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

Updated Jan 18, 2018

Reported In

Hardware

  • NI-9205
  • NI-9206

Issue Details

I see noisy and unsettled measurements when reading NI 9205/9206 analog input 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.
 
  1. 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.

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