6/1/16: Slide 9 - Noise Analysis in Precision Analog Designs

The op amp has very high gain, so when there's negative feedback, the voltage at the inverting input is fixed. Therefore any noise generated by R1 must show up at the output, being that it can't modify the voltage of the inverting input. Still, I tend to find it easier to use the simplified method shown in later slides, where you take the RTI noise due to these resistors as the noise of the parallel combination of the resistors connected to the inverting input (R1 and R2 in this case) and you use the noise gain to translate that noise between RTI and RTO.