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3.4.2 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 A/D Converter Card 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 minimum 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.
3.4.2.1 Modes
The A/D converter operates in three different modes:
? the single-ended mode,
? the differential mode, and
? the 4–20 mA current mode
The calibration and read routines provided correspond to these three modes.
3.4.2.2 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.
To get the best results from the A/D converter, it is necessary to calibrate each mode for
each of its gains. The following table provides a grid for each possible set of calibration
constants.
User ’s Manual
59
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