Datasheet

AD7804/AD7805/AD7808/AD7809
–17–REV. A
Table VII shows the twos complement transfer function for the
Sub DAC. Figure 23 shows the Sub DAC transfer function for
twos complement coding. Any Sub DAC output voltage can be
expressed as:
V
OUT
" = V
BIAS
/16 × (NB/256)
where NB is the decimal equivalent of the twos complement
input code. NB ranges from –128 to +127.
Table VII. Twos Complement Code Table for Sub DAC
Digital Input Analog Input
MSB . . . LSB
01111111 (V
BIAS
/16) × (127/256)
01111111 (V
BIAS
/16) × (126/256)
00000001 (V
BIAS
/16) × (1/256)
00000000 0
11111111 (–V
BIAS
/16) × (1/256)
10000001 (–V
BIAS
/16) × (127/256)
10000000 (–V
BIAS
/16) × (128/256)
127
3
V
BIAS
256 16
0
DAC OUTPUT VOLTAGE
128
3
V
BIAS
256 16
80 81
DAC INPUT CODE
FF
00
01
7E
7F
Figure 23. Sub DAC Output Voltage vs. DAC Input Codes
(HEX) for Twos Complement Coding
The total output for a single channel when using twos comple-
ment coding is the sum of the voltage from the Main DAC and
the Sub DAC.
V
OUT
= V
OUT
' + V
OUT
"
= V
BIAS
+ 1.875 × V
BIAS
× (NA/1024) + V
BIAS
/16 × (NB/256)
= V
BIAS
× (1 + 1.875 × NA/1024 + NB/4096)
where NA ranges from –512 to +511 and NB ranges from –128 to
+127. Figure 28 shows a pictorial view of the transfer function for
any DAC.
Configuring the AD7805/AD7809 for Twos Complement Coding
Figure 24 shows a typical configuration for the AD7805/AD7809.
The circuit can be used for either 3.3 V or 5 V operation and uses
the internal V
DD
/2 as the reference for the part and 10-bit paral-
lel interfacing is used. The following are the steps required to
operate the Main DACs in this part.
0.01mF
AD7805/
AD7809
AV
DD
DV
DD
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
AGND
DGND
LDAC
CLR
COMP
MODE
D9
D0
CS
WR
DV
DD
REFIN
REFOUT
0.1mF
0.1mF
10mF
A1
A0
0.01mF
A2*
*USED ON THE
AD7809 ONLY
DIGITAL
INTERFACE
+3.3V/+5V
Figure 24. Typical Configuration for AD7805/AD7809
System Control Register Write:
MODE = 0, address inputs (A2, A1, A0) are don’t cares.
Write 020 Hex Configure part for 10-bit parallel, twos
complement coding, normal operation
Channel Control Register Write:
MODE = 0, address inputs (A2, A1, A0) select desired channel.
Write 011 Hex Internal V
DD
/2 selected as V
BIAS
for
DAC, and any DAC data writes that
follow are to the Main DAC.
DAC Data Register Write:
MODE = 1, address inputs (A2, A1, A0) select desired channel.
Write XXX Hex With MODE = 1 all data writes are to
the selected DAC. XXX is the required
data. 200 Hex will give zero scale and 1FF
Hex will give full scale from the DAC.