Eliminating ghost voltage and breaking ground loops in a system
That ghost voltage is caused by the switch mode power supply's internal EMI reduction capacitors AND it is likely that the SMPS you have used either doesn't require an earth connection for safety or EMC reasons.

If the SMPS did use earth then it would route EMI reduction capacitors directly to earth instead. This would kill-off the ghost voltage you are seeing.

As an alternative, try earthing the negative SMPS output lead. 

You should also consider operating the DAQ with balanced inputs so that any common-mode earth voltage problem is much alleviated. I know this means halving the number of avaliable channels (in some DAQs) but you will get better measurements and in the collection of telemetry information most of the industry I know of goes down the differential route if they can (even if the signals to be measured are single ended).