I've designed a 5th order low pass antialiasing filter using the filter wizard. The design I obtained is based on the AD8022 dual opamp. I've constructed the circuit and the result I got today are alright.
I also need to model the circuit using SPICE (LTSpice) in order to compute its noise behaviour. However it seem that the SPICE model of the AD8022 as some error, or that my Spice simulator is not configured correctly.
What I did :
- Get the computed noise from the filter wizard. It predict a noise floor around 10 nV/sqrt(Hz) at low frequency, that is the correct one given the circuit configuration and the AD8022 datasheet (and my experimental results).
- download the AD8022 spice model, draw the circuit using it in LTSpice, and lunch a noise analysis. The noise is a very wrong 240 nV/sqrt(Hz) at low frequency.
- directly use the .cir file I got by clicking the "get design files" boutton on the last page of the filter wizard, also in LTSpice. The modeled noise is also 240 nV/sqrt(Hz).
Can you provide a corrected model for the AD8022, or help me detect a possible problem with my Spice simulator?
Attached file : output from the filter wizard and a screenshot of my Spice simulation