The reason I'm asking this is that when I read about CMRR issue in amplifiers I only see differential amplifiers as if it is only a diff. amp. issue. But isn't a single ended amplifier also a differential amplifier where one input is zero with respect to the other input? Can we talk about CMRR issues when it comes to single ended inputs for a data acquisiton or it only applies to differential inputs?
Answer
does cmrr value make sense for a differential amplifier if its inputs are separate and totally different voltages?
Yes, it makes sense. Consider two independent voltage sources as the inputs to a differential amplifier:
simulate this circuit – Schematic created using CircuitLab
This is equivalent to:
simulate this circuit Where we've defined the differential and common mode voltages to be:
$$V_d = V_{in1} - V_{in2}$$
$$V_{cm} = \dfrac{V_{in1} + V_{in2}}{2}$$
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