What I hope is final word on subject, with more risk of appearing even stupider --- when I think of it, isn't what I want to do very similar to the logic in an auto-ranging multimeter? If so, what's so damned impossible, or stupid or unreasonable about it? Here we go, KE7GKP --- lay it on me. Waiting....
Seriously, it's all very esoteric. The concept of earth seems to be relative only, and there is no absolute standard earth other than some arbitrary less than a higher potential. What I read about differential input in an adc seems to my poor old brain to push the idea of being able to select a range in which the bottom input is still higher than the chip earth. If I've got it that wrong maybe I should take up something simpler before I electrocute myself.