I'm using a DAQ from National Instruments. Testing my LabView software I noticed that when I plug a 1.5V DC battery in the analog input channels if I connect an RC filter (R=56 kΩ, C=10 uF) to the batter then my DAQ instead of measuring the expected voltage it measures a lower value, 1.22V, as if there was a load of about 300 kΩ. The declared imput impedance is instead 144 kΩ and this happens only if I measure the voltage in differential mode, if I measure it in RSE (Reference Single Ended) that doesn't happen (i.e. I read 1.5V). Can you explain why?
Furthermore: in the lower schematic, what is the purpose of the +2.5 Vref and of the resistors?