AD7804 datasheet(19/28 Pages) AD

19 / 28 page

AD7804/AD7805/AD7808/AD7809

–19–

REV. A

Table IX. Offset Binary Code Table for Sub DAC

Digital Input

Analog Output

MSB . . . LSB

11111111

VBIAS/16

× 127/256

11111110

VBIAS/16

× 126/256

10000001

VBIAS/16

× 1/256

10000000

01111111

–VBIAS/16

× 1/256

00000001

–VBIAS/16

× 127/256

00000000

–VBIAS/32

127

VBIAS

128

VBIAS

DAC INPUT CODE

Figure 26. Sub DAC Output Voltage vs. DAC Input Codes

(HEX) for Offset Binary Coding

Configuring the AD7804/AD7808 for Offset Binary Coding

Figure 27 shows a typical configuration for the AD7804/AD7808.

This circuit can be used for both 3.3 V or 5 V operation and

uses an external AD589 as the reference for the part and serial

interfacing with offset binary coding is used. The MX1 and

MX0 bits in the system control register have to be set to enable

selection of the AD589 as the reference. The following are the

steps required to operate the DACs in this part. Figures 4 to 7

show the contents of the registers on the AD7804/AD7808.

AD589

0.01 F

AD7804/

AD7808

AVDD

DVDD

VOUTA

VOUTB

VOUTC

VOUTD

AGND

DGND

LDAC

CLR

COMP

SDIN

FSIN

DVDD

REFIN

REFOUT

0.1 F

10 F

0.01 F

SERIAL

INTERFACE

+3.3V/+5V

6.8k

CLKIN

Figure 27. Typical Configuration for AD7804/AD7808

Using an AD589 1.23 V Reference for the AD7804/AD7808

System Control Register Serial Write:

Write 0060 Hex

Mode bits select system control register

and configure system for offset binary

coding and normal operation.

Channel Control Register Serial Write:

Write 4210 Hex

Mode bits select channel control register,

channel A is configured for operation with

external reference.

Main DAC Data Register Serial Write:

Write 23FF Hex

This 16-bit write selects writing to channel

A and writes full scale to the Main DAC.

Sub DAC Data Register Serial Write:

Write A3FF Hex

This 16-bit write selects writing to channel

A Sub DAC and writes full scale to the

Sub DAC.

Table VIII and Figure 25 show the analog outputs available for

the above configuration when writing to the Main DAC only

while Table IX and Figure 26 show the contributions from the

Sub DAC to the overall transfer function. The total output for a

single channel when using offset binary coding is the sum of that

from the Main DAC and the Sub DAC.

VOUT = VOUT' + VOUT"

= VBIAS + 1.875

× V

BIAS

× ((NA-512)/1024) + V

BIAS/16

× [(NB-128)/256]

= VBIAS

× (1 + 1.875 × ((NA-512)/1024) + (NB-128)/

4096)

where NA ranges from 0 to +1023 and NB ranges from 0 to

+255. Figure 28 shows a pictorial view of the transfer function

for any DAC channel.

AD7804 Datasheet(PDF) 19 Page - Analog Devices

AD7804 Datasheet(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL