When I want to simulate the input referred noise of the op-amp, should I perform the test under open loop condition or under closed loop condition with unity gain buffer for example
I see that Allen holberg is simulating it fromstabilized open loop as shown in figure below
How are you measuring noise in this circuit? What's the purpose of Vin?
The open loop configuration can work with limited open gain, but you need to correct for frequency dependent gain, so it's at least inconvenient and inaccurate. Input noise can be best measured with finite high closed loop gain, depending on GBW and noise bandwidth of interest, e.g. G = 100 .. 1000.
They are same in small signal noise simulation.
However close loop configulation such as voltage follower is required in actual measurement not EDA Tool Play.
I think in the picture from Allen slides he missed to put the ac source on the plus_input.
Dear Fvm, For sure working with high gain open loop is not easy thing this is why he using the RC network to stabilize the circuit for DC but it will be open for ac.
It would be more easier to simulate it with closed loop, and as Pancho said my feedback should not add noise, meaning by simulation using ideal resistor network or the simple buffer connection........ indeed that is what I usually do until I read this slide so I just wanted to confirm from you...
Now According to your explanation I can do it from buffer connection... in this case the output noise is equal to the input noise (G=1)
G=1 closed loop configuration can be used, but in a real measurement you want to amplify the DUT noise above the noise level of the noise meter, which is typically not much below that of the DUT.