Chapter 6
Calibration Procedures
© National Instruments Corporation
6-5
AT-MIO-64F-5 User Manual
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 20¡ 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-64F-5 onboard reference needs to be measured to
±0.012% (±120 ppm) accuracy. According to standard practice, the equipment used to calibrate
the AT-MIO-64F-5 should be 10 times as accurate; that is, the equipment should have ±0.001%
(±10 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-64F-5 is ±0.003% (±30 ppm). To redetermine the value of the
reference on the AT-MIO-64F-5 board you will need the following equipment:
A precision DC voltage source (usually a calibrator)
Voltage:
5.0 to 10.0 V
Accuracy:
±0.001% (±10 ppm) standard
±0.003% (±30 ppm) sufficient
It is important to realize that inaccuracy of the internal voltage reference results only in gain
error. Offset error is unaffected. If an application can tolerate slight gain inaccuracy, there
should not be a need to redetermine the value of the onboard reference.
Calibration DACs
There are eight 8-bit DACs (CALDAC<0..7>) on the AT-MIO-64F-5 that are used for
calibration. These DACs are described in Table 6-2.
Table 6-2. Calibration DACs
Analog
DAC
Function
Adjustment
Range
Incremental
Effect
CALDAC0
Pregain offset trim
2.4 mV
-9.4 µV
Input
CALDAC1
Postgain offset trim
41 LSB
-0.16 LSB
CALDAC2
Unipolar offset trim
41 LSB
-0.16 LSB
CALDAC3
Gain trim
1.0%
-39 ppm
CALDAC4
DAC0 offset trim
200 mV
-0.78 mV
Output
CALDAC5
DAC0 gain trim
0.47 %
-18 ppm
CALDAC6
DAC1 offset trim
200 mV
-0.78 mV
CALDAC7
DAC1 gain trim
0.47 %
-18 ppm
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