AD394TD/883B View Datasheet(PDF) - Analog Devices

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AD394TD/883B

uP Compatible Multiplying Quad 12-Bit D/A Converter

Analog Devices 

AD394

8-Bit Processors

The circuit of Figure 13 shows the general principles for

connecting the AD394 to an 8-bit data bus. The 74LS244 buffers

the data bus; its outputs are enabled when the DAC address

appears on the address bus. The first byte sent to the DAC is

loaded to the 74LS373 octal latch and, when the second byte is

sent to the DAC, it is combined with the first byte to create a

12-bit word. The connections shown are for right-hand justified

data. CS and WR inputs to the DAC are also gated, and when

active, the DAC is loaded. Pull-up resistors at the output of the

74LS244 buffer ensure that the inputs to the DAC do not float at

an ill-defined level when the DAC is not being addressed. This

method of connecting 12-bit DACs to an 8-bit data bus is most

cost effective when multiple DACs are utilized for 8-bit data bus

applications.

the ADC function since the processor can perform the required

digital operations under software control. A suitable circuit is

shown in Figure 14. The AD311 comparator compares the

unknown input voltage to one of the AD394 outputs for the

analog-to-digital conversion, while the other three outputs are

used as normal DACs. The diode clamp shown limits the

voltage swing at the comparator input and improves conversion

speed. With careful layout, a new compar-ison can be

performed in less than 15 µs, resulting in a 12-bit successive

approximation conversion in under 180 µs. The benefit of using

the AD394 in this application is that one ADC and three DACs

can be implemented with only two IC packages (the AD394 and

the comparator).

Figure 14. Using One AD394 Output for A/D Conversion

Programmable Window Comparator

The AD394 can be used to perform limit testing of responses to

digitally controlled input signals. For example, two DACs may

be used to generate software-controlled test conditions for a

component or circuit. The response to these input conditions

can be either completely converted from analog to digital or

simply tested against high and low limits generated by the two

DACs in the AD394.

Figure 13. 8-Bit Data Bus Interface

APPLICATIONS

The functional density of the AD394 permits complex analog

functions to be produced under digital control, where board

space requirements would otherwise be prohibitive. Multiple-

output plotters, multichannel displays, complex waveform

generation, and multiple programmable voltage sources can all

be implemented with the AD394 in a fraction of the space that

would be needed if separate DACs were used.

Using the AD394 for Analog-to-Digital Conversion

Many systems require both analog output and analog input

capability. While complete integrated circuit analog-to-digital

converters (such as the AD574A) are readily available, the

AD394 can be used as the precision analog section of an ADC

if some external logic is available. Several types of analog-to-

digital converters can be built with a DAC, comparator, and

control logic, including staircase, tracking, and successive-

approximation types. In systems that include a micropro-cessor,

only a comparator must be added to the AD394 to accomplish

Figure 15. Programmable Window Comparator

Used in Power-Supply Testing

In the circuit shown in Figure 15, two AD311 voltage compar-

ators are used within the AD394 to test the output of a 5 V

power-supply regulator. The AD394 VOUT1 output (through an

appropriate current booster) drives the input to the regulator to

simulate variations in input voltage. The output of the regulator

is applied to Comparators 1 and 2, with their outputs wire-

Rev. A | Page 10 of 12

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