Tool/software:
For the noise performance of an amplifier for high impedance sources not only the 2.3nV/sqrt(Hz) performance but also the current noise density is important. For a Rs=1MOhm source resistance the induced voltage noise density from the JFET input current density is:
Vnd(Ind-JFET)=1e6*1.5e-12=1500nV/sqrt(Hz)
The voltage noise density of the source resistance themselve is:
Vnd(Rs)=4kT*1Mohm=sqrt(1.66e-20*1e6)=128.8nV/sqrt(Hz)
The BUF802 would be a bad choice as it have a nois figure at Rs=1MOhm:
NF=20*log10(1500e-9/128.8e-9)=21.3dB
If we look at the shot noise of the leakage current:
Ind(Ileak)=sqrt(2*q*Ileak)=sqrt(2*1.6e-19*3e-12)=0.98fA/sqrt(Hz)
The number from the shot noise is more than 3 orders lower than the sepcified number of the BUF802 JFET input.
I looked also at this post:
where the customer as for the current noise. TI support also post a noise simulation plot, unlucky with very low resolution and without vertical markers.
The current noise plot is not easy to read below 0.5pA/sqrt(Hz) and it should be mention that the voltage noise to the left and the current noise scale to the right do not match there zero axis. I am not shure what is going on here but the current noise density of the JFET seams very bad and also the 1.5pA/sqrt(Hz) could not be read on a input noise plot from another post. Anyway both make the JFET for 1MOhm sources look bad.
2nd board result:
