Page 239 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes
Figure 12.24 shows one method of implementing the control circuitry for the converter shown in
Figure 12.23. The waveforms for one conversion are also shown. A conversion is initiated by the
receipt of a START signal. The positive edge of the START pulse is used to reset all the flip-flops
in the counter and to trigger the one-shot. The output of the one-shot sets the control flip-flop,
which makes the AND gate true and allows clock pulses to advance the counter.

The delay between the RESET pulse to the flip-flops and the beginning of the clock pulses (ensured
by the one-shot) is to ensure that all flip-flops are reset before counting begins. This is a definite
attempt to avoid any racing problems.

With the control flip-flop set, the counter advances through its normal count sequence until the
staircase voltage from the ladder is equal to the analog input voltage. At this time, the comparator
output changes state, generating a positive pulse which resets the control flip-flop. Thus the AND
gate is closed and counting ceases. The counter now holds a digital number which is equivalent
to the analog input voltage. The converter remains in this state until another conversion signal
is received.

Figure: 12.25: (a) Digitizing an Analog Voltage, (b) Reconstructed
Signal from the Digital Data

Voltage Analog input voltage

Ladder voltage Time
()
a

Voltage

b
() Time
If a new start signal is generated immediately after each conversion is completed, the converter
will operate at its maximum rate. The converter could then be used to digitize a signal as shown
in Figure 12.25a. Notice that the conversion times in digitizing this signal are not constant but
depend on the amplitude of the input signal. The analog input signal can be reconstructed from
the digital information by drawing straight lines from each digitized point to the next. Such a
reconstruction is shown in Figure 12.25b; it is, indeed, a reasonable representation of the original
input signal. In this case, it is important to note that the conversion times are smaller than the
transient time of the input waveform.
On the other hand, if the transient time of the input waveform approaches the conversion time,
the reconstructed output signal is not quite so accurate. Such a situation is shown in Figure 12.26a
and b. In this case, the input waveform changes at a rate faster than the converter is capable of
recognizing. Thus the need for reducing conversion time is apparent.

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