National Instruments NI 784xR User Manual

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National Instruments NI 784xR User Manual Manual pdf 54 page image

Chapter 3 Calibration (NI 783xR/784xR/785xR Only)R Series Intelligent DAQ User Manual 3-2 ni.comapplication. Internal calibration minimizes the effects of any offset andgain drifts, particularly those due to changes in temperature. During theinternal calibration process, the AI and AO channels are compared to theNI 783xR/784xR/785xR onboard voltage reference. The offset and gainerrors in the analog circuitry are calibrated out by adjusting the CalDACsto minimize these errors.Note The NI 78xxR Calibration Utility does not support NI 781xR devices.If you have NI-RIO installed, you can find the internal calibration utility atStart»All Programs»National Instruments»NI-RIO»Calibrate 78xxRDevice. Device is the NI PXI-783xR/784xR/785xR or NI PCI-783xRdevice.Immediately after internal calibration, the only significant residualcalibration error is gain error due to time and temperature drift of theonboard voltage reference. You can minimize gain errors by performingan external calibration. If you are primarily taking relative measurements,then you can ignore a small amount of gain error and self-calibration issufficient.The flash memory on the NI 783xR/784xR/785xR stores the results of aninternal calibration so the CalDACs automatically load with the newlycalculated calibration constants the next time the NI 783xR/784xR/785xRis powered on.External CalibrationAn external calibration refers to calibrating your device with a knownexternal reference rather than relying on the onboard reference. TheNI 783xR/784xR/785xR has an onboard calibration reference to ensurethe accuracy of self-calibration. The reference voltage is measured at thefactory and stored in the flash memory for subsequent internal calibrations.Externally calibrate the device annually or more often if you use it atextreme temperatures.During the external calibration process, the onboard reference value isre-calculated. This compensates for any time or temperature drift-relatederrors in the onboard reference that might have occurred since the lastcalibration. You can save the results of the external calibration process toflash memory so that the NI 783xR/784xR/785xR loads the new calibrationconstants the next time it is powered on. The device uses the newlymeasured onboard reference level for subsequent internal calibrations.