issues.  

Ground Loops: When there is more than one ground reference point in a system, it can lead to ground loops. Ground loops can cause common mode currents to flow, resulting in noise. Proper grounding practices are essential to minimize ground loop issues

Poorly Shielded Cables: Inadequate/poor shielding of cables can make them susceptible to external EMI, leading to common mode noise. Shielded cables are commonly used to minimize this effect.

Switching Power Supplies: The switching action in power supplies (e.g., SMPS) can generate high-frequency common mode noise on conductors. Filtering and proper design techniques are employed to reduce the impact of this noise.

Voltage Spikes and Surges: Sudden voltage spikes or surges on the power lines can induce common mode noise in electronic circuits. Transient protection devices are often used to mitigate the impact of such events.

Common Mode Chokes: Common mode chokes can be used to mitigate common mode noise (Figure 5). The choke is formed by winding the signal or supply wires to one ferrite core. When the common mode current (noise) flows through the wires, the magnetic flux caused by this current is accumulated (i.e., NOT cancels each other), providing high impedance to this current flow. i.e., The common choke works as an inductor against common mode current (noise) and provides high impedance against common mode current. Thereby, it suppresses the common mode noise.   

While common mode chokes are often used to mitigate common mode noise, they can also be a source of problems if poorly designed or if their impedance characteristics are not suitable for the application.

Figure 5: Common mode noise suppression using common mode choke

Note: In the common mode choke, when differential mode current (useful signals) flows through the wires, the magnetic flux caused by this current does NOT cancel out each other, nor provide any impedance to current flow. i.e., The choke works as a simple wire and allows the flow of differential mode current (useful signals). 

Poorly Designed PCB Layouts: The layout of a printed circuit board (PCB) can affect common mode noise. A well-designed layout considers the placement of components, traces, and grounding to minimize common mode noise.

To address common mode noise signals, designers often employ techniques such as using twisted-pair cables, use of common mode chokes and filters, adding common mode inductors or ferrite cores/ferrite beads on power cables, and implementing proper grounding and shielding practices.

Note: The differential mode noise can be suppressed by installing a standard filter (e.g., LC filter) on the power supply lines.