Z-World eDisplay OP7200, Руководство пользователя

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24 eDisplay (OP7200)
3.4.3 Calibrating the A/D Converter Chip
Manufacturing tolerances for resistors, bias currents, offset voltages, gain, and the like
introduce errors into the A/D conversions. Ideally there would be a one-to-one straight-
line relationship between the input voltage and the output of the A/D converter, and a
graph of such a line would have a slope of 1 and would pass through the (0,0) coordinate.
However, the errors arising from manufacturing tolerances introduce a deviation between
the applied input voltage and the voltage that is output by the A/D converter. The actual
plot of voltage in vs. the voltage out from A/D converter is not actually a straight line.
However, a straight line is a very good first-order approximation, and the calibration rou-
tines provided for the OP7200 are based on a straight line with a slope of 1 and an offset
from (0,0). The calibration routines use two known measurement points on the voltage-in
vs. voltage-out line as the basis to calculate calibration constants that will be used to adjust
for the slope of the line and the offset from (0,0). The calibration routines typically use
input voltage points that are 10% less then the maximum and 10% more than the mini-
mum readings possible for the A/D converter on any given range.
Quality calibration procedures are extremely important in obtaining good A/D converter
results. No matter how high a resolution the A/D converter has, it cannot compensate for
improper calibration. A/D converter results will never be more accurate than the meter
used in the calibration process. Therefore, use the best digital volt and milli-amp meter
available that meets or exceeds the accuracy of the A/D converter chip.
3.4.3.1 Modes
The OP7200 A/D converter operates in three different modes:
•
the single-ended mode,
•
the differential mode, and
•
the milli-amp mode
The calibration and read routines provided correspond to these three modes.
3.4.3.2 Calibration Constants
The A/D converter has eight individual input channels, and each channel has eight pro-
grammable gains. Additionally, the A/D converter has the capability for adjacent inputs
to be paired to make differential measurements with eight different gains, and provision is
also made to convert 4–20 mA analog current measurements.