Below is a link to a current loop isolator that Onset has tested with our data loggers.
Because the analog inputs on all HOBO analog loggers and modules are single-ended (returns are tied common), it is suggested to use loop isolation when monitoring multiple analog inputs from different power sources. This will help mitigate the possibility of electrical noise generated by a ground loop.
http://www.api-usa.com/pdf/api/api_dpi-2.pdf