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Noisy Measurements When Using NI 9205/9206 Modules in CompactRIO Scan Interface Mode

Hardware: PXI/CompactPCI>>Controllers

Problem:
I see noisy and unsettled measurements when reading NI 9205/9206 AI channels on a CompactRIO System configured to use the CompactRIO Scan Interface.  If I leave the signals connected and read the channels from the LabVIEW FPGA Interface I do not see the same noisy measurements.  Why does this occur and how can I fix it?

Solution:
Noisy measurements when reading AI channels on NI 9205/9206 modules in CompactRIO Scan interface mode can occur for several reasons.

1. COM has not been connected If you leave COM unconnected, the signals float outside the working input range of the NI 9205. This may result in unreliable measurements because there is no way to ensure that the input signal is within 10 V of COM.
2. Unused Channels are not tied to ground   If one or more of the unused channels is not wired to ground, it floats and will potentially affect the channels that are read.  This is due to how the NI 9205/9206 internally muxes inputs.  When using in the CompactRIO Scan Interface, the software reads all channels, where-as in LabVIEW FPGA Interface mode only the used channels are muxed.

Related Links:
Products and Services: NI 9205

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Report Date: 08/28/2008
Last Updated: 02/28/2009
Document ID: 4OR9PRLX

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