Both of these are the form of conducted electromagnetic interference (EMI), which can be generated or emitted by electrical and electronic equipment (e.g., switched mode power supply) during operation or other natural causes, potentially affecting other connected or nearby devices. In most circuits, both CM and DM noise currents are typically present.

It is important to note that the common mode noise is a primary contributor to radiated emissions from a device or conductors/cables. Proper mitigation strategies are essential for ensuring electromagnetic compatibility (EMC), i.e., ensuring equipment operates reliably without causing interference to nearby devices or systems.   Let’s understand these noises in more detail.

Understanding common mode noise:

Common mode (CM) noise refers to noise currents that flow through both the signal and return lines in the same direction, with equal magnitude and phase, with respect to a common reference ground. These currents complete their circuit through parasitic capacitances and the ground path (Figure 1). The common mode noise does not contain any useful information and is often referred to as common mode voltage (CMV).

Figure 1: Understanding common mode noise currents flow and its radiated emission

The CM noise currents can disturb the operation of other connected equipment on the same power line and/or signal lines. For example, in the case of a computer, it may cause lockups, bad data transfer, component failure, and other failures/disturbances. Also, When CM noise currents flow through power lines or conductors, they generate a magnetic field around the conductors, which leads to unwanted radiated emissions. This radiation emission from the conductors could interfere with the performance of other nearby devices or radio communication systems (e.g., AM radio), Figure 1. 

Note: The magnitude of CM currents is usually small as compared to differential mode (DM) currents, but they can be the main cause of radiated emissions from power lines, unshielded cables, and electrical and electronic equipment. Differential Mode (DM) current refers to the normal operational current that carries useful information or desired signal and flows through signal and return lines with equal magnitude and opposite directions.

Common causes of common mode noise:

• Differences in ground potential between connected devices create ground loop issues, leading to CM noise currents. 
• Ungrounded sources (e.g., ungrounded remote power supply)
• External electromagnetic radiation disturbance from nearby devices or RF transmitters can induce common mode noise voltage/current in power and signal lines/unshielded cables through inductive/capacitive coupling. It leads to crosstalk in the electrical circuit/system. 
• Switched mode power supply (SMPS), non-linear load operation (e.g., motors)
• The other causes of common mode noise currents include switching power supplies, non-linear load operation (e.g., motors), power utility switching, noise from electrical distribution systems/grid, high-speed switching operations of power semiconductor switches in power electronics converters, and AC line transients, such as line surges due to lightning strikes.

Calculation of radiated emission level (V/m) caused by common mode noise currents:

As discussed above, common mode noise currents can cause radiated emissions when flowing through the power line or unshielded cables. Figure 2 shows radiation caused by CM mode currents flowing through the cable.

Figure 2: Radiation caused by CM mode currents flowing through the cable

The radiation level or electric field intensity (Ec) at a point located at a distance r from the cable is calculated by the following formula.Here, Ic -common mode noise current magnitude, r - is the distance to the observation point from the conductor, and f is the noise signal frequency. The formula shows that the radiated emissions level (Ec) due to common mode noise currents is proportional to the noise current magnitude (Ic), signal frequency (f), and line/cable length (L).

 Sample Calculation 1: When the common mode noise of 1 µA with frequency 100 MHz flowing in a 20 m cable, then the value of radiated emission level, or electric field intensity (Ec) at a distance of 1m (90^o) is:

Figure 3: Calculation details for radiated emission due to CM currents

Solutions for common mode noise:

• Proper grounding practices reduce common mode interference by ensuring consistent ground potential across devices. 
• Installing common mode chokes on power or signal lines suppress common mode currents by offering high impedance to noise currents while allowing normal differential signals (i.e., normal signals) to pass. 
• Using shielded cables helps block external radiated EMI, preventing inducing common mode currents in the cables due to external EMI.
• Using shorter cables also minimizes CM noise 
• Placing Ferrite beads around cables suppresses high-frequency common mode noise flowing through the cables by absorbing and dissipating the noise energy into the surrounding environment.
• Using a power line filter can suppress both CM and DM noises.
• Surge protective devices protect against transient voltage spikes that can cause common mode EMI.
• Proper PCB layouts, such as placement of components, traces, and grounding practices, can minimize CM noise.

