I was learning about noise analysis of opamp with the help of this document.It says that for Noise Gain In op-amp circuits, noise is commonly assigned to the non-inverting op-amp input. But no explanation why.

 

What is the question, if any ?

 

The input noise is actually referenced to be in series with two input nodes.
You can put it in series with the inverting input if you like, but if you do the calculations you will see that it will still be amplified by the non-inverting gain for either the non-inverting or inverting configuration.

Simple example:
Put a 1mV noise source in series with the (-) input for the inverting (left) configuration with an inverting gain of 1.
Since the feedback volage is reduced by 1/2 at the (-) input (with the input resistor at AC ground), this means the output noise will be twice the noise source or 2mV, (thus amplified by the non-inverting gain of 2).

Make sense?

 

Understood thank you...