Chapter 5 Calibration
PCI-4451/4452/4453/4454 User Manual 5-2 www.ni.com
Self-Calibration
Your PCI-445X can measure and correct 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 the preferred method of
assuring accuracy in your application. Initiate self-calibration to minimize
the effects of any offset and gain drifts, particularly those due to
temperature variations.
Your PCI-445X has an onboard calibration reference to ensure the accuracy
of self-calibration. Its specifications are listed in Appendix A,
. The reference voltage is measured at the factory and stored
in the EEPROM for subsequent self-calibrations.
Immediately after self-calibration, the only significant residual calibration
error could be gain error due to time or temperature drift of the onboard
voltage reference. This error is addressed by external calibration, which is
discussed in the External Calibration section. If you are interested
primarily in relative measurements, you can ignore a small amount of gain
error, and self-calibration should be sufficient.
If you calibrate your PCI-4451/4452 while it is connected to a BNC-2140
accessory, set each input channel to SE and connect each channel
+ terminal to a channel – terminal through a BNC shunt. In addition, make
sure that ICP
®
power is turned off on the BNC-2140 to avoid affecting the
reference voltage reading. If you calibrate your PCI-4453/4454 while it is
connected to the BNC-2142 accessory, connect each + terminal to its shield
through a BNC shunt. You can also calibrate your PCI-445X by removing
the external cable connected to the BNC-214X accessory.
External Calibration
The onboard calibration reference voltage is stable enough for most
applications, but if you are using your device at an extreme temperature or
if the onboard reference has not been measured for a year or more, you
might want to externally calibrate your device.
External calibration refers to calibrating your device with a known external
reference rather than relying on the onboard reference. You can store the
results of an external calibration in the user area of the onboard EEPROM.
Externally calibrate your device by calling the NI-DAQ calibration
function. When you perform an external calibration, be sure to use a very