equivalent input noise

Hi can anyone pls explain to me what equivalent input referred noise is. I found this
"A microphone converts acoustical energy to electrical energy. Microphones have some level of electrical noise at their output. This noise may have contributions from random diaphragm movement, thermal noise, or a dozen other sources, but those can all be thought of as an imaginary acoustic noise source injecting sound into the (now noiseless) microphone."

I also found "Equivalent noise simply represents a noise source (current, voltage, or resistor) at the input, output, or any other terminal of a noiseless circuit, which replaces the existing (real) noise at the respective otherwise open terminal of the real circuit.

Hence the equivalent input noise is identical (or equivalent) to the input noise of the open input circuit (meaning: no other noise sources present than that/those from the circuit itself); accordingly the same is valid for the open (no load) output, or any other open terminal of the circuit."

Pls correct me if wrong. I can see that initially a noisy circuit has normal input current/voltage source and the output noise is generated by the circuit. So equivalent input referred noise is nothing but considering that output noise that was generated by the noisy circuit is now a input noise current/voltage source which is given into a noiseless circuit..My doubts are

1. we do noise anaylsis to measure the output noise. so even before knowing the output noise how can we assume the current/voltage source taken is equivalent to the output noise. (it can be more or less right). How can we set it to the equivalent measure of noise?

2. Even if we set the input noise source fine. What are we gonna determine from a noiseless circuit. After all i want to know how much noise the circuit (its components) has. So considering it as noiseless and just injecting noise how is it anyway different from injecting normal current/voltage source to a noisy netwrok. How is it different???
Its so confusing.. Can anyone pls help out!!!

Just view the equivalent input noise as the output noise divided by the total circuit gain.
No need to get more complicated than that.

Oh thank you for the reply!!! I actually need to do noise analysis in cadence for my circuit. So the final noise of a circuit is it equivalent input noise or equivalent output noise. I am having options to choose between these two. so which one gives us the actual noise of the circuit we r looking for???