Page 242 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 12: A/D and D/A Converters

A waveform typical of this type of converter is shown in Figure 12.28. You can see that this Notes
converter is capable of following input voltages that change at a much faster rate.
Figure 12.28: Continuous A/D Conversion

12.13: Quite often, additional circuitry is added to a continuous converter to
ensure that it cannot count off scale in either direction. For example, if the counter contained all
ls, it would be undesirable to allow it to progress through a count-up cycle, since the next count
would advance it to all Os. We would like to design the logic necessary to prevent this.

Solution:
The two limit points which must be detected are all 1s and all 0s in the counter. Suppose that we
construct an AND gate having the 1 sides of all the counter flip-flops as its inputs. The output
of this gate will be true whenever the counter contains all 1s. If the gate is then connected to the
reset side of the up flip-flop, the counter will be unable to count beyond all 1s.
Similarly, we might construct an AND gate in which the inputs are the 0 sides of all the counter
flip-flops. The output of this gate can be connected to the reset side of the down flip-flop, and the
counter will then be unable to count beyond all 0s. The gates are shown in Figure 12.29.

Figure 12.29: Count-Limiting Gates for the Converter

12.8 A/D Techniques

There are a variety of other methods for digitizing analog signals-too many to discuss in
detail. Nevertheless, we shall take the time to examine two more techniques and the reasons
for their importance.

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