National Instruments NI 785xR - Manual (Page 54)

Chapter 3 Calibration (NI 783xR/784xR/785xR Only)
R Series Intelligent DAQ User Manual 3-2 ni.com
application. Internal calibration minimizes the effects of any offset and
gain drifts, particularly those due to changes in temperature. During the
internal calibration process, the AI and AO channels are compared to the
NI 783xR/784xR/785xR onboard voltage reference. The offset and gain
errors in the analog circuitry are calibrated out by adjusting the CalDACs
to 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 at
Start»All Programs»National Instruments»NI-RIO»Calibrate 78xxR
Device. Device is the NI PXI-783xR/784xR/785xR or NI PCI-783xR
device.
Immediately after internal calibration, the only significant residual
calibration error is gain error due to time and temperature drift of the
onboard voltage reference. You can minimize gain errors by performing
an external calibration. If you are primarily taking relative measurements,
then you can ignore a small amount of gain error and self-calibration is
sufficient.
The flash memory on the NI 783xR/784xR/785xR stores the results of an
internal calibration so the CalDACs automatically load with the newly
calculated calibration constants the next time the NI 783xR/784xR/785xR
is powered on.
External Calibration
An external calibration refers to calibrating your device with a known
external reference rather than relying on the onboard reference. The
NI 783xR/784xR/785xR has an onboard calibration reference to ensure
the accuracy of self-calibration. The reference voltage is measured at the
factory and stored in the flash memory for subsequent internal calibrations.
Externally calibrate the device annually or more often if you use it at
extreme temperatures.
During the external calibration process, the onboard reference value is
re-calculated. This compensates for any time or temperature drift-related
errors in the onboard reference that might have occurred since the last
calibration. You can save the results of the external calibration process to
flash memory so that the NI 783xR/784xR/785xR loads the new calibration
constants the next time it is powered on. The device uses the newly
measured onboard reference level for subsequent internal calibrations.