AD7804/AD7805/AD7808/AD7809
Table IX. Offset Binary Code Table for Sub DAC
+3.3V/+5V
Digital Input
MSB . . . LSB
Analog Output
0.1F
0.1F
10F
6.8k⍀
11111111
11111110
10000001
10000000
01111111
00000001
00000000
V
BIAS/16 × 127/256
0.01F
0.01F
AV
DV
DD
DD
VBIAS/16 × 126/256
VBIAS/16 × 1/256
0
–VBIAS/16 × 1/256
–VBIAS/16 × 127/256
–VBIAS/32
COMP
REFIN
AD589
AD7804/
AD7808
FSIN
REFOUT
SERIAL
INTERFACE
SDIN
CLKIN
V
A
B
OUT
V
OUT
V
C
D
OUT
V
127
128
BIAS
32
V
DV
DD
CLR
OUT
LDAC
DGND
AGND
0
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
V
BIAS
32
coding and normal operation.
Channel Control Register Serial Write:
00
01
7F 80 81
FF
DAC INPUT CODE
FE
Write 4210 Hex Mode bits select channel control register,
channel A is configured for operation with
external reference.
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.
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 × VBIAS × ((NA-512)/1024) + VBIAS/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.
REV. A
–19–
ETC [ ETC ]
