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Chapter 5 Calibration
PCI-6110E/6111E User Manual 5-2
©
National Instruments Corporation
This method of calibration is not very accurate because it does not take
into account the fact that the board measurement and output voltage
errors can vary with time and temperature. It is better to self-calibrate
when the board is installed in the environment in which it will be used.
Self-Calibration
The 611
X
E board can measure and correct for almost all of its
calibration-related errors without any external signal connections. Your
National Instruments software provides a self-calibration method. This
self-calibration process, which generally takes less than a minute, is
thepreferred method of assuring accuracy in your application. Initiate
self-calibration to minimize the effects of any offset, gain, and linearity
drifts, particularly those due to warmup.
Immediately after self-calibration, the only significant residual
calibration error could be gain error due to time or temperature drift
ofthe onboard voltage reference. This error is addressed by external
calibration, which is discussed in the following section. If you are
interested primarily in relative measurements, you can ignore a small
amount of gain error, and self-calibration should be sufficient.
External Calibration
The 611
X
E board has an onboard calibration reference to ensurethe
accuracy of self-calibration. Its specifications are listed in AppendixA,
Specifications
. The reference voltage is measured at the factory and
stored in the EEPROM for subsequent self-calibrations. This voltage is
stable enough for most applications, but if you are using your board at
an extreme temperature or if the onboard reference has not been
measured for a year or more, you may wish to externally calibrate your
board.
An external calibration refers to calibrating your board with a known
external reference rather than relying on the onboard reference.
Redetermining the value of the onboard reference is part of this process
and the results can be saved in the EEPROM, so you should not have to
perform an external calibration very often. You can externally calibrate
your board by calling the NI-DAQ calibration function.
PCI_E.book Page 2 Thursday, June 25, 1998 12:55 PM