Unexpected Offset, Scaling, or Drift on Analog Input Channels of R Series or DAQ Hardware

Updated Jul 31, 2019

Reported In

Hardware

  • USB-7845
  • USB-7846
  • USB-7855
  • USB-7856
  • NI-9205

Issue Details

I see unexpected offset, scaling, or drift on the analog input channels of my device when I provide a known voltage source. Does my device need to be replaced or repaired? Why is this happening when I provide a known voltage?

Solution

The unexpected measurement may be caused by an incorrect setting of the analog input terminal mode configuration. 

The default analog input terminal mode is differential (DIFF). To correctly measure a differential signal, use the AI+ and AI- pins of the device. For example, use pin 68 and 34 as shown in the figure below to measure a signal on the analog input channel 0 differentially.
It's also important in differential configurations to confirm whether you also require a connection to COM ground following the guidance in Table 1 of the document Field Wiring and Noise Considerations for Analog Signals. As an example, the following DAQ device measuring a differential floating signal would need to have a channel connected to AI0 (+) / pin 1, AI8 (-) / pin 20, and COM, with each consecutive pair (pins 2 & 21, 3 & 22, etc.) for a given channel also connecting to a COM pin.

If your application requires single-ended measurements, configure the analog input channels to reference single-ended (RSE) or non-referenced single-ended (NRSE). See the NI R Series for USB Getting Started Part 2: Advanced tutorial for information on how to configure the analog input terminal mode programmatically or through the project explorer.

Additional Information

You may refer to How to Determine If an Input Signal Is Ground-Referenced or Floating should you need to clarify whether the source is ground-referenced or floating, which impacts whether you need to connect to COM on a DAQ device. 

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