I don't understand why output is considered ground when calculating Vin's superposition at amplifier input pin.
http://www.ti.com/lit/an/slyt157/slyt157.pdfFrom the above TI notes, I learnt that I should apply superposition
Vn=(Vin-)*R2/(R1+R2) + (Vout+)*R1/(R1+R2) (1)
Vp=(Vin+)*R4/(R3+R4) + (Vout-)*R3/(R3+R4) (2)
Then based on the differential amplifier equation (Vp-Vn)*Adiff=(Vout+)-(Vout-), I can calculate the closed loop gain.
My question is about the superposition equations (1). Why can I assume Vout+ node is grounded when considering (Vin-)'s superposition, which give me Vn=(Vin-)*R2/(R1+R2). If the amplifier is OTA, the output node is high impedance.
Can anyone explain to me why in this superposition, I can obtain Vn=(Vin-)*R2/(R1+R2)? I can understand the other way when calculating Vn=(Vout+)*R1/(R1+R2) because Vin- voltage source should be grounded in Vout+'s superposition. What is missing in my understanding?