Common-mode input definition — op-amp inputs tied to the same source: When both inputs of an op-amp are connected to the same signal source (i.e., the same waveform appears at v+ and v−), is this a common-mode input condition?

Introduction / Context:
Common-mode and differential-mode inputs are fundamental in understanding what an op-amp amplifies versus rejects. Noise immunity, instrumentation amplifier design, and sensor interfacing depend heavily on these definitions. This item asks you to recognize the common-mode condition: the same signal applied to both inputs simultaneously.

Given Data / Assumptions:

• Both op-amp inputs see the same time-varying signal.
• Linear operation assumed; no saturation.
• Focus on definition, not on numeric rejection performance.

Concept / Approach:
Common-mode input is the component of the input that is identical at both terminals. Differential input is the difference between them. An ideal differential amplifier responds only to the differential component, rejecting common-mode signals (output ideally zero for pure common-mode). Real devices have finite CMRR, so some common-mode leaks into the output, but the definition of common-mode remains the same.

Step-by-Step Solution:

Verification / Alternative check:
Instrumentation amplifiers specify high CMRR because they must measure small differences riding on large common-mode voltages, precisely the case of identical signals at both inputs plus a tiny difference.

Why Other Options Are Wrong:

• Incorrect: Contradicts the definition of common-mode.
• Only true for differential pairs without feedback: Feedback does not alter the definition of common-mode.
• Only true for DC offsets: AC or DC can be common-mode if identical at both inputs.

Common Pitfalls:
Confusing “what is common-mode” with “how much is rejected”; the former is a definition, the latter is device performance.

Final Answer:
Correct