I am still targeting 6 or 6.5 digits. I just posted here where this specific ADC and Opamp buffer was in discussion.
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If target would be 5 digit I would just add digital filter on top and would be there.
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My target is to monitor other references with a mux(MAX4051) and as such low impedance was not absolutely required, but I also want possibly to use it behind that, and as such high impedance is required. I have two temperature NTC rigged to the Arduino which I can log in parallel, and as such with single equipment I would be able to monitor multiple references and temperature variation with a single equipment.
I will test other AZ with this circuit. From the simulations I saw that Ad8628 is spread spectrum and current noise is all over the place, so maybe this is why I see noise. Since other AZ are constant current noise, maybe this will translate in constant offset which I can then get rid with calibration.
If this will not result will focus on the JFET with bootstrapping. Then I will need to change architecture, power supply, maybe negative and positive rail, and new boards. Maybe even LM399 as reference, but then other requirements for power supply. A lot of more work just to monitor a few references in parallel, and more cost as well.
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My immediate next step when I get back home is: Reduce C20 back to 2nF or something, remove 10nF capacitor between digital and analog ground to make sure it is not bringing noise from laptop to analog side; use other AZ; test speed of PC817 for SPI as I have a bunch around, and maybe can replace isolator with a solution based on it in case noise is traveling through the capacitive isolator