07-29-2010 04:24 PM
I have a LVDT connected in differential mode in NI 9205 which is in NI cDAQ 9172. The full scale output signal is -10 VDC to + 10 VDC. The excitation voltage is +- 15VDC. I have positioned the LVDT to read -1400 mVDC on the digital multimeter (DMM), well it fluctuates between -1399 mVDC and -1400 mVDC. When I read that in Labview 8.5 the reading is around -1.410 VDC, fluctuates between -1.407 VDC and -1.414 VDC. I know a 10 mVDC offset is not that much when my full scale output is -10 to 10 VDC and I am not losing my sleep over it. But I just wanted to know if this was normal.
The LVDT output signal is ground referenced (- terminal of the excitation and - terminal of output signal are tied; this is required) and I have connected this ground to COM port of NI 9205.
I also have a pressure transducer with floating ground whose output is 0 - 100 mVDC. I have connected this in differential mode too The + and - terminals of the output are connected to the ground/COM port using 100k ohm bias registers. In this case there is no offset at all. The DMM and Labview show the same reading. By the way I am simultaenously viewing the readings in Labview and DMM.
Solved! Go to Solution.
07-30-2010 04:29 PM
You can figure your offset error using pages 23 and 24 of the NI 9205 Operating Instructions and Specifications. 10 mV seems a little high for that particular card, but as you said, 0.1% is not that bad. I would say you are right not to lose sleep over it.
Have a great weekend!