Voltage drift while using ADC in USB 6341

[MountainEater]

Hello, I am using NI DAQ USB 6341 in ADC (channel AI0) for recording analog signal from Thorlabs PDA36A2 power diode. This PD has a BNC output and i tried connecting the negative terminal to ai0- and aignd. However, when i use RSE mode, the voltage keeps on increasing and then saturates. In differential mode, the voltage increases and then decreases again so after a minute, the data seems to be ok but it is not absolute value, which is what i require. Cant seem to fix the issue by changing connection of the negative pin from ai_gnd to ai_0-.  

[altenbach]

Maybe you need some bias resistors?

[AnalogKid2DigitalMan]

Probably wants a 50 ohm load instead of the high input impedance of the DAQ.

Try putting a 50 ohm resistor across the two input terminals of the DAQ.

-AK2DM