The unexpected measurement may be caused by an incorrect setting of the analog input terminal mode configuration.
The default analog input terminal mode is differential (DIFF). To correctly measure a differential signal, use the AI+ and AI- pins of the device. For example, use pin 68 and 34 as shown in the figure below to measure a signal on the analog input channel 0 differentially.
It's also important in differential configurations to confirm whether you also require a connection to COM ground following the guidance in Table 1 of the document Field Wiring and Noise Considerations for Analog Signals
. As an example, the following DAQ device measuring a differential floating signal would need to have a channel connected to AI0 (+) / pin 1, AI8 (-) / pin 20, and COM, with each consecutive pair (pins 2 & 21, 3 & 22, etc.) for a given channel also connecting to a COM pin.
If your application requires single-ended measurements, configure the analog input channels to reference single-ended (RSE) or non-referenced single-ended (NRSE). See the NI R Series for USB Getting Started Part 2: Advanced
tutorial for information on how to configure the analog input terminal mode programmatically or through the project explorer.