My Front panel displays incorrect voltage that is accquired from the DAQ

[Rohit Narula]

Hello All

I am new user of the LabVIEW software and am finding it amazing. I am using this software as a past of my thesis project. I am currently trying to read in voltage from the DAQ from one of its channel. I am using a NI 6008 USB DAQ and LabVIEW 7.1 Student Edition. when I try to read the voltage from the output of an Op-AMP, the value that comes up on the front panel is incorrect and also varying a lot. When I check this same voltage using a multimeter at the input of the DAQ, it is the same as the ouput from the Op-AMP and very stable. I am just taking the input from the DAQ, taking the mean of it (reduction factor 50) and finally displaying it on the Waveform Graph.

The value from the Op-AMP actually reads at 5 Volts DC, but the front panel display shows 3.74 Volts DC. I am taking in the input from AI0 and I have put the GND pin of my DAQ to the common ground of my external circuit.

Any suggestion will be greatly appreciated.

Thanks

Rohit

[devchander]

Hi Rohit,

Try two things,

First run an example code that's shipped with LabVIEW and see if you are still getting unstable readings.

If your still are, connect your signal in differential mode and check.

Hope this helps

Regards

Dev

[Rohit Narula]

Hi

Thanks for the quick reply. I tried caputing it in differential mode but still the value is varying by about 50 mV. But this time when I accquired the voltage, the front panel was displaying it fine. But still the capturing is very unstable.

Rohit

[WM_John_Weeks]

If you use differential mode, you must connect the source to both terminals of the differential inputs. If you use AI0, the positive connection from the source goes to AI0 and the negative side goes to AI8.

You could also use RSE (Reference Single Ended) mode. Then the source connects to AI0 and ground.

If you use differential mode and leave AI8 open, you will see unstable readings.

[Rohit Narula]

Hi John

I have tried connecting it in differentail mode, the source to AI0+ (Terminal 2) and negative side to AI- (Terminal 3), but still it was giving a lot of variation. And in the RSe mode, I just put the source to Terminal 2 and Ground to Terminal 1. But still no difference.

rohit

[devchander]

Yes those connections are correct for your hardware USB 6008

Just post your VI and also tell what ADC range you have set( High and low limits for input)

Message Edited by devchander on 01-19-2006 10:27 PM

[Rohit Narula]

Hi Dev

I am uploading my VI file here. My upper limit for voltage in the DAQ Assistant settings is 10 Volts and lower limit is 0 Volts.

[devchander]

Hi Rohit,

considering that all seems fine on the signal connection end, this  is my observation

Why not use an example VI provided with labVIEW for performing continous DAQmx voltage acquisition?

Here is one VI for performing DAQ mx voltage acq

Try running this VI on your setup and see if you still get that 50 mV variation

regads

Dev

[GDE]

Rohit,

Another thing you might want to try is checking the inputs in Measurement and Automation Explorer (MAX). You didn't say, but I will assume that your OS is WinXP. If this is the case then you will have MAX installed with your drivers, and you can use this utility to check the signals, and to make sure the device is working without having to use any LabView code.  You will want to use the Test Panels to monitor the analog input lines of your DAQ-USB-6008 device. If everything is working there, then follow devchanders advice and use a NI provided example (which can be found through Help Menu >> Find Examples). A good example to use is: Hardware Input and Output >> DAQmx >> Analog Measurements >> Voltage >> "Cont Acq&Graph Voltage-Int Clk.vi"

Other resources that you may find useful:
DAQ Tutorial
http://zone.ni.com/devzone/conceptd.nsf/webmain/48B4C2DC01295874862570D7007915DA?opendocument&Submitted&node=201181_US
Wiring Considerations
http://zone.ni.com/devzone/conceptd.nsf/webmain/01f147e156a1be15862568650057df15#6

-GDE

[Rohit Narula]

Thnaks for the suggestion GDE, I am gonna try all that and then try devchander's method. Thanks a lot guys, ou gave me a lot of support for my project

Rohit