Identifying Common-Mode and Differential-Mode Disturbance in Electronic Circuits

Key Takeaways

• How to identify common-mode disturbance.

• How to identify differential-mode disturbance.

• Comparing common-mode and differential-mode disturbance.

Disturbance interrupts the normal functioning of a circuit

Think of a time when someone disturbed you while you were working on something that required precision and focus. Your work was disrupted, right? In the same way, disturbance interrupts the normal functioning of a circuit. And, repeated disturbance is even capable of causing permanent damage to the electronic system.

Identifying the type of disturbance is the first step towards limiting it. The disturbance experienced in electronic circuits can be common-mode or differential-mode disturbance, inductively or capacitively coupled disturbance, or radiated noise. In this article, we will focus on common-mode and differential-mode disturbance and some mitigation methods to consider.  

Types of Disturbance in Electronic Circuits

In electronic circuits, rapid variations in the voltage and current generate electromagnetic disturbance called electromagnetic interference (EMI). EMI affects the source as well as nearby electronic systems.

There are various means by which the disturbance generated is transmitted from the source to the victim. According to the type of coupling path, electromagnetic disturbance can be classified as conducted disturbance, radiated disturbance, inductively coupled, and capacitively coupled disturbance. 

Conducted disturbance can be injected into either the currents or voltages in a circuit. The disturbance injected deviates the voltages and currents from the specification required for the proper and normal operation of the circuit. According to how the disturbance is conducted into the circuit, it is further classified into common-mode disturbance or differential-mode disturbance. 

Common-Mode Disturbance

Common-mode disturbance is conducted into a circuit via power lines or communication lines, which are common to the components in the circuit. Common-mode disturbance is any current or voltage disturbance measured between the power lines and ground. Whenever a disturbance source shares a common connection (usually a ground connection) with the victim, any current generating from the source flows through the common connection to the victim circuit. The impedance of the common line develops a voltage drop that disturbs the victim circuit. As the length of the common connection increases, its impedance per length increases and, in turn, increases the common-mode voltage disturbance. If the source and victim are different sections of the same circuit, a spurious feedback path is formed through the common impedance. Common-mode disturbance of this kind produces oscillations in the circuit. 

Common-mode disturbance is conducted in the same direction. Whether it is a power line or signal line, the conducted noise of common-mode disturbances flow in the same direction as in the ground. In the case of a DC power supply connected to an electronic circuit, the direction of common-mode currents remain in the same direction in the cables connecting to the positive and negative terminals of the supply. From the description above, we can conclude that common-mode disturbance never appears between the power lines and flows in the same direction. 

Differential-Mode Disturbance

When two electronic circuits are interconnected and current flows between the circuits in differential mode through cables, then the disturbance affecting such types of current and voltage is called differential-mode disturbance. For differential current flow, there will be ‘go’  and ‘ return’ wires between the circuits. Differential-mode disturbance appears in these go and return wires and take direction opposite to each other.

Compared to common-mode disturbance, there is no influence of the ground reference plane in differential-mode disturbance. The coupling path involved in differential-mode disturbance is two power lines or two signal lines. The radiated fields are also capable of inducing differential-mode disturbance in wires and cables present in electronic circuits. The closed-loop formed by the go and return lines experiences differential-mode disturbance. Differential-mode disturbance appears in series with the desired signals. 

Identifying Common-Mode and Differential-Mode Disturbance

The mitigation methods for common-mode and differential-mode disturbance differ. Generally, common-mode chokes are used for eliminating common-mode disturbance. Incorporating shunt capacitors between the power lines and grounds is also a mitigation method for common-mode disturbance. The combination of inductors in series with line and shunt capacitors between the differential lines is a common filter solution recommended for mitigating differential-mode disturbance. 

Filters designed for eliminating common-mode disturbance do not work well with differential-mode disturbance. The type of disturbance is an important parameter for designing the appropriate filter. Cadence’s suite of design and analysis tools can help you design filters to mitigate common-mode as well as differential-mode disturbance.

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