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Chapter 5 Calibration
PCI-4451/4452 User Manual 5-2
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National Instruments Corporation
not very accurate because it does not take into account the fact that the
device measurement and output voltage errors can vary with time and
temperature. It is better to self-calibrate when you install the device in your
environment.
Self-Calibration
Your PCI-4451/4452 device 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 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 warmup.
Your PCI-4451/4452 device has an onboard calibration reference to ensure
the accuracy of self-calibration. Its specifications are listed in Appendix A,
Specifications. 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 following section, External Calibration. 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 device 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. You can
also calibrate your PCI-4451/4452 device by removing the external cable
connected to the BNC-2140 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 may
wish to externally calibrate your device.
An external calibration refers to calibrating your device 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
User.book Page 2 Tuesday, April 14, 1998 10:20 AM