data acquisition - problem with NI 9215

[gaetano@uva]

I just got a input voltage reader module NI 9215. I have to read a voltage coming from a flow controller which I control using another module (NI 9263).
The output module correctly writes the imposed voltage, but the input module does two strange things:

1) when the sample rate is very low (about 10), the voltage reading (after a randomly  pretty long or short transient) reaches the correct values and stays there for a few seconds, then it drops down and maybe sometimes later cycles that behaviour (with a lot of noise, but I wont' be worried about this...);

2) when the sample rate is higher (100) then the voltage reading crazy goes between 0 and the correct value. (On a chart this results in a filled white rectangle plot, given the huge number of oscillations between 0 and the value). In this case, this behaviour seems to be steady in time.

By the way, my 5 $ voltmeter reads the right voltage ...

I have used NI DAQmx express and also NI DAQmx vis, it's always the same problem.

My question is: Is the module defective, or is there any Labview related issue ( I don't know, maybe a wrong setting)

I hope to have been clear, please ask any clarification.

[J_Thomas]

Hi gaetano,

I have a couple of suggestions and a couple of questions.

The first suggestion is to take LabVIEW out of the equation until you can verify that the NI-9215 can read the correct signal.  You can use a Test Panel in Measurement & Automation Explorer for the NI-9215 to read you analog input signal.  Open Measurement & Automation Explorer (MAX) and find the entry for your device.  It can be located by looking under My System >> Devices and Interfaces >> NI-DAQmx Devices >> NI 9215.  Right-Click on the entry for the NI 9215 and select Test Panels.   You can select a sampling rate and other parameters as needed to test your analog input signals.

Anytime noise large noise levels are observed on a signal it is always a good idea to check for possible grounding issues.

How do you have your signals wired? (differential, single-ended ground referenced, etc)

Is the voltage that you are measuring floating (not connected to ground) or grounded?

Depending on your signal some simple steps may need to be taken.  For example if you are measuring a floating signal in differential mode it is recommended to use bias resistors to reduce noise by providing a path to ground for any bias currents.

The following document is an excellent resource for setting up your measurement:

Field Wiring and Noise Considerations for Analog Signals

Take a look at your connections and setup and feel free to post back with the results of your investigation.

[gaetano@uva]

Indeed it was a grounding problem! Thanks a lot, experience is always one of the most important knowledge!

gaetano