of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 267
- 3
DM2504 EOC 23
BTi
21
2 MHz o------!l- CLOCK 811
20
810
19
START~ 89
START 88
---u--
18
17
87 16
86
9
f ENA8LE 85 8
84
+5 7
83 6
B2
470 J1,
5
81
~ DATA
4
80
T
I 2 3 4 5 6 7 8 9 10 111121
8118108988 B7 8685 B4 83828180
10 K 15 - I ref
~
DAC= 312
+Vref 10 K 14
110 V.)
Lti- lre/ lout I lout 2 Vee Vee Ceomp Vlth
19 18 3
7~3 t
f;f,~ +5
17ticJI6
_15 0 . 01 }JF
1
=
I +
OR
ffi2 4 ~ "IN4148 +15
ANALOG
TO
-IOTO+IOV.
5K +15 -15 (KEEP THIS
NODE SMALL!
END OF CONY.
BTr (TWOS COMP.I
811 (OFFSET 8IN.)
810
89
88
87
86
85
84
83
82
81
80 LS8
Fig. 7-34. 6-tLsec 12-bit ADC using DAC-312
One very reasonably priced (under $5 for Y2 LSB linearity) unit is the
ADC0801 series from National Semiconductor. It uses a resistor string DAC
internally along with CMOS circuitry and is capable of conversion times in
the 60-100-J.Lsec range. It operates entirely from a +5 V power supply and
accepts analog input voltages from 0 to + 5 Vas well. The 8-bit output value
goes through an internal tri-state buffer and thus can be connected directly to
the microprocessor data bus in a small dedicated system. Figure 7-35 shows
a typical hookup for the ADC0803, which is the Y2 LSB grade unit.
Although the analog input is a very high impedance when the unit is idle, it
becomes finite and variable during conversion and therefore should be driven
from an op-amp output. Internally, a 2-to-1 voltage divider provides a
+ 2. 5-V reference from the + 5-V power input, but an external reference can
be connected to the Vrefpin if desired. The analog input range is twice the
reference voltage but must not be larger than the supply voltage or negative.
The sequencing clock is generated by a built-in RC oscillator and a
conversion requires from 66 to 73 clock cycles to complete. The components
shown will produce approximately 1 MHz or an external clock can be
connected to pin 4. A negative pulse on the start input while chip select is