Incorrect Readings When Using a Scale in DAQmx Channels

Updated Oct 19, 2018

Reported In

Software

  • Measurement & Automation Explorer (MAX)

Driver

  • NI-DAQmx

Issue Details

  • My custom scale is displaying the wrong values, or is not reaching the limits of my data. Without scaling my device works fine and I can read the input channel normally.
  • I defined a custom scale for my task. I then set the input limits of my channel. However, when I read in a voltage near the top of my input range, the values appear to cutoff at a certain point and I cannot read my maximum value.
  • I get an out of range error when working with custom scales and then a none of the min and max values that I can read in.  If I set my voltage to that range, I can read it in, but where do these min and max values come from?
  • How does the range work with custom scales?

Solution

If you use a custom scale, the Signal Input Range in the task refer to the values of the input after scaling (Scaled Units).

For example: With a Pre-Scaled Values range of 0 - 10 V and a Scaled Values range of 0 - 50 V, set the Signal Input Range for the task to 0 - 50 V, as shown in the image below. If you set the Signal Input Range to 0 - 10 V and read in 10 V on your device, 10 V is outside the range and the gain amplifier could become saturated, which results in an incorrect reading.

Additional Information

Pre-scaled Units refer to values expressed in the unit of the channel prior to the custom scale being applied. Usually, pre-scaled units are volts or amps since most channels will natively measure or generate signals of this kind. However, it is possible to associate a scale with a channel that contains a transducer explicitly supported by NI-DAQmx. In this case, the pre-scaled units are those used by the supported transducer channel.

For example, if you associate a scale with an analog input resistance channel, the pre-scaled units would be ohms and the scale would specify how to convert from ohms to the desired scaled units.

Scaled Units refer to values expressed in the final unit after NI-DAQmx applies the custom scale. You are able to define scaled units however you see fit for your application.

For example, if you assign a linear-position-to-voltage scale on a voltage output channel, the pre-scaled samples are in volts while the scaled samples could be specified in meters.

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