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9 AUTOMOTIVE INSTRUMENTATION

INPUT AND OUTPUT SIGNAL CONVERSION

Most sensors provide an It should be emphasized that any single input can be either digital
analog output, whereas switched or analog, depending on the technology used for the sensor. A typical
computers require digi- instrumentation computer is an integrated subsystem that is designed to accept
tal inputs. A/D convert- all of these input formats. A typical system is designed with a separate input
ers convert analog from each sensor or switch. An example of an analog input is the fuel quantity
signals to digital codes sensor, which is normally a potentiometer attached to a float, as described in
appropriate for signal detail later in this chapter. The measurement of vehicle speed given in Chapter
processing by the com- 8 is an example of a measurement that is already in digital format.
puter. The analog inputs must all be converted to digital format using an analog
to digital (A/D) converter as explained in Chapter 4 and illustrated in Figure
9.3. The digital inputs are, of course, already in the desired format. The
conversion process requires an amount of time that depends primarily on the
A/D converter. After the conversion is complete, the digital output generated
by the A/D converter is the closest possible approximation to the equivalent
analog voltage, using an M-bit binary number (where M is chosen by the
designer and is normally between 8 and 32). The A/D converter then signals
the computer by changing the logic state on a separate lead (labeled “conversion
complete’’ in Figure 9.3) that is connected to the computer. (Recall the use of
interrupts for this purpose, as discussed in Chapter 4.) The output voltage of
each analog sensor for which the computer performs signal processing must be
converted in this way. Once the conversion is complete, the digital output is
transferred into a register in the computer. If the output is to drive a digital
display, this output can be used directly. However, if an analog display is used,
the binary number must be converted to the appropriate analog signal by using
a digital-to-analog (D/A) converter (see Chapter 4).

Figure 9.3
Analog-to-Digital
Conversion

FPO

300 UNDERSTANDING AUTOMOTIVE ELECTRONICS