I am reading this article (https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf) in order to understand more how traditional instrumentation rejects common-mode voltage and why op-amp can't.
With this diagram, the article states "Because of feedback applied externally between the output and the summing junction, the voltage on the “–” input is forced to be the same as that on the “+” input voltage. Therefore, the op-amp ideally will have zero volts across its input terminals. As a result, the voltage at the op-amp output must equal VCM, for zero volts differential input." I don't understand how the output happened to equal the common-mode voltage. The output should equal to zero, as Vin is zero. Do I miss something here?