Analoge input not constant/ NI 6221/ DAQmx

Question

Dear Sir
I use DAQmx (labview7.1).
I would like to use NI6221 to read 32 analoge inputs by using 4 input channels (ai0-ai3). 
Each input will read 8 signals by using digital output to select.
I try to look my result by giving constant input voltage to A/D (ai0-ai3).I use input terminal RSE, and NRSE.
My problems are
(1) The reading voltage are changing (some channel is increased and some channel is decreased).
(2) The reading voltage differ from the given value.
How should I solve these problems?
Best regards,
Thongchai

Answer 1

Thongchai,
In reviewing your code, I noticed that you are setting up and clearing your DAQmx tasks with every iteration of the loop, this could be causing you some problems in a couple of ways.  First, setting up and clearing the tasks takes some time, so you could be introducing delay and getting the wrong measured values as a result of this.  Second, when you clear the digital output task in the loop, the lines will return to a default state, which will likely not be the actual values you want -- this could be causing you to measure the wrong channels.  The best practice for using DAQmx tasks inside loops is to perform the setup (DAQmx Create Channel, DAQmx Timing, and DAQmx Start) and the DAQmx Clear outside the loop -- the only functions that should be inside the loop are the DAQmx Read and DAQmx Write.  Along with this, you will need to make sure you set up the timing for continuous sampling instead of finite.  Refer to the LabVIEW example finder and look at the DAQmx examples for continuous acquisition to learn more about this.
Next, you will want to use the Measurement and Automation Explorer (MAX) to configure a task and run a test panel on the channels using a known voltage source to verify that your hardware is working properly.  If the values are measuring properly there, then the source of the error may be within the code.
As far as your measurements themselves, it looks like you are trying to do strain measurements of some sort, these typically can be measured more accurately using differential measurements instead of RSE or NRSE.  I will include a link to a page about the differences between the measurement types and noise considerations for analog measurements.  I will also attach some references to a few documents about strain measurement methods and principles for you review.
Finally, I would recommend looking into an alternative for this system -- either a multifunction DAQ board with more analog input capability (like a PCI-6229, 32 analog inputs), or if you are wanting more accurate strain measurements I would recommend something in our SCXI line for the best strain measurement results.
Field Wiring Considerations for Analog Measurements:
http://zone.ni.com/devzone/conceptd.nsf/webmain/01f147e156a1be15862568650057df15
Strain Measurement References:
http://zone.ni.com/devzone/conceptd.nsf/webmain/04bd243edda22dd386256d75007900d5
http://zone.ni.com/devzone/conceptd.nsf/webmain/C18F65CE920C115086256D720058325B
http://zone.ni.com/devzone/conceptd.nsf/webmain/2d5849b3108e416b86256da2007a8e7b
Regards,
Erik Fogleman
Applications Engineer