Hardware Reference
In-Depth Information
Time
(a) Original analog signal
Time
(b) Signal after sampling and DAC process
Figure 7.34 Analog signal and the resulting signal after sampling, ADC, and
DAC process
the DAC also at the uniform intervals. As a result, all practical DACs output a sequence of
piecewise constant values or rectangular pulses. An example of this process is illustrated in
Figure 7.34.
A DAC has many applications. Examples are digital gain and offset adjustment, programma-
ble voltage and current sources, programmable attenuators, digital audio, closed-loop positioning,
robotics, and so on. In the last few years, digital video is also getting more and more popular. As
more and more flat panel displays come with DVI and HDMI interfaces, digital video will become
the norm in a few years. Although there are a few microcontrollers incorporating the D/A con-
verter on the chip, most microcontrollers still need to use an off-chip D/A converter to perform
the D/A conversion function. The HCS12 is no exception. A D/A converter may use a serial or
parallel interface to obtain digital code from the microprocessor or microcontroller.
There are several factors to consider when choosing a DAC.
Resolution. This is the number of possible output levels the DAC is designed to
reproduce. The resolution is usually stated as the number of bits it uses and is the
base 2 logarithm of the number of levels. For example, an 8-bit DAC can represent
256 levels.
Dynamic range. This is a measurement of the difference between the largest and
smallest signals the DAC can reproduce represented in decibels. This characteristic
is related to the DAC resolution and noise floor (the sum of all the noise sources).
Number of channels. A DAC may have more than one output channel to satisfy
the needs of the applications that require more than one channel.
 
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