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Reading slightly off in differential mode in NI 9205

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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.

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Accepted by topic author sharmaa

Hi sharmaa-

 

     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!

Gary P.
Applications Engineer
National Instruments
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