I am suspicious of the results because a baffle was not used. This looks a lot like the different results we got with the same part number at different times because the lead frame material changed resulting in very different thermocouple effects with the same assembly.
Is there a baffling technique that you would recommend?
Any minimum volume low profile enclosure over the operational amplifier and its feedback network works. I usually end up using card stock but fishpaper or a wax paper Dixie cup cut down to size works well also. The problem is convective air currents. I used to print out a 1:1 folding diagram with slots and tabs on card stock using an impact printer to make perfect little baffles.
I suspect the major improvement comes from preventing turbulent flow of the air as it rises.
Is there a particular IC that you think I should retest?
Any of the ones which showed high flicker noise. Pick one and test it again with and without a baffle.
I have not looked at all the chips but most list a maximum of 15nV / deg C for temp co. Of course if they used the wrong material in the lead frame then they could have any temp co.
That specification excludes thermocouple effects and represents the potential performance under ideal conditions. It does not include the thermocouple junctions between the leads and circuit.
If there was a 2 deg C peak to peak temperature change, then you could have 30nVpp noise from thermocouple effects. This would convert to around 5nV rms at a low frequency. I am seeing less than 1 deg C change over a test series.
External thermocouple effects will always ruin the low frequency noise performance of even a low noise precision bipolar part (0.1uV/C) with its higher albeit low 1/f noise never mind a chopper stabilized part.
The gain setting resistors are 0805 SMD parts with 10ppm / deg C. With the offset voltages I have and the limited temperature change, I have computed less than 1nV of noise from the gain resistors.
Resistor selection is of course important but you probably cannot go wrong with precision metal film or wire wound parts unless they are defective which sometimes happens. A thermally symmetric layout with phantom parts to equalize the number of junctions in series with both inputs can help.
Once upon a time I opened an old Zener reference, in a North Hills CC source, and it was full of sand. That's one way to kill convection currents and add thermal mass. However, I would rather not do that here.
Sand would sure work and it would dampen vibration. Potting in gel or epoxy also works. I am not so sure about immersion in oil but I know that has been done also. An air baffle is the simple if perhaps lower performance way and allows an easy comparison of performance with and without the baffle. In production make the baffle conductive for free shielding at the expense of more common mode capacitance.