Sigrity – Tip of the week: Common Mode vs. 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.

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. Click here to know more detail on this.

Team SimTech

Cadence Design Systems