What is the Absolute Accuracy for Half the Voltage Input Range?Hardware: CompactDAQ>>Analog Input Modules>>NI 9205
Problem: I would like to know the formula to determine the absolute accuracy for the half the input voltage range. For example, I would like to know the accuracy for the 0 - 10V range of the NI 9205, but only the full range (+/- 10 V) is mentioned in the Product Manual. Solution: The range and gain selections available to the DAQmx driver are specific to each card, and are listed in the card's specification sheet as the Nominal Input Ranges. If your signal’s maximum and minimum values are not the extremes of one of the hardware’s nominal ranges, then the driver will select the nearest range that the signal fits within. In the instance of the NI 9205 module, it has four possible bipolar ranges (varying across zero): +/- 10 V, +/- 5 V, +/- 1 V, and +/- 0.5 V. When you input the unipolar 0-10 V range, the driver will automatically convert this to the +/- 10 V range and use those settings. However, some DAQ cards are capable of both bipolar and unipolar ranges. The absolute accuracies for the DAQ card will be listed in the AI Absolute Accuracy Tables and Formulas of the Operating Instructions and Specification manual. Something to note is that these values are the maximum inaccuracies you can expect to see in that range, i.e. in the case of the +/- 10 V it is the error in a 10 V reading. If you wanted to know what you might expect the error to be if you were to read 6 V signal in this range you could use the equations from same AI Absolute Accuracy Tables and Formulas page. For the 9205, since this card is not capable of a unipolar 0-10V range, the correct absolute accuracies listed by full range on page 22 of the NI 9205 Operating Instructions and Specifications, linked below. The equations can be found on page 24 and the values for the applicable ranges on page 23. NOTE: When you use the DAQmx Create Channel.vi in LabVIEW, the two inputs where you can select the maximum and minimum value are not the range you want the hardware’s ADC to use, but instead define the range that you expect your input signal to vary between. The NI-DAQmx driver will select nominal range and gain settings to use behind the scenes based on these inputs so as to maximize the resolution capabilities of the hardware. How this is done can be seen in the Knowledge Base Documents linked below. Related Links: Product Manuals: NI 9205 Operating Instructions and Specifications Knowledge Base 3IHCT5LE: Absolute Accuracy of Dynamic Signal Acquisition DevicesKnowledge Base 3KK935D3:How Do I Set the Analog Input Gain using DAQmx? Attachments:
Report Date: 08/28/2008 Last Updated: 09/02/2008 Document ID: 4ORIR79G |
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