Manual 104-AIO16A and 104-AIO16E
35
Breakdown: Calibrating the A/D
A/D Calibration, while fundamentally different than the DAC calibration process described above, is also
very similar. In the A/D calibration you are determining the amount of calibration error, adjusting the digital
potentiometers until the error is eliminated, and storing the adjustment for later use. Unlike the DAC, there
are two digital potentiometers for the A/D (offset adjust and gain adjust). The same two steps apply:
Step 1. Determine the calibration constants.
Step 2. Write the calibration constants into the calibration potentiometers.
First, let’s expand Step 1 into its component sub-steps for the A/D:
Step 1.
Determine the Calibration Constants for the A/D
1.1 Offset
Adjust
1.1.1 Apply Ground to the A/D input.
1.1.2 Acquire the voltage using the A/D converter.
1.1.3 Adjust the value in the digital calibration potentiometer for the A/D until the voltage read
by the A/D indicates 1 count
1
.
1.1.4 Store the value from the digital calibration potentiometer into a spot
2
in the EEPROM for
use on the next and subsequent board initializations.
1.2 Gain
Adjust
1.2.1 Apply a known voltage
3
to the A/D Input.
1.2.2 Acquire the voltage using the A/D converter.
1.2.3 Adjust the value in the digital calibration potentiometer for the A/D until the voltage read
by the A/D matches the input voltage.
1.2.4 Store the value from the digital calibration potentiometer into a spot
2
in the EEPROM for
use on the next and subsequent board initializations.
Step 2.
Write the Calibration Constants into the Calibration Potentiometers
For details on step 2, please refer to the section “Step 2" near the end of this appendix.
Note 1: Zero calibration is performed to a value of “1" count instead of “0" to avoid railing the inputs, a
condition where you calibrate the board off the end of a range due to the inability of the device to report
voltages above the maximum or below the minimum.
Note 2: The correct spot in EEPROM varies with the A/D range and single-ended/differential selection
being calibrated. We recommend you use the same locations as our provided Calibration program,
drivers, and samples, as shown in
Table B-1
, below.
Note 3: The known voltage to use varies with the jumper selected A/D input range. For best results apply
a voltage within 5% of the full scale voltage for your selected range.
