Agreed, and we've tried that with other USB DAQ systems. The +5 rail is still referenced to the inferior ground. You can split that guy out too, and bring in external power; but then there's the question of the signal lines - to whom are they referenced, and will they function reliably?
It's so much more satisfying to know that you have truly isolated connections than to worry about whether you've gotten a complicated system laid out correctly. I don't trust me to get the details right :).
(As you say, for cleaning up AC from the mains, isolation transformers are fabulous.)
It's so much more satisfying to know that you have truly isolated connections than to worry about whether you've gotten a complicated system laid out correctly. I don't trust me to get the details right :).
(As you say, for cleaning up AC from the mains, isolation transformers are fabulous.)