Common mode (CM) interference is a form of conducted electromagnetic interference (EMI) that can degrade the performance of or even damage the electrical and electronic equipment connected to power or signal lines as well as nearby devices/systems. It is caused by common mode (CM) noise currents that flow through the signal and return lines in the same direction with equal magnitude and phase with respect to a common reference ground. The CM noise currents do not carry any useful information and complete the circuit through the parasitic capacitances and ground path (Figure 1). 

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

For example, in the case of a computer, the CM interference or noise currents may cause intermittent reboots, lockups, bad data transfer, component failure, and other failures/disturbances. Also, it is important to note that when the CM currents flow via the power line conductors or unshielded cables, they create a magnetic field around the conductor with equal magnitude and polarity. These magnetic fields add together in the center region between the two conductors and lead to unwanted electromagnetic radiation emission from the conductors or unshielded cables (known as radiated EMI). This radiated emission can interfere with the other nearby conductors/electronic devices or radio communication systems in the surrounding environment. 

Hence, common mode interference not only affects the equipment connected to power or signal lines but also impacts nearby devices and communication systems through radiated EMI caused by CM noise currents flowing through conductors. It is important to note that the common mode noise current is a primary contributor to radiated emissions from a device or conductors/cables. The radiated emission caused by CM mode current is known as common mode radiation.

Common causes of common mode interference or common mode noise:

Ground Loop Issues: Differences in ground potential between connected devices can create common mode noise, particularly in systems with multiple, physically separated grounds. These differences in ground potential are the primary cause of common mode noise currents in electrical circuits. 

Ungrounded sources: It is the second most common cause of common mode interference. In some systems, particularly in industrial or remote applications, a field device (such as a sensor or actuator) may be powered by a separate power supply located away from the main control system. If this remote power supply is not grounded, differences in ground potential between the power supply and connected devices can arise, leading to common mode noise currents.

External electromagnetic radiation disturbance: Common mode EMI can occur when radiated EMI signals (i.e., unwanted external electromagnetic radiation fields) from nearby equipment are coupled to the conductors or unshielded cables of a system, particularly over long cable runs. The radiation signals from the external environment couple equally to both conductors or cables, resulting in CM noise currents flowing through them with the same direction, magnitude, and phase. 

Improper Shielding: Inadequate shielding of cables and nearby devices can allow external electromagnetic fields to couple into the cables, contributing to common mode interference.

Switching Power Supplies: High-frequency switching components in power supplies can generate common mode noise, which propagates through power lines or across the ground.

Other causes: Additional sources of common mode interference include conducted noise from electrical distribution systems/grid, power utility switching, loads being switched on and off, high-speed switching operations of power semiconductor switches in power electronics converters, and AC line transients, such as line surges due to lightning strikes.

Common mode radiation level calculation formula:

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

Let’s consider Figure 2 which shows radiated emission due to common mode noise current flow through the cable. The radiation level or electric field intensity (Ec) at the observation 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. From the formula, understand that the radiated emissions level (Ec) due to common mode noise is proportional to the noise current magnitude (Ic), signal frequency (f), and line/cable length (L). 

The common mode radiation level (Ec) is 100 times larger than the differential mode radiation level (Ed) for the same noise current flowing through the cables or conductors. Hence, common-mode interference or noise is more harmful than differential-mode noise (or normal-mode noise) in electrical and electronic systems. Differential mode radiation occurs due to the normal circuit current along with noise (which carries useful information) flowing through the main and return lines. The differential mode current flows through the signal and return lines with equal magnitude and opposite directions, and does not take the ground path (figure 3).

Figure 3: Understanding differential mode noise currents flow and its radiated emission Differential mode radiation level formula:

Figure 4 shows radiated emission due to differential mode noise current flow through the cable.  The radiation level or electric field intensity (Ed) at the observation point located at a distance r from the cable is calculated by the following formula.

Figure 4: Radiated emission due to differential mode noise

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

Here, Id – magnitude differential mode noise current, r - is the distance to the observation point, f is the noise signal frequency, and S is the loop area. From the formula, understand 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. 

Solutions for common mode interference:

Proper Grounding: Proper grounding techniques, including a single-point ground system or equalizing ground potentials between devices, can reduce the voltage differences that cause common mode noise.

Common Mode Chokes: Installing common mode chokes on power or signal lines helps suppress common mode currents by offering high impedance to unwanted noise while allowing normal differential signals to pass.

Shielding cables:   Using shielded cables helps block external EMI, preventing it from inducing common mode currents in the cables.

Shorter Cable Runs: The use of shorter cables minimizes the chances of external electromagnetic fields coupling into the conductors, reducing CM interference.

Ferrite Beads: Placing ferrite beads around cables helps suppress high-frequency common mode interference by absorbing and dissipating the energy. 

Filtering: Installing EMI filters/power line filters at the power supply and signal line entry and output port of a device (e.g., SMPS) helps to block or attenuate conducted noise before it affects system performance. 

Surge Suppressors: Surge suppression devices can protect against transient voltage spikes that can cause common mode EMI.

Properly designed PCB Layouts: A well-designed PCB layout, such as the placement of components, traces, and grounding practices,  can minimize common mode noise.