Page 240 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 12: A/D and D/A Converters

Notes
Figure 12.26 (a) Digitizing an Analog Voltage, (b) Reconstructed Signal from the Digital Data

Voltage

Time
()
a

Voltage

b
() Time

Prepare a truth table and circuit diagram for D/A converter.

12.7 Continuous A/D Conversion

An obvious method for speeding up the conversion of the signal as shown in Figure 12.26 is to
eliminate the need for resetting the counter each time a conversion is made. If this were continuous-
type done, the counter would not begin at zero each time, but instead would begin at the value
A/D converter of the last converted point. This means that the counter would have to be capable
of counting either up or down. This is no problem; we are already familiar with the operation of
up-down counters.
There is, however, the need for additional logic circuitry, since we must decide whether to count
up or down by examining the output of the comparator. An A/D converter which uses an up-
down counter is shown in Figure 12.27. This method is known as continuous conversion, and
thus the converter is called a continuous type A/D converter.
The D/A portion of this converter is the same as those previously discussed, with the exception
of the counter. It is an up-down counter and has the up and down count controls lines in addition
to the advance line at its input.
The output of the ladder is fed into a comparator which has two outputs instead of one as before.
When the analog voltage is more positive than the ladder output, the up output of the comparator
is high.

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