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An ungrounded differential signal source can float outside the common mode range of the logger inputs.  Possible ways to prevent this are:

• grounding one of the signal lines at the logger
• grounding one of the signal lines at the logger through a moderate value resistor, eg. 10k (http://www.ni.com/tutorial/7113/en/)
• grounding both of the signal lines through high value resistors, eg. 100k, 1M, 10M depending on the output impedance of the source (http://www.ni.com/white-paper/3344/en/)

I have seen the first suggestion in a CS pyranometer manual, but I have not seen any mention of this or any other methods in the logger manuals.  It would seem that the high imput impedance of the loggers would easily allow the a differential signal to "float" out of range. 

Should one of these methods be used with Campbel loggers?

Does the AM25T provide any paths to ground to prevent ungrounded thermocouples from floating out of range?

We recently saw this issue using a separate voltage source to power a load cell. The load cell output was mV/V, and we were seeing +8.123 volts on the low side, and +8.134 volts on the high side, and yes, the high impedence of the input channels on our CR300 kept these floating voltage out of measurement range. After reading some forums, I tied the low side of the voltage supply to ground on the CR300. This worked. However, it was a bit risky because I could not find documentation on our voltage supply, and it looks like it is from the 1980s.