Signal jitter in Analogue input using NI 9201

[AndrewElite]

Have a signal issue in the system we're developing. I am using several NI 9201 analogue input modules in a 4unit chassis connected to a 7841R fpga with a 8110 Pxi controller. The 9201s are being used to measure the output of several contactless rotary sensors plus some force transducers and other signals. All sensors have the same isolated DC supply and noise is relatively small. The first rotary sensor of the six however at infrequent random occaisions gets quite large jitter, about +/-0.2v at a random frequency and the voltage appears to increase gradually. Measuring the input signal directly with a multimeter does not seem to have the same jitter or rise in voltage and no other signals are effected. When the sensor is unplugged and replugged the signal is normal again or after the FPGA is reset. It probably only happens once every 8 or so hours of use.

We had an identical issue with a previous system which used poentiometers instead of contactless rotary sensors. The only common hardware factor was that the problem only occurred on the first analogue input in a NI 9201 which is housed in the first slot on a 4u chassis. It is not the same 9201 module or chassis. Even the FPGA was a different model. All signals undergo the same filtering etc in the FPGA.

Has anyone had similar issues or have any suggestions on how I should investigate/resolve the issue.
Thanks in advance,
Andrew

[Jamie_S.]

Hi Andrew,

Thanks for your forum post regarding the jitter you are experiencing with your analogue measurements using the NI 9201. Having researched your issue I have come across the help file article here which details a potential cause of jitter due to resource contention on board the FPGA.

Basically the article describes that multiple requests on the same analogue input resource within the same FPGA clock cycle can introduce jitter, the jitter occurs most often when you use a shared subVI in two independent parts of the VI, or when you access a resource interface from parallel loops.

Please let me know if this assists in the debugging of your issue.

Many Thanks

Jamie S.