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AD7846
(Rev.:RevE)

LC2MOS 16-Bit Voltage Output DAC

Analog Devices 

AD7846

POSITION MEASUREMENT APPLICATION

Figure 22 shows the AD7846 in a position measurement appli-

cation using an LVDT (Linear Variable Displacement Trans-

ducer), an AD630 synchronous demodulator and a comparator

to make a 16-bit LVDT-to-Digital Converter. The LVDT is

excited with a fixed frequency and fixed amplitude sine wave

(usually 2.5 kHz, 2 V pk-pk). The outputs of the secondary coil

are in antiphase and their relative amplitudes depend on the

position of the core in the LVDT. The AD7846 output interpo-

lates between these two inputs in response to the DAC input

code. The AD630 is set up so that it rectifies the DAC output

signal. Thus, if the output of the DAC is in phase with the

VREF+ input, the inverting input to the comparator will be posi-

tive, and if it is in phase with VREF–, the output will be negative.

By turning on each bit of the DAC in succession starting with

the MSB, and deciding to leave it on or turn it off based on the

comparator output, a 16-bit measurement of the core position is

obtained.

In a multiple DAC system, the double buffering of the AD7846

allows the user to simultaneously update all DACs. In Figure

24, a 16-bit word is loaded to the input latches of each of the

DACs in sequence. Then, with one instruction to the appropri-

ate address, CS4 (i.e., LDAC) is brought low, updating all the

DACs simultaneously.

ALE

8086

DEN

RD

WR

AD0–AD15

16-BIT

LATCH

ADDRESS BUS

ADDRESS

DECODE

CS

AD7846*

LDAC

DATA BUS

R/ W

CLR +5V

DB0–DB15

ASIN ␻ t

LVDT x ASIN ␻ t

–(1–x)ASIN ␻ t

*ADDITIONAL PINS

OMITTED FOR CLARITY

VREF+ VOUT

RIN

AD7846*

VREF–

DGND

DB15 DB0

SIGNAL

GROUND

PROCESSOR DATA BUS

R1

100k⍀

C1

1␮F

AD630*

TO

PROCESSOR PORT

Figure 22. AD7846 in Position Measurement Application

MICROPROCESSOR INTERFACING

AD7846-to-8086 Interface

Figure 23 shows the 8086 16-bit processor interfacing to the

AD7846. The double buffering feature of the DAC is not used

in this circuit since LDAC is permanently tied to 0 V. AD0–

AD15 (the 16-bit data bus) are connected to the DAC data bus

(DB0–DB15). The 16-bit word is written to the DAC in one

MOV instruction and the analog output responds immediately.

In this example, the DAC address is D000H.

ADDRESS BUS

ALE

8086

DEN

RD

WR

16-BIT

LATCH

ADDRESS

DECODE

CS

LDAC

+5V CLR

AD7846*

R/ W

CS

AD7846*

LDAC

R/ W

CLR +5V

DB0–DB15

*LINEAR CIRCUITRY

OMITTED FOR CLARITY

CS

AD7846*

LDAC

R/ W

CLR +5V

DB0–DB15

Figure 24. AD7846-to-8086 Interface: Multiple DAC System

AD7846-to-MC68000 Interface

Interfacing between the AD7846 and MC68000 is accom-

plished using the circuit of Figure 25. The following routine

writes data to the DAC latches and then outputs the data via the

DAC latch.

1000

MOVE.W #W, D0

The desired DAC data, W,

is loaded into Data Regis-

ter 0. W may be any value

between 0 and 65535

(decimal) or 0 and FFFF

(hexadecimal).

MOVE.W D0, $E000 The data, W, is transferred

between D0 and the DAC

register.

MOVE.W #228, D7

TRAP #14

Control is returned to the

System Monitor using

these two instructions.

AD0–AD15

DATA BUS

*LINEAR CIRCUITRY

OMITTED FOR CLARITY

DB0–DB15

Figure 23. AD7846-to-8086 Interface Circuit

REV. E

–11–

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