Chapter 4
Calibration
4-2
ni.com
minimize the effects of any offset and gain drifts, particularly those due to
temperature variations.
The NI 447
X
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 or during an external
calibration operation 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.
External Calibration
The onboard calibration reference voltage is stable enough for most
applications, but if you are using the device at an extreme temperature
or if the onboard reference has not been measured for one year or more,
you should externally calibrate the device.
External calibration refers to calibrating your device with a known external
reference rather than relying on the onboard reference. The new calibration
constants are stored in the onboard EEPROM, overwriting the factory
calibration constants.
Externally calibrate the device by calling the DSA Calibrate VI or the
Calibrate_DSA
function. When you perform an external calibration,
be sure to use a very accurate external DC reference. The reference should
be several times more accurate than the device itself. For example, to
calibrate the NI 447
X
, the external reference should have a DC accuracy
better than ±115 ppm (±0.001 dB).
Note
When you calibrate the NI 447
X
, make sure the inputs are DC-coupled and that
IEPE power is turned off to avoid affecting the reference voltage reading.
