Calibration Procedures
Chapter 6
AT-MIO-64F-5 User Manual
6-6
© National Instruments Corporation
Calibration Channels
Table 6-3 lists the calibration channels for the AT-MIO-64F-5, showing what connects to each
input of the PGIA when each channel is selected. To measure these channels, the board must be
in Calibration mode. See Chapter 4,
Register Map and Descriptions
, for more information.
Table 6-3. Calibration Channels
Effect
Channel
PGIA (+)
PGIA (-)
0
AIGND
AIGND
1
AOGND
AIGND
2
AOUT0
AOGND
3
AOUT1
AOGND
4
AIGND
AIGND
5
REF5V
AIGND
6
AOUT0
REF5V
7
AOUT1
REF5V
Reference Calibration
The AT-MIO-64F-5 has a stable voltage reference to which gain can be calibrated. The value of
this voltage reference is determined through the reference calibration routine, which requires a
known external voltage between 5 and 9.99 V to be connected differentially on any desired input
channel. The routine calibrates the circuitry to the external reference and then reads the internal
reference. This value is stored as a two's complement binary number in the onboard EEPROM
for subsequent use by the analog input calibration routines.
Because the onboard reference is very stable with respect to time and temperature, it is seldom
necessary to use the reference calibration routine. Every year should be sufficient, 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. Factory calibration is performed at
approximately 25¡ C.
Analog Input Calibration
To null out error sources that compromise the quality of measurements, your input calibration
routine should calibrate the analog input circuitry by adjusting the following potential sources of
error:
¥
Pregain offset (offset error at the input of the PGIA)
¥
Postgain offset (offset error at the input of the ADC)
¥
Unipolar offset (additional postgain offset present only in unipolar mode)
¥
Gain error of the analog input circuitry
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