Page 123 - Embedded Microprocessor Systems Real World Design

(for a DAC) will be no better than the cumulative errors of the ADC/DAC, the ref-
erence, and any analog amplification. Using 5 percent resistors in the front end
will completely defeat the purpose of having a 10-bit ADC.
Even though the accuracy of results is limited to accuracy of the components,
you can sometimes achieve better accuracy by calibrating each system. An ADC
system might be calibrated by driving the analog input with a known, precise voltage
and letting the microprocessor store a calibration constant that is used to correct
all future measurements. This sort of calibration can compensate for part-to-part
variations, but it won’t compensate for things like aging or temperature drift. And,
of course, this is an extra and possibly expensive manufacturing step.
To perform calibration, the system will need:
A means to input a calibration signal. This must be processed using all the same
parts that the system will use in normal operation (input amplifiers, filters, and
so on) to insure an accurate result.
Nonvolatile memory to store the calibration results.
Sufficient processing power to perform the required correction in normal
operation.
Software for calibration. This can be permanently part of the operating software,
or it could be special software that is loaded only during the calibration
operation.
If field replacement of parts must be supported, there must be a means to cali-
brate field replacements before they are shipped or a means to calibrate them
in the field.
Of course, calibration cannot compensate for temperature drift unless it is
performed at various temperatures. In addition, calibration cannot compensate
for EM1 or other noise inputs that cannot be duplicated during the calibration
step.

Internal Analog-to-Dlgffal Converters

Many microprocessors include an internal analog-todigital converter. The
Microchip PIC16C7x series is a typical example. The parts in this family have an
internal &bit A/D converter. The parts have from four to eight analog inputs. There
is only one A/D converter, but an internal analog multiplexer allows the A/D con-
verter to process any of the inputs, one at a time. Any microprocessor design that
uses an A/D converter, whether internal or external, must take into account some
considerations.
The Microchip A/D converter handles multiple inputs by selecting one at a time
under software control. Once an input is selected, a settling time must elapse before
the A/D conversion can start. If the conversion is started immediately, the result
will be incorrect. The software must take this delay into account.

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