Before revision: Applications based on subtracting from In these applications, the output voltage is inserted with an opposite polarity in respect to the input voltage travelling along the loop (but in respect to ground, the polarities are the same).
After revision: Applications based on subtracting V2 from V1 In these applications, the output voltage is inserted with an opposite polarity in respect to the input voltage travelling along the loop (but in respect to ground, the polarities are the same).