Analog input issues

[Bob1324]

Hello,

I am seeing some strange errors when I take analog input data with my compact DAQ system.

I am using a cDAQ-9172 back plane wiith a 32 channel 9205 module on a wondows vista computer running Labview 8.2.1.

The first channel I am using works fine in MAX but seems to freeze at a single value when I run my Labview software. I used a multimeter to confirm the voltage on the wire was, in fact, fluctuating. If I disconnect this channel it slowly (over a minute) crawled to zero.

The second channel had the inputted voltage from different device drop sharply to zero every 30 to 60 seconds. And again I confirmed with a multimeter that the real voltage on the wire was not going to zero. This drop also occurred when I tested this channel in MAX.

These two channels seem to have the correct input configurations selected, at least changing them did not improve anything.  I also have tried resetting the DAQ system, changing the module slot, and using different channels. And a thermocouple module I tested worked fine with the same backplane and same Labview program. 

I am really stumped by this especially since the thermocouple module worked fine after only plugging it in. Any advice or tips would be appreciated.

Thanks,

~Chris

[Jason_D]

Bob1324,

If the first channel is working in MAX then we probably have a problem in software.  In order to get help with this you might want to post your code so myself and others can help you debug it.  You could also try and describe it more in depth but it is likely a problem with how you are updating the indicator/graph or how the sampling is being processed.

This second channel is much more troubling for me.  The fact that test panels in MAX is not working indicates that it will never work in software.  When you say drops sharply to 0 are you talking about several samples or for a significant period of time (a few seconds).  If it is simply a few samples your dmm may not be accurate/sample fast enough to notice the change.  I would suggest that you hook up the other signal to channel two and see if it experiences the same behavior.  Additionally does changing this signal source to a different channel show the same results?  If not this may be a bad channel. 

[Bob1324]

Thanks for the response Jason,

I ended up playing with this a little more today and found the value freezing problem turned out to be due to the max and min values on the channel being set with respect to the voltage values not the scaled values, causing it hang.

But fixing this let be me see that this channel was, in fact, having the same signal drop as the other channel. It appears the problem occurs on all the channels on this module. I included a screen shot of both channels running simultaneously. The large gaps are were the signals mysteriously stop and then start again.

Both these channels are reading continuous analog signals connected in NRSE input modes and were continuously sampled at a 1Hz for about 7 seconds.

As was suggested, I tried different channels on this module and they all seem to have the same effect. Since this also occurs in MAX I think it is a safe bet that this is some kind of a hardware problem. I do find it odd that both signals are being effected that same way, especially since NRSE removes common ground noise.

Again, thanks for the help with this.

[Jason_D]

Bob1324,

When you say these values are the same in MAX do you mean through Test Panels.  If not you should try Test Panels.

Also could you try hooking up a steady simple source like a battery and see if that gets the same results.

Your minimum value here is not 0 correct?

[Bob1324]

A new configuration this morning was tried with our hardware and the signals started giving correct values. It seems I was mistaken in the types of devices we were using and they should have been connected in RSE input modes not NRSE (Sorry, we are all learning here).

Jason, I appreciate the time you took to reply to my posts. Your suggestions were helpful during this troubleshooting.

For those following this thread, http://zone.ni.com/devzone/cda/tut/p/id/3394 is a good document to read on the different analog input configurations.

For my specific problem, I noticed that when reading an empty channel there were steady signal fluctuations as high as 5V. This channel must be picking up some internal voltage changes in the module that happen at a specific rate. This voltage fluctuation went away when that channel was wired directly to the common ground. When I attached my signals without properly grounding them, I think this internal signal was picked up, This caused my signals to jump beyond their voltage limits (0 to 5V) at a frequency determined by the rate of that internal signal and my sampling rate. That would explain why my signals appeared to be modulated at a fairly specific rate and why the problem disappeared when they were grounded correctly.