National Instruments AT-MIO-16X - Manual (Page 219)

Chapter 6 Calibration Procedures
© National Instruments Corporation 6-7 AT-MIO-16X User Manual
be XXXXX000. If the analog input section is calibrated using the utility
library functions and the constants are saved to User Area 7, then the
ADC Range bit in the area information field for User Area 7 is set or
cleared according to the mode in which the analog input section was
calibratedolar. The same holds true for the analog output section.
Calibration Equipment Requirements
Normal self-calibration requires no external calibration equipment.
However, because the internal voltage reference drifts slightly with
time and temperature, it may be necessary to redetermine its value every
year, or whenever operating the board at an ambient temperature that is
more than 10° C from the temperature at which the reference value was
last determined. The value of the reference is initially determined at the
factory at a room temperature of 25° C. After the value of the reference
is determined, the value should be stored in the EEPROM so that it can
be used by the input and output calibration routines. The calibration
procedure which determines the reference value is explained in the
Reference Calibration section later in this chapter. Locations have been
provided in the EEPROM to accommodate user calibration constants
(see Figure 6-1).
For best measurement results, the AT-MIO-16X onboard reference
needs to be measured to ±0.0015% (±15 ppm) accuracy. According to
standard practice, the equipment used to calibrate the AT-MIO-16X
should be 10 times as accurate; that is, the equipment should have
±0.00015% (±1.5 ppm) rated accuracy. Practically speaking,
calibration equipment with four times the accuracy of the item under
calibration is generally considered acceptable. Four times the accuracy
of the AT-MIO-16X is ±0.000375% (±3.75 ppm). To redetermine the
value of the reference on the AT-MIO-16X board you will need the
following equipment:
A precision DC voltage source (usually a calibrator):
Voltage: 5.0 to 9.99 V
Accuracy: ±0.00015% (±1.5 ppm) standard
±0.000375% (±3.75 ppm) standard