Datasheet

AD7398/AD7399
Rev. C | Page 19 of 24
PROGRAMMABLE DAC REFERENCE VOLTAGE
With the flexibility of the AD7398/AD7399, one of the internal
DACs can be used to control a common programmable V
REFX
for the remainder of the DACs.
The circuit configuration is shown in Figure 37. The relationship of
V
REFX
to V
REF
is dependent upon the digital code and the ratio of
R1 and R2, and is given by
1
2
21
2
1
R
RD
V
R
R
VV
N
REFXREFREFX
××−






+×=
(4)








×+






+×
=
1
2
2
1
1
2
1
R
R
N
D
R
R
V
V
REF
REFX
(5)
where:
D = decimal equivalent of input code.
N = number of bits.
V
REF
= applied external reference.
V
REFX
= reference voltage for DAC A to DAC D.
Table 7. V
REFX
vs. R1 and R2
R1, R2 Digital Code V
REFX
R1 = R2 0000 0000 0000 2 V
REF
R1 = R2 1000 0000 0000 1.3 V
REF
R1 = R2 1111 1111 1111 V
REF
R1 = 3R2 0000 0000 0000 4 V
REF
R1 = 3R2 1000 0000 0000 1.6 V
REF
R1 = 3R2 1111 1111 1111 V
REF
The accuracy of V
REFX
is affected by the quality of R1 and R2.
Therefore, tight tolerance, low tempco, thin film resistors
should be used.
02179-037
AD7398/AD7399
DAC A
V
REF
A
V
OUT
A
R1 ±0.1%
R2 ±0.1%
V
REF
VIN
DAC B
V
REF
B
V
OUT
B
ADR293
DAC C
V
REF
C
V
OUT
C
DAC D
V
REF
D
V
OUