National Instruments AT-MIO-16X - Manual (Page 222)

Chapter 6 Calibration Procedures
AT-MIO-16X User Manual 6-10 © National Instruments Corporation
CALDAC1 until the measured voltage is equal to the value of the
reference as stored in the onboard EEPROM. Once the board is
calibrated at a gain of 1, there is only a small residual gain error
(±0.02% maximum) at the other gains. The gain error is always
calibrated immediately after the offsets are calibrated.
Analog Output Calibration
To null out error sources that affect the accuracy of the output voltages
generated, the output calibration routine calibrates the analog output
circuitry by adjusting the following potential sources of error:
• Analog output offset error
• Analog output gain error
In order to read the analog output voltages, the output calibration
routine requires that the AT-MIO-16X analog outputs be wrapped back
to the analog inputs as follows:
1. Connect DAC0 to a channel in ACH<0..7>
2. Connect DAC1 to another channel in ACH<0..7>
3. Connect AO GND to the negative sides of the channels selected in
steps 1 and 2. Do not tie AO GND to AI GND as this will complete
a ground loop, potentially introducing offset calibration errors of
several LSBs.
The output calibration routines require that the input is already
calibrated, because it uses the input circuitry as the source of
calibration.
Offset error in the analog output circuitry is the total of the voltage
offsets contributed by the components in the output circuitry. This
error, which is independent of the DAC output voltage, is the amount of
voltage generated by the DAC when it is set to produce 0 V. To correct
this offset error, the routine writes a value of 0 to each DAC and adjusts
CALDAC6 and CALDAC7 until it measures 0 V between each analog
output and AO GND.
Gain error in the analog output circuitry is the sum of the gain errors
contributed by the components in the output circuitry. This error is a
voltage difference between the desired voltage and the actual output
voltage generated that is proportional to the DAC output voltage. To
correct the gain error, the output calibration routine sets each analog