multiple channels in daq assistant

[ariemeir]

Hi guys,

My first message on this forum, would appreciate some advice.

I'm using labview 8.2  with PCI-6071E acq. card.

I am observing an unexpected behavior when trying to read analog data from my card.

If i define in the daq assistant a single channel to be read, all is great, and the data is ready as expected. i can see it in the preview window (express task)

when i open the daq assistant.

But, the moment i add one more channel (from the same physical pci-6071e card), my measurements are totally bogus - i am getting some weird signal...

The sampling rate is more than enough (i sample with 64khz a signal which is 1khz arriving at only one of my channels).

What am i missing here ? 

Thanks for any advice you might have.

Lenny

[ariemeir]

Hi guys,

Since i've posted my initial question, i have been looking some more into the problem. Perhaps this information can help pinpoint problem faster:

If i create a blank VI, and use a daq assistant, select measure analog input signal and select 2 signals (differential mode) : ai0 and ai2.

It is interesting that the preview would only show me the first one in the order: if i move the ai2 to the top , it will be shown, if ai0 is at the top of the list,

only it will be shown. (i do see the graph line for the other signal, but it is flat, as if nothing is there)

I am observing some additional discrepancy, so i am wondering if i am missing something big : i feed a 1k sinusoid 2v amplitude, 0-dc bias, into the signal (ai0 or ai2).

The appropriate differential end (ai8 or ai10) is connected to ground.

if i change the sampling frequency (in the daq assist settings ,  i am in continuous mode, 1000 samples). from 37k to 38k, i lose all my signal (the line goes from 1k sine wave to flat). It actually happens over a certain range: my dc offset starts drifting and then the signal goes flat.

Since my 6071e card is supposed to support 1.25MS/s I wouldn't think it has to do with sampling rates...

I believe it is some software related issue because if i connect a scope on the input of the data acquisition card, the signal is nice and strong... no matter what i do with the software settings.

Would greatly appreciate any advice !!

Lenny

[RavensFan]

Software is not going to cause that.

My guess is it is hardware.  You have a mismatch between how you have it wired and how you have the channels defined  (differential, referenced single-ended, or non-referenced single ended.)

You say you have it set for differential mode.  Check how you are supposed to have it wired.  You shouldn't be wiring the other end to ground.

[ariemeir]

Dear RavensFan, thanks for your reply.

both of my channels are defined as differential.

To test it connect the output of a sine signal generator (this one: http://goo.gl/9FQuH) 

into the + and - respectively (ai0 and ai8 in my board).  i have noticed that when i don't ground the (-) side of the signal generator, the signal is noisy.

That is the only reason i connected the ground in my differential setup. When i use it for my "regular" signals , i do not use ground. 

following your post, 

I have just now tried to define both channels as RSE, and similar behavior occurrs : when i change the order of the channels

in the daq assistant settings, the output in the preview window changes: from a happy 1k sine to a flat line....

Thanks for any advice,

Lenny

[RavensFan]

Something is wrong with your link.

If something is noisy, then I think you need to figure out what is causing the noise.  You might need to ground the shields of the wire.  Or possibly tie the grounds together between the device and the DAQ card without grounding the AI- side of the differential signal.

[ariemeir]

Sorry about the link:

http://www.ebay.com/itm/3MHz-DDS-Sweep-Function-TTL-Output-Signal-Generator-UDB1103S-Meter-DC-5V-PSU...

RavensFan,

I was able to simplify my problem further: i removed the function generator from the "equation".

Now i have my hardware device generating current and running it through a resistor.

i take the resistor ends and connect it to a differential channel of my DAQ PCI 6071E.

If i have only one channel in my measurements, all works well. If i add another channel, all hell breaks loose....

It doesn't seem to be a software issue, as you mentioned, at least not on application level. I've used the example vi provided by NI and

it behaves the same way.... 

What am i doing wrong ? 

[johnsold · Accepted solution]

With the signal generator (TTL output) or the PCI-6071E AO the gsignla is NOT differential. Use one of the single ended input configurations.

Ground all unused inputs.

Lynn

[ariemeir]

Lynn , thanks for your reply and willingness to help.

I am testing the system in both modes : RSE and differential.

In both cases i can get the signal beautifully when only one channel is read.

when i select multiple channels in the daq assistant, i can see only the first signal well,

the second signal is flat.

i am going to restart my pc now, this is very strange behavior.

If you have a suggestion i could test, thank you !

Lenny

[ariemeir]

Lynn, grounding the unused channels worked.

Kind regards for your help.

Lenny