OK I don't have a DVM for the moment but I'll post my measurement as soon as I get one.
I really don't understand, I've been reading Campbell manuals for a week now and I didn't find anything about my problem.
Even better, the software provided with the datalogger is able to generate code automatically in order to make measurements with several widely known sensors, including the NRG #200 ; this is the code I used ! I tried to change some parameters, the range of the values, the excitation channel, but it's still the same...