Resistor Noise Calculation using FFT and Noise Analysis is different

Hello,

I am doing a simple simulation for a circuit noise and found that the FFT spectral measurement gives different results compared to noise simulation. This test bench shows the difference:

• The circuit is simply a 1kOhm resistor with enabled noise at (Fmin = 1, Fmax = 320kHz) and 300K temp.
• On running a noise simulation from 1mHz to 10GHz, and plot the output noise of the resistor, I get a noise floor of 20log(sqrt(4kTR)) V/sqrt(Hz) = -167.8 dB (which is correct with high accuracy).
• On running transient for 998.4msec at a fixed step of 1/320k sec, then measuring the FFT spectrum as shown below, I get the noise floor = -161.9 dB (nearly twice as much noise [i.e. additional 6dB]).

Is there a setting that I am missing?

OS: Redhat 7

Virtuoso: IC6.1.8-64b.500.15

sub-version  19.1.0.541.isr14

Regards,

Nader Sh.