I am experiencing noise on my PCI-6238 card on all analog inputs.

[MHP]

My company is manufacturing a machine using LabView and a PCI-6238 multifunction board to handle all the I/O.  The PC is a Lenovo desktop.  We connected our analog inputs from our sensors.  The 6238 was reading the signals except that the signal was noisy.  We checked the signal with an milliAmp meter.  The signal is fine.  It is not changing as much as the Labview is indicating.  On the Labview NI Max we are seeing the signal as varying about +/-0.3mA on the graph.  The noise seems to be around 2-4hz.

I have tried the following with no success:

1)     Disconnected all wiring from connection block except for a Fluke loop calibrator that is outputing 4-20mA.  The signal is still noisy.

2)     Reseated the 6238 card.  The signal is still noisy.

3)     Removed and readded the 6238 card from the configuration.  The signal is still noisy.

4)     Connected the DC - on the loop calibrator to the analog input ground.  The signal is still noisy.

5)     Removed the 480 VAC from the system and are only operating the PC on a extension cord.  The signal is still noisy.

Any help with this would be greatly appreciated.

[Mark_E]

Hi MHP,

I suspect that you may have some grounding issue that is causing the noise. If you generate an analog output signal and feed it to the analog input, is the noise gone? Here’s a useful link for wiring and noise considerations.

Calibration is something to consider when concerned about noise. How old is the card? When was the last time it had an external calibration? Internal calibration?

Will you please include the versions of all the software you are using, including LabVIEW, DAQmx, Measurement & Automation Explorer (MAX), and what operating system you have?

Will you please include a screenshot of the noise if none of these suggestions cleans it up?

[Mark_E]

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@MHP wrote:

The PC, card, and machine are new.  I disconnected all external wiring and used a 4-20mA loop calibrator.  The signal is still the same amount of noise.  I then wired an analog output to one of the analog inputs.  The noise was the same or worse.  I was experiencing a 0.6mA amplitude on the noise with a constant output.  Today I can get a reasonable signal, but then it starts ramping and then rails.  Sometimes the signal is an occilation that spans from 4mA to 20mA.  My signals incoming are constant.

I even took the PC to another location(my house) and used the analog output to power the analog input.  The results were the same noisy signal.  I was trying to eliminate the fact that my office may have "dirty" power with harmonics on the lines.

I have the negative on the analog input tied to DC common and the AI GND tied to DC common.  I am concerned that there is a problem with the PCI-6238 card.  It passes all the tests and has been calibrated.

I have isolated the breakout board from all field wiring and the noise is still eradict.  Please offer any assistance.

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Do you have an identical board that you could try in order to verify that it is just this board that is malfunctioning?
It sounds like this would best be handled off-line as a possible RMA. Please visit www.ni.com/ask to request phone support. Inform the representative of this discussion forum so they can see the troubleshooting steps already taken. Please provide the versions of all the software in use (LabVIEW, MAX, DAQmx, etc.). Thank you.

Message Edited by Mark E on 11-01-2007 03:16 PM

[MHP]

Can I get a sample wiring diagram for a PCI-6238 board.  I want to make sure I am grounding correctly.  I have the un-used channels jumped + to -, but I don't have them connected to AIGRD.  Should they be?

[Mark_E]

Hi MHP,

One of the easiest and most visual ways to see how signals should be connected is to create a task in MAX. This is done by right-clicking Data Neighborhood » Create New… » NI-DAQmx Task and choosing the type of task (current input). You then choose your device and channel(s). Once it is created, you can click on the Connection Diagram and choose the channels individually to see how each should be wired. For devices with an accessory, you’ll need to choose the correct one (the picture is of the first choice).

Let me know if you have more questions.

Message Edited by Mark E on 11-02-2007 08:39 AM