Arduino: How to read sensor without common ground?
You could use a digital isolation technique that can power a remote ADC. ADI have some chips that can do this: -



The ADuM5401 provides bi-directional isolated digital lines (SPI) and a little bit of isolated "juice" that can be used to power the ADC (AD7793). This is just a general idea that I have used before but if you check out what ADI have to offer on the different range of ADC chips (AD77xx) you will find other options like thermocouple inputs, RTDs etc..

You can of course use an isolation amp like this: -



But the slight down side is that you need to provide isolated power using an external DC/DC converter. Here's a clearer picture of the internals of one type of isolation amplifier: -