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AD7112JR

LC2MOS LOGDAC Dual Logarithmic D/A Converter

Analog Devices 

AD7112

MICROPROCESSOR INTERFACING

Figures 18 to 20 show interfaces between the AD7112 and

three popular 8-bit microprocessor systems, the MC68008,

8085A/8088 and the 8051. In the MC68008 and 8085/8088 in-

terfaces, the AD7112 is memory mapped with separate ad-

dresses for each DAC.

AD7112-8085A/8088 INTERFACE

Figure 18 shows a connection diagram for interfacing the

AD7112 to both the 8085A and the 8088 microprocessors. This

scheme is also suited to the Z80 microprocessor, but the Z80

address/data bus does not have to be demultiplexed. The

AD7112 is memory mapped with separate memory addresses

for DAC A and DAC B.

A15 – A8

DEN

8085A / 8088

ADDRESS BUS

ADDRESS

DECODE

LOGIC

A+1** A**

WR

ALE

AD7 – AD0

8-BIT

LATCH

DATA BUS

DAC A / DAC B

CS

WR

AD7112*

DB7 – DB0

* ANALOG CIRCUITRY HAS BEEN OMITTED FOR CLARITY.

** A = DECODED ADDRESS FOR AD7112 DAC A

A+1 = DECODED ADDRESS FOR AD7112 DAC B

Figure 18. AD7112–8085A/8088 Interface Circuit

AD7112–68008 INTERFACE

Figure 19 shows a connection diagram for interfacing the

AD7112 to the 68008 microprocessor. The AD7112 is again

memory mapped with separate memory addresses for DAC A

and DAC B.

A23 – A1

AS

68008

DTACK

R /W

D7 – D0

ADDRESS BUS

ADDRESS

DECODE

LOGIC

A+1** A**

DATA BUS

DAC A / DAC B

CS

WR

AD7112*

DB7 – DB0

* ANALOG CIRCUITRY HAS BEEN OMITTED FOR CLARITY.

** A = DECODED ADDRESS FOR AD7112 DAC A

A+1 = DECODED ADDRESS FOR AD7112 DAC B

Figure 19. AD7112–68008 Interface Circuit

AD7112–8051 INTERFACE

Figure 20 shows a connection diagram between the AD7112

and the 8051 microprocessor. The AD7112 is port mapped in

this interface. The loading structure is as follows: Data to be

loaded to the DAC is output to Port 1: P3.0, P3.1 and P3.2 are

bit addressable port lines and are used to control the DAC

select, CS and WR inputs. A sample routine for writing to DAC A

is shown below.

MOV A,DATA;

CLR 3.2;

CLR 3.0;

CLR 3.1;

MOV A,P1;

SET B 3.1;

SET B 3.0;

Data to be written is loaded to the accumulator.

Select DAC A.

Bring CS low.

Bring WR low.

Write data to DAC.

Deactivate WR.

Deactivate CS

P3.0

P3.1

P3.2

8051

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

CS

WR

DAC A / DAC B

AD7112*

DB0

DB1

DB2

DB3

DB4

DB5

DB6

DB7

* ANALOG CIRCUITRY OMITTED FOR CLARITY

Figure 20. AD7112–8051 Interface Circuit

APPLICATIONS

Automatic Gain Control

In an automatic gain control system an input signal is attenuated

or amplified so that its average output level remains constant.

The AD7112 D/A converter is used here as a variable gain or at-

tenuation element that adjusts the output signal relative to the

input level.

A feedback loop consisting of a detector, comparator, and up/

down counter continuously adjusts the contents of the counter

and hence the gain or attenuation of the circuit so that the signal

level at the output remains constant and equal to the reference

input signal. The negative feedback action of the loop ensures

that the average output voltage of the automatic gain control

system remains constant. Figure 21 shows a block diagram of a

typical AGC control loop using 1/2 AD7112 as the gain/ attenu-

ation element.

Whenever the input signal is outside the dynamic range of the

programmable gain element in the AGC loop, there should be a

stable, well defined input output relationship.

–10–

REV. 0

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