User manual

Chapter 5 Calibration
NI 6034E/6035E/6036E 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 device measurement and output voltage errors can
vary with time and temperature. It is better to self-calibrate when the device
is installed in the environment in which it is used.
Self-Calibration
The NI 6034E/6035E/6036E device can measure and correct for almost all
of its calibration-related errors without any external signal connections.
Your NI software provides a self-calibration method. This self-calibration
process, which generally takes less than two minutes, 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.
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. 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 NI 6034E/6035E/6036E 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. This
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
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 device
by calling the NI-DAQ calibration function.
To externally calibrate your device, be sure to use a very accurate external
reference. The reference should be several times more accurate than the
device itself.
UM.book Page 2 Monday, May 14, 2001 10:32 AM