Understanding Common Mode vs. Differential Mode Signals

Key Takeaways

• Conducted noise can be in the form of either common mode signals or differential mode signals. For both common mode and differential mode signals, two lines are required for the propagation of signals.

• Parasitic capacitance between circuit components and between the components and ground is the leading cause of common mode noise in circuits. 

• Decreasing the loop area is the best solution to reduce differential mode noise. 

 Noise in electronic circuits can be from either common mode or differential mode signals

Common mode and differential mode signals are associated with both op-amps and interference noise in circuits. Common mode voltage gain results from the same signal being given to both the inputs of an op-amp. If both signals flow in the same direction, it creates common mode interference, or noise. Differential mode is the opposite of common mode, in that the direction of the signals are different.

Let’s take a closer look at common mode vs. differential mode signals in the context of interference and noise.

Comparing Common Mode vs. Differential Mode Signals

Electromagnetic interference is either radiated or conducted from the source to the victim, and conducted noise can be either common mode signals or differential mode signals. Both common mode and differential mode signals require two lines to propagate the signals. 

Common Mode Signals

With common mode signals, the signals flow through the two lines in the same direction and phase.

When applying the right-hand thumb rule, remember:

1. The magnetic fields associated with common mode signals might be opposite to each other.
2. Common mode signals find a return path through the common ground.

Parasitic capacitance between circuit components and between the components and ground are the leading cause of common mode noise in circuits. Parasitic inductance exacerbates the generation of common mode interference in circuits. 

Differential Mode Signals

Consider two wires in close proximity, carrying signals in opposite directions. These signals correspond to differential mode signals. Differential mode noise or interference in electrical equipment is measured between the line-to-line voltages and line currents. This noise is transmitted through the input lines to the utility system. The stray capacitance between the circuit components and the magnetic coupling between the circuits can generate differential mode noise. 

How to Fix Common Mode and Differential Mode Noise

Differential interference is dominant at lower frequencies, and as the frequency increases, common mode noise is more active. The following solutions can help reduce differential mode noise:

• Choose the right capacitor - Choosing the capacitors in a circuit with low parasitic inductance and resistance can help minimize differential mode interference. On the input side, a series inductor and parallel capacitor can attenuate the differential mode interferences. 
• Reduce the loop area - Decreasing the loop area is the best solution to reduce differential mode noise. Twisted-wire cables are an alternative to two separate cables to reduce the loop area in circuits. EMI filters help minimize both differential mode and common mode noise. 
• Common mode chokes - Common mode interference reduction is possible by reducing the length of the cable. Common mode chokes are effective at reducing common mode interference. They operate as inductors against common mode noise and work as simple wires against differential mode noise. 

Suppressing Common Mode Noise With Common Mode Chokes

In industrial and telecommunication systems, common mode chokes work to suppress common mode noise. A common mode choke is an inductor with more than one coil wound around a single magnetic core. The windings share a series connection between each other. When energized, the common mode choke offers high impedance to noise signals. The wanted signals pass through the coil, whereas the unwanted signals are attenuated using the common mode choke. Common mode chokes are designed for low resistance, leading to limited power loss.

It is important to understand the difference between common mode vs. differential mode signals and how they cause interference in electrical and electronic circuits to build a better-performing system. Cadence’s software can help design filters suppressing common mode and differential mode noise.

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