TINA/Spice/OPA627-DIE: OPA627

Jubayer,

I built up a circuit based on the Word document you provided.
The 10G resistor on the input will contribute no noise because the signal source in parallel with it will conduct the resistor noise to AC ground.
That leaves the feedback resistor and the op amp model to as noise contributors. To see if the model & simulator are okay, we will do some approximate noise calculations and compare the results to simulations.

The first step is to look at just the op amp noise interacting with the (noiseless) capacitors and the 1G feedback resistor.
The 1G feedback R is made noiseless by setting its absolute temp to -273.15C. This results in the output noise plot shown in the first plot below.
The first plot shows a max output noise amplitude of 1.6uV/rt(Hz).
It turns out the major output noise contributor is the input current noise interacting with the large impedances of the external C's and R.
Using the noise parameters from the datasheet, an approximate calculation predicts about 2uV/rt(Hz) of output noise.
This calculation ignored the op amp's input C's and R's. If they were included, it would lower the max noise to be closer to the simulated result of 1.6uV/rt(Hz).
So an approximate analysis of an ideal case (noiseless feedback resistor) gives results that somewhat agree with simulation.

Next, the 1G feedback resistor is made noisy by setting its absolute temp to 27C. It should contribute 4uV/rt(Hz) to the output noise.
Combining the approximated current noise (2uV) at the output with the 4uV feedback resistor noise predicts about 4.4uV/rt(Hz) of total output noise.
The second plot below shows a simulated max output noise of 4.37uV/rt(Hz), which is pretty close to our approximate calculations.
So it looks like the model and simulator are giving results that are close to some approximate calculations.

I hope this helps. Please let me know if you have any more questions.
Regards,
John

OutNoiseNoiselessRF.emf

OutNoiseNoisyRF.emf