24 bit ADCs are an item of specmenship. Only in the most ideal circumstances will you get away without an isolated supply and isolated data lines.

Next the type of configuration is a consideration. Ratiometric measurements are one thing where the reference must be stable for the period on one conversion. Measurements against a standard (eg. thermocouple) are a very different thing and here you will need a rock solid reference which will be pricy and component layout does-in many a designer.

I have been able to get 20 bit resolution against a standard with repeated single conversions. I don't know how to get 24 bit resolution without averaging, and then I am not yet convinced that the result would be meaningful.

As a side note, when attempting resolutions of this magnitude, do not plan on using surface mount resistors. Thermal warping of the board puts a mechanical strain on them which distorts their value.

TomB