Page 150 - Understanding Automotive Electronics

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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

Figure 4.23
Typical Automotive
Instrumentation
Architecture

FPO

sampled data is “smoothed” to a suitable continuous-time voltage by means
of a special filter known as a reconstruction filter. The continuous-time
output of this filter drives the continuous-time display.
In a great many applications, the display is digital (e.g., automotive
speed measurement). In this case, the conversion from digital to analog is not
required, and the computer output data can directly activate the digital
display.

Digital Filters

Digital filters can be pro- As an example of computer-based instrumentation signal processing
grammed on a micro- applications, consider the relatively straightforward task of filtering the output
computer to reject of a sensor. Low-pass filters pass low-frequency signals but reject high-frequency
specific types of signals signals. High-pass filters do just the reverse: They pass high-frequency signals
while passing other sig- and reject low-frequency signals. Bandpass filters pass midrange frequencies but
nals. reject both low and high frequencies. Analog filters use resistive, capacitive, and
inductive components, and sometimes operational amplifiers. Digital low-pass
and bandpass filters can be programmed on a microcomputer to perform
basically the same functions as their analog counterparts.
A digital low-pass filter could be used, for instance, to smooth the output
of an automotive fuel level sensor. The fuel level sensor produces an electrical
signal that is proportional to the height of the fuel in the center of the tank.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 137