Wrong samples from NI PCI 6251 ADC board

[daniele.nicolodi]

Hello.

I configured an NI PCI 6251 ADC board for 50 kHz continuos sampling on channels 6 and 7 configured as differential analog inputs. I'm testing the acquisition acquiring, on both channels, the same sine wave signal produced by a function generator. The system works as expected, but every now and then, with a frequnecy of some points per hour, I obtain an obviously wrong sample from one of the channels: the read value is much different than the expected one by a quantity huge respect to the measurement noise.

I'm unable to understand what can be the source of those wrong samples. Has someone else seen something similar? How can I investigate this problem? 

Thanks. Cheers.

D.

[Logan_081]

Hello Daniele,

You shouldn't get back invalid points during you acquisition. Did you plot on a Chart/Graph these results, or better can you add a snapshot of these plotted data? Are you able to reproduce the issue also on different channels or you receive bad value from the same one? In this way we can figured out that there's something wrong with the hardware and it need to be repaired.

Please let me know if you could provide for these additional informations. It would be a pleasure to help you with your issue.

Best regards 

[daniele.nicolodi]

Hello Matteo. Thank for your answer.

I've checked the issue and it is present on all channels. The bad sample is always in the last sample of a scan. If i sample the same channel multiple times in a scan, the bad sample is only on the last sampling in a scan. It looks like the bad behavior is not influenced by the sampling rate or by the time between adc conversions in the same scan.

I think the issue must be due to some FIFO synchronization problem.

Cheers.

D.

[Logan_081]

Hello Daniele,

If we assume that you are facing with a software error, i suggest you to run an example program located in the NI Example Finder (open LabVIEW >> Help >> Find Examples...). Unfortunately i'm not sure about your code architecture. At least you can post your source code, so i can run it and point you to the correct solution.

I wait for your feedback.

[daniele.nicolodi]

I think that the software platform I'm using is not supported by NI. I'm using Analogy: a port of the Comedi drivers on top of the Xenomai real-time Linux extension. Does NI offer support for the use of different drivers than the Microsoft Windows ones?