Max/Min Readings Are Lower Than Expected When Using Custom Scaling

Updated Apr 10, 2019

Reported In

Software

  • Measurement & Automation Explorer (MAX)
  • LabVIEW

Driver

  • NI-DAQmx

Issue Details

  • I have set up custom scaling in a DAQmx task but I am not seeing values as high or as low as I am expecting but when I look at the voltage readings everything seems fine.
  • I am trying to apply a custom scale to my DAQmx task using a PXI 6220, but it appears my signal is railing or hitting a maximum value at 10V. I have the Maximum and Minimum values set to -10 and 10 for the DAQmx task as those are the hardware limits for the PXI-6220.  When my custom linear scale has a slope of 9.9, the signal comes through fine, but whenever I change the slope to be greater than 10, the signal looks like it is railing. I have measured the incoming signal and it is within spec for the device and isn't railing. I see this behavior in NI Measurement & Automation Explorer (NI-MAX) and in LabVIEW. How do I fix this?

Solution

Make sure that the max and min settings for the signal input range are expressed in the scaled units.

For example, if a custom scale, which converts volts to meters, is assigned to a voltage channel, the channels minimum and maximum attributes are expressed in meters.  Or if you set a custom scale to multiply your pre-scaled reading by 100, then the units for your scaled signal will have a max input range of 1000 and a min input range of -1000 because +/-1000 / 100 = +/-10 Volts. 

Figure 1. Max and Min Configuration in NI-MAX

Figure 2. Max and Min Configuration in LabVIEW

Additional Information

After a scale is assigned to a channel, the scale applies to all attributes normally expressed in the pre-scaled units of the channel. This includes the maximum and minimum values assigned to the channel.

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