Page 142 - Understanding Automotive Electronics

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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

Figure 4.18
Staircase Output
Voltage of the DAC in
Figure 4.17

FPO

produce the desired shape and smoothness in the analog signal so that it is a
reasonable duplication of the variations in the digital levels.
Analog-to-Digital Converter

In addition, microcomputers can measure analog voltages by using a
special interface component called an analog-to-digital converter (ADC).
Analog-to-digital converters convert an analog voltage input into a digital
number output that the microcomputer can read.
The ADC performs a Figure 4.19 shows a conceptually simple, but not very practical, way of
function opposite of the making an ADC by using a DAC and a voltage comparator. The input to the
DAC: It converts an DAC is a binary number generated by the microcomputer that starts at the
analog signal into digi- minimum value and increases toward the maximum value, as seen in Figure
tal form for processing 4.18. This binary number is generated at the parallel output of the
by the microcomputer. microcomputer. The output of the DAC, V , is one input to the comparator.
out
The other input is the input voltage, V , that the ADC is measuring. When the
in
V voltage of the DAC is less then V , the output of the comparator, V comp , is
in
out
a low logic level. When V is greater than V , the output of the comparator is
in
out
a high level.
As soon as the binary number generated by the microcomputer causes
V from the DAC to be greater than V , the comparator output goes high and
in
out
stops the microcomputer from changing the binary number input further. The
binary number is used by the microcomputer as the equivalent of the analog
input voltage V . The microcomputer then resets and starts the binary number
in
generation again to make another match to the V voltage. In this manner, a
in
binary number, equivalent to a V and analog voltage, is produced at a selected
in
UNDERSTANDING AUTOMOTIVE ELECTRONICS 129