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As you all know, most Inst. Amp. examples on the Internet have a 'Differential Amplifier' part as shown below.

But, what would happen if a summing amplifier was used instead ? What kind of disadvantages I would have ?

Noise elimination and gain are very similar like Diff. Amp. does if my calculations are correct.

Any comment will be appreciated. Thank you for your consideration.

enter image description here

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    \$\begingroup\$ Um.. you would end up cancelling out your differential signal and amplifying the common mode noise instead... \$\endgroup\$ – Trevor_G Oct 12 '17 at 15:26
  • \$\begingroup\$ I think you need to look up the core reasoning in using a differential amplifier (differential mode amplification, common-mode rejection) \$\endgroup\$ – Joren Vaes Oct 12 '17 at 15:29
  • \$\begingroup\$ "... if my calculations are correct." Let's see the calculations! See my answer to electronics.stackexchange.com/questions/332390/… which may help. \$\endgroup\$ – Transistor Oct 12 '17 at 19:33
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With a differential amplifier you are removing the noise and amplifying the signal. With a summing amplifier you are removing the signal and amplifying the noise. This might be useful in some exotic application, but certainly not common.

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