Page 236 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 12: A/D and D/A Converters

Flash converter: Remember that three comparators were necessary for defining four ranges. In Notes
general, it can be said that 2 – 1 comparators are required to convert to a digital signal that has
n
n bits. Some of the comparators have inverters at their outputs since both C and C are needed
for the encoding matrix.
The encoding matrix must accept seven input levels and encode them into a 3-bit binary number
(having eight possible states). Operation of the encoding matrix can be most easily understood
by examination of the table of outputs in Figure 12.22.
The 2 bit is easiest to determine since it must be high (the 2 flip-flop must be set) whenever C
2
2
4
is high.
The 2 line must be high whenever C is high and C is high, or whenever C is high. In equation
1
4
6
2
form, we can write 2 = C C + C .
1
4
6
2
The logic equation for the 2 bit can be found in a similar manner; it is
0
2 = CC + 12 CC + 3 4 CC + C 7
0
56
The transfer of data from the encoding matrix into the register must be carried out in two steps.
First, a positive reset pulse must appear on the RESET line to reset all the flip-flops low. Then,
a positive READ pulse allows the proper READ gates to go high and thus transfer the digital
information into the flip-flops.
Interestingly, a convenient application for a 9318 priority encoder is to use it to replace all the
digital logic as shown in Figure 12.21b. Of course, the inputs C , C , ..., C must be TTL-compatible.
2
7
1
In essence, the output of the 9318 is a digital number that reflects the highest-order zero input;
this corresponds to the lowest reference voltage that still exceeds the input analog voltage.
The construction of a simultaneous A/D converter is quite straightforward and relatively easy to
understand. However, as the number of bits in the desired digital number increases, the number
of comparators increases very rapidly (2 – 1), and the problem soon becomes unmanageable.
n
Even though this method is simple and is capable of extremely fast conversion rates, there are
preferable methods for digitizing numbers having more than 3 or 4 bits. Because it is so fast, this
type of converter is frequently called a flash converter.
Figure 12.22: Logic Table for the Converter
Input voltage Comparator for level Binary output
C 1 C 2 C 3 C 4 C 5 C 6 C 7 2 2 2 1 2 0
0 to V/8 Low Low Low Low Low Low Low 0 0 0
V/8 to V/4 High Low Low Low Low Low Low 0 0 1
V/4 to 3V/8 High High Low Low Low Low Low 0 1 0
3V/8 to V/2 High High High Low Low Low Low 0 1 1
V/2 to 5V/8 High High High High Low Low Low 1 0 0
5V/8 to 3V/4 High High High High High Low Low 1 0 1
3V/4 to 7V/8 High High High High High High Low 1 1 0
7V/8 to V High High High High High High High 1 1 1
The outputs are tri-state TTL = compatible. The flash A/D converter is capable of operation with
a 25-MHz clock! It comes in a 24-pin DIP and requires two dc supply voltages—typically +5 Vdc
and –5 Vdc. Possible applications include radar signal processing, video displays, high-speed
instrumentation, and television broadcasting.
A time-stretch analog-to-digital converter (TS-ADC) digitizes a very wide
bandwidth analog signal that cannot be digitized by a conventional electronic
ADC, by time-stretching the signal prior to digitization.

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