I am intending to measure a quadrant photodiode output voltage by a PCI 6040E connected to a BNC2110. I have just found that the voltage in test panel (by NI max or Labview) is a little higher than that I measured by a voltmeter. I have some questions:
1. Does this DAQ hardware has a offset? can I change it to zero? or it is such kind of amplification of voltage?
2- Quadrant photodiode has connected to a power supply with a ground. Which is better in my case, the floating or ground measurement? I have just tried both, but there is no different between amounts of measured signal. In case of ground, Do I need to connect the ground of power supply to some places like BNC 2110, or computer?
You can perform a Self-Calibration from Measurement and Automation Explorer. This will -probably- correct for temperature influences in the measured signal. If the signal is still different from the multimeter and you are sure that the multimeter is correct, a calibration of your DAQ device might be needed (http://sine.ni.com/nips/cds/view/p/lang/nl/nid/207957)
When it comes to grounding: if the signal source is grounded as in your case, your DAQ device should have a differential or single-ended non-referenced input configuration to avoid ground loops. Connecting a ground wire from the photodiode to the BNC connector is not needed. Have a look at: http://www.ni.com/white-paper/3344/en/