Page 244 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 12: A/D and D/A Converters

Notes
Successive-approx imation register: When dealing with conversion times this short, it
is usually necessary to take into account the other delays in the system (e.g., switching
time of the multiplexer, settling time of the ladder network, comparator delay, and settling
time).
Free-running mode: All the logic blocks inside the dashed line in Figure 12.30a, or
some equivalent arrangement, are frequently constructed on a single MSI chip; this
chip is called a successive approximation register (SAR). For example, the Motorola
MC6108 shown in Figure 12.30c is an 8-bit microprocessor-compatible A/D converter
that includes an SAR, D/A conversion capabilities, control logic, and buffered digital
outputs, in a 28-pin DIP.

12.8.2 The ADC0804
The ADC0804 is an inexpensive and very popular A/D converter which is available from
a number of different manufacturers, including National Semiconductor. The ADC0804 is
an 8-bit CMOS microprocessor-compatible successive-approximation A/D converter that is
supplied in a 20-pin DIP. It is capable of digitizing an analog input voltage within the range
0 to +5 Vdc, and it only requires a single dc supply voltage—usually +5 Vdc. The digital
outputs are both TTL- and CMOS-compatible.

Figure 12.31: Block Diagram of an ADC0804

The block diagram of an ADC0804 is shown in Figure 11.31. In this case, the controls are wired
such that the converter operates continuously. This is the so-called free-running mode. The
10–kW resistor, along with the 150–pF capacitor, establishes the frequency of operation according
to f ≈ 1/1.1(RC). In this case,

f ≈ 1/(1.1 * (10 * 1.5 * 150pF))
4
=1/(1.1 * (10 * 1.5 * 10 ))
–12
4
A momentary activation of the START switch is necessary to begin operation.

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