Although the following was concerning the ADI SPICE model for the AD8676 opamp, this could be relevant to many other opamps:
Using PSpice, the noise from the AD8676 opamp model from ADI was much higher than the expected data sheet value of 2.8nV/rt(Hz).
It seems the explicit noise sources are contributing less than the thermal noise from each resistor. As shown below, by replacing every resistor (except the one used to generate the 2.8nV) with a resistor model fixed to absolute zero temperature, the broadband noise was reduced to around 3.5nV. This value was the sum of 1)thermal, 2) flicker and 3) shot noise of the input transistors, as shown in the attachment.
1) Is this a feature in many SPICE models?
2) Is there a method to remove the input transistor shot noise?
.SUBCKT AD8676 1 2 99 50 45
Q1 15 7 501 NIX
Q2 6 2 502 NIX
RE1 501 5 RZERONOISE 1.731E+03
RE2 502 5 RZERONOISE 1.731E+03
IOS 1 2 500E-12
* VOLTAGE NOISE REFERENCE OF 2.8nV/rt(Hz)*VN1 80 98 0RN1 80 98 16.45E-3HN 81 98 VN1 2.80RN2 81 98 RZERONOISE 1
*flicker noise *D5 69 98 DNOISEVSN 69 98 DC 0.6551H1 70 98 VSN 12RN 70 98 RZERONOISE 1
.MODEL RZERONOISE RES (R=1,T_ABS=-273.15)
Apologies for the delayed response. The AD8676 spice model is currently being modified. For the meantime, you may use the AD8675 model instead. AD8676 is a dual of AD8675.