I am using a NI PCI 6251 multi functional DAQ card to acquire an anolog signal from a floating signal source (a PMT, photomultiplier tube). The PMT is connected to a custom built circuit board through a BNC cable. The PCI 6251 card is connected to the circuit board using a matching connecting cable from NI. The circuit board is wired in such a way so that the center pin from the BNC cable (signal) is connected to channel AI 0 of the PCI 6251, whereas the ground of the BNC is connected to channel AI 8. AI 8 is not connected to a AI ground channel. DAQmx VIs used in the program. In the program, AI 0 is configured as "differential". Through simple testing, it seems like the program and the program are working: as we increase and decrease the signal, the DAQ card sees it, reads it and displays it. However, the noise level is huge and seems to increase as the magnitude of the signal increases. Also the noise doesn't seem to be random, instead, it is showing a zig-zag pattern. Couldn't figure out where this "patterned noise" is coming from. I was reading the manual for PCI 6251 yesterday. While reading it, I had the impression that when a channel is configured as "differential" and the signal is coming from a floating signal source, I need to connect the reference channel (in this case AI 😎 to an AI Ground. Does this has anything to do with the "patterned noise" I am seeing? Thanks!
