grounded signal source

[samik]

    hi ,

  i am using a NI PCI 6035 E daq board along with labview 8.2. i have got a pressure sensor connected to a carrier demodulator( grounded source) which give output 0-10 v dc. i connected the output + and - lead to the ai0 channel of the patch board connected to daq board (in differential mode and  as per measurement and automation software ai0 connection diagram shows +  to 68 and - to 34, by the way i am keeping the voltage level below 5 volt always). now in the labview programme when i use the daqmx  ai read. vi, (with the same task name and channel name as declared in measurement and automation software), it works only for N channel option. as a result when i export the waveform to a spreadshit file, along with date and time column it has 2 voltage columns. i found that those 2 column reading are almost same , say 1.254 &1.248.......and  the carrier demodulators display panel reads say 1.258 volt output.
                                    now my question is which reading i should consider or do i need to take the average and what exactly is happening. i will appreciate any help in this regard


Message Edited by samik on 06-08-2008 01:27 PM

[Corby_B]

Hello samik,

Thanks for your post. I see that you are using the NI PCI 6035 E DAQ Board in LV 8.2. If I am reading your correctly are you setting up a task in MAX for continuous acquisition but only taking N samples in your LabVIEW program? How do you have your AI read configured? Since you are only using one AI channel (AI0) make sure it is set up for "Analog 1D DBL 1 Chan NSamp" This way you will know that there is only a 1D array of information coming out of the AI read. I do not believe that you should have two columns of voltages as you are only reading one. Are you using the "write to spreadsheet.vi" to save your data? For instance if I take N samples it simply appends to the last entry each time I take more samples so still giving you only 1 column.

What I would recommend is to try acquiring the data using an example program first and then adding in the routines for saving data. If you go to Help >> Find Examples Look for a program called "Cont Acq&Graph Voltage-Int Clk.vi". You can find this in the example finder under Hardware Input and Output >> DAQmx >> Analog Measurements >> Voltage. There are also other examples that deal with saving data to files as well. If you could also post a screen shot of your code that might also be helpful to figure out why you have two voltage columns.

Please post back if you have any more questions.

Corby_B
http://www.ni.com/support

[samik]

hi corby,
thanks for your help. here are the screenshots of my vi.
1) 1.png is the error i get when i use 1 channel Nsamples option
2) 2.png is the programme that runs correctly with N channel N samples option  
3) 3. png is the corresponding channel declaration in measurement and automation software


i went through the const acq volt int clk write data to TDMS file.vi...there it use N channel N sample option .
thanking you again
samik

[Dennis_Knutson]

Only use DAQmx Create Channel if you want to add a channel to an existing task. That is what you are doing and that is why you get the error when you use the single channel DAQmx Read. It is perfectly legitmate to have a task with the same channel entered twice. So, as I said, skip the Create Channel or if you want to use the Create Channel with a physical channel input, don't wire up an existing task.