Russell,
It isn't just the device though is it ? 16 bits is STILL hard to obtain, in the presence of thermoelectric effects, magnetic induction, and thats before the converter noise.

I am probably interpreting this wrongly. This is from your website, my comments.

# PRECISION ON-CHIP VOLTAGE REFERENCE:
Accuracy: 0.2%
Drift: 5ppm/°C

2.5 bits gone, for one degree change, 7 bits in an industrial range. 17 bits left.

# ON-CHIP OFFSET/GAIN CALIBRATION
# OFFSET DRIFT: 0.02PPM/°C
Alright thats pretty stable !

# GAIN DRIFT: 0.5PPM/°C
5 bits of uncertainty ?

And now the converter is looking a little less good. We're back at 16 bits again - or worse!

Steve