Understanding differential mode noise:

Differential Mode (DM) current refers to the normal operational current that flows through the signal and return lines, carrying the useful signal or information. The currents in both lines have the same magnitude but flow in opposite directions (180° phase shift).

Figure 4: Understanding differential mode noise (current) flow

Differential mode noise current is the noise current that arises due to the normal circuit operation. It flows in the same direction as the power supply current, i.e., it also flows through the signal and return lines with equal magnitude and opposite directions, but it is unwanted noise. This noise current does not take the ground path. Differential mode noise is also referred to as normal mode noise because it originates from the normal operation of the circuit.

The differential mode noise current can be caused by noise on the power line, a noise source present either at the power supply side (e.g., SMPS) or within the load, variable speed drive control, and harmonics of digital logic circuits.  

Figure 5: Differential mode radiation from a PCB

For example, the output of a switched mode power supply (SMPS) may contain harmonics currents at switching frequency, which leads to differential mode noise current. This noise current can affect the performance of the connected electrical and electrical equipment. Additionally, when these noise currents flow the signal and return line, creates a loop antenna, resulting in unwanted radiation emission from the conductors. This unwanted radiated emission is known as differential mode radiation that can interfere with other nearby conductors/devices. Since signal loops are essential for circuit operation, the loop size and area should be carefully controlled during the design stage to minimize radiated EMI.

Note: In an ideal system, the equal magnitude currents flowing through the signal and return conductors in opposite directions create magnetic fields around each conductor that are 180° out of phase. These opposing fields cancel out each other, resulting in NO radiated emission.

Calculation of radiated emission level (V/m) caused by differential mode noise currents:

As discussed above, differential mode noise currents can cause radiated emission when flowing through the transmission line. Figure 6 shows radiation caused by differential mode currents. 

The radiation level or electric field intensity (Ed) at a point located at a distance r from the conductor is calculated by the following formula.

where, 

Id – magnitude differential mode noise current
r - is the distance to the observation point
f is the noise signal frequency
S is the loop area.

The formula indicates that the radiation level, or electric field intensity (Ed), is directly proportional to the square of the frequency, as well as the magnitude of the noise current and the loop area.

The above formulas are taken from the book titled - Electromagnetic Compatibility Engineering, Author: Henry W. Ott, Publisher: John Wiley & Sons. 

Sample calculation 2: When the differential mode noise current of 1 µA with frequency 100 MHz flows in a loop area of (0.2 x 0.01 m²) cable (Figure 6), the value of radiated emission level, or electric field intensity (Ed) at a distance of 1m (90^o) is: 

The above sample calculation results show that the common mode radiation level (Ec) is greater than the differential mode radiation level (Ed) for the same noise current value (i.e., 1 µA). Hence, common-mode noise is more harmful than differential-mode noise (or normal-mode noise) in electrical and electronic systems. 

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

Electrical and electronic equipment must comply with the conducted and radiated emission limits and immunity requirements specified by the relevant EMC standards (like FCC Class A and MIL-STD-461). 

How to mitigate differential mode noise currents?

• The differential mode noise can be suppressed by installing a standard filter (e.g., LC filter) on the power supply lines. 
• Use of twisted pair cable reduces loop area, reducing differential mode noise. 

Note: Power line filters are commonly used in electrical and electronic systems to suppress both common mode and differential mode noise, in addition to proper system design. The figure shows a general power line filter topology, featuring an X capacitor for attenuating differential mode (DM) noise, and a common mode choke and Y capacitors for attenuating common mode (CM) noise. 

Figure 7: General power line filter topology with typical component values

 Common mode noise vs. differential mode noise: