acquire multi analog input with PCI 6221 M series

[deskod]

Hello,

I have to acquire   6 channels for measuring voltage in the differential mode.

Could you tell me what wrong with my VI?

Thanks

[Dennis_Knutson]

It looks okay to me though you're only indexing the first two waveforms out of the results and not 6. You don't have any values in the controls so you could be setting something wrong. What values are you using? What exactly is the problem you're having? Do you get an error or something else?

[deskod]

Hello Dennis ,

 I have no error message .
 I use PCI 6221 to acquire two voltage signal in differential mode.I use two sensors (one to acquire voltage another acquire current)I suppose to have a sinusoidal for the 2 signal.I use the 6 channel to make the differential mode.Am I on the right way?With an oscilloscope I easily have the 2 signal mais in the labview graph I have not the same sinusoidal signal .This my problem .

Thanks for your help and excuse for my  english

[Mcdan]

deskod,

I am a little confused about your setup.  From your origional post it would appear that you are trying to measure 6 seperate signals in differential mode.  I have modified your example and added some additional error handling (see attachment).  I have also demonstrated two simple methods you can use to display your data.  In you second post it seemed as though you were only trying to acquire 2 signals (two channels).  From your 2nd post it seemed as though you were not recieving an error from your VI, but rather you were not seeing the signal as you would expect it to be.  There are several reasons that can happen.  The first thing to check is your signal connections.  For measuring 6 channels in differential mode, I would reccomend the followin:

When using differential mode, your 6221 basically pairs channels in groups of 8.  Channels 0 through 7 are paired with channels 8 through 15 to form differential pairs.  Therefore if you want to read from channel 0 in differential mode, you would connect the signal across channels 0 and 8.  DAQmx will automatically group these pairs of channels.  For a 6 channel setup, I have created a table with some possible signal connections.
Channel            Signal(+)            Signal(-)
0                        ai0                      ai8
1                        ai1                      ai9
2                        ai2                      ai10
3                        ai3                      ai11
4                        ai4                      ai12
5                        ai5                      ai13
If you are to attach your signals in this manner than to read these, you would simply read physical channels ai0:5 in differential mode.

If your signal connections are correct, then another thing to check is your sample rate.  Ensure that you sample fast enough to capture the signal which you are trying to measure.  For example, if the signal you are trying to measure is an 11 kHz sine wave, then you would need to sample at a minimum rate of 22 kHz (this would allow you to see the correct frequency of your wavefrom, but to see its shape you'd really want to sample considerably faster than that).

If both your physical connections are correct, and your sample rate is correct, then ensure that you are setting your minimum value and maximum value appropriately.  If the signal you are measuring is +/- 2 mV, then you are going to want to set you minimum or maximum accordingly.

These are the key points that come to mind.  I hope you find them useful.  Please reply back if you have any further questions.
Hope this helps,
Dan

[deskod]

Hello,

 Thanks for your VI.

 Without an oscilloscope I get a good signal.(Goodtension.bmp)

 With an oscilloscope I get a distorted signal (tensiondeforme.bmp)

[deskod]

 When I measure with an oscilloscope the VI gave this signal  distorted.If  I remove the probe I get the good signal.

 Have you any idea of this distortion signal

Thanks

[WM_John_Weeks]

Well...

You are using differential inputs. Is the scope grounding something that shouldn't be grounded?

Can the signal drive a 50-ohm scope input? Are you using the AC pass filter on the scope input? That could put a reactive load on the signal. You may need an isolation probe on the scope to avoid these problems.

Do you have to use the scope? If your signals are good without it, maybe you shouldn't use the scope.

[deskod]

 Hello ,

 wihout a scope the signal look good.

 Thanks