Integrated noise for opamp is not converged.

Hello,

I am simulating a traditional two-stage operational amplifier with miller and lead compensation. The DC gain is 70dB, and gain bandwidth is 580MHz, with the phase margin 115 deg.

After I achieve the specifications above, I start to simulate the integrated noise. The opamp is fully differential, the load is pure capacitor in each branch with the value 350fF. According to the first-order filter theory, the final integrated noise should be KT/C. For each branch, the integrated noise is 1.08e-4 volt.

When I use ADE L/print/noise summary to plot the integrated noise, the results are as follows,
When integrated band is from 1KHz to 10MHz,
total summarized noise = 0.132026V, total input referred noise = 6.20415e-5V.

When integrated band is from 1KHz to 100MHz,
total summarized noise = 0.132042V, total input referred noise = 1.10047e-4V. According to the simulation results, it seems the total summarized noise is converged, and the input referred noise is increasing when the band is higher (not converged).

When I show this results to my boss, he told me how the total summarized noise can be so large, it is around 132mV!!! For the opamp, the noise cannot be so high. Compared with the value of KT/C, it seems the integrated noise is huge.

    1. My first question, why the integrated noise of the opamp is so large. How does spectre calculate the input referred noise?

According to Ken's view from designer guide "the input-referred noise is the ratio of the output noise to the gain". When the integrated band is larger, the gain of the opamp is dropping, maybe it is the reason why the input referred noise becomes worse as integrated band is higher. But my boss told me when the band is larger, the gain of the opamp is dropping, the noise of the opamp at higher frequency is dropping as well, therefore the input referred noise should be converged too even for the infinite integrated band.

    2. My second question is how to get the converged integrated input referred noise if the integrated band is going to infinite. How Cadence Spectre calculate the input referred noise in ADE L->results->print->noise summary?

  
    I am confused now. Anybody can tell me give me some help. Thanks!