User`s manual

XP8500 I/O Configurations s 31
Step 7. Calibrate the A/D Converter
Regardless of whether the mathematically derived resistance values or the
scaled resistance values are found, the inherent component-to-component
variations of 5% or 1% resistors can completely swamp the 0.25%
resolution of the A/D converter. To achieve the highest accuracy possible,
the A/D converter itself must be calibrated.
The software drivers for the A/D converter provide routines to compute
calibration coefficients, given two reference points, and then to store the
calibration coefficients in a defined location in nonvolatile memory. Each
reference point consists of a pair of values: the actual applied test voltage
and the raw converted A/D value (a 12-bit integer). Z-Worlds software
will automatically use these calibration coefficients to correct all subse-
quent A/D readings.
Op-Amp Test Points
The factory-installed gain and bias resistors (R1, R3, R5, R7 = 2370 Ω and
R2, R4, R6, R8 = 39.2 kΩ) have a 2% tolerance. These resistors yield a
gain of 0.25 for a unipolar input-signal range of 0 V to 10 V.
Figure 3-10 shows some convenient points at which to make voltage
measurements of the op-amp.
Figure 3-10. XP8500 LMC660 Op-Amp Test Points
AN0 AN1 AN2 AN3
AGND AGND AGND AGND
VR0- VR1- VR2- VR3-
VR0+ VR1+ VR2+ VR3+