Problem in acquiring analog signals from PCI-6251

[mhs100]

Hi all,

I am using NI PCI-6251 Multifunction Daq card on lab windows and using the shipping example
"ContAcqIntClk.prj"  for continuously acquiring the two sine wave signals without any DC offset.

The problem which i am facing in acquiring  is that there is always some negative offset voltage (  about -5 to -7 volts)  appearing on which the two sine waves are over riding but this NEGATIVE OFFSET VOLTAGE should not
occur.

The analog signals and ground are properly connected.

Can anyone tell me what might be the reasons of this problem?  

Any help will be greatly appreciated.

[Roryriffic]

mhs100,

I hope this doesn't sound silly, but make sure that the two sine wave signals are indeed correct before ever reaching the PCI-6251.  If you have an oscope handy, probe the two waveforms to be sure they have no DC offset.  Next, try to take Labview out of the equation for simple testing.  Open up a Test Panel for the PCI-6251 in MAX, select the channel you are acquiring data on, configure the parameters, and click Start.  You should see the same signal you see on the oscope.  If you do not see the same signal, try changing analog input channels on the PCI-6251.  If you do see the same signals, then something is wrong with parameters in the LabVIEW code.

Also, see this forum post and reply by Neal M where another user saw a DC offset in analog acquistion.

http://forums.ni.com/t5/Measurement-Studio-for-VB6/why-do-i-get-an-offset-while-acuiring-analog-inpu...

Rory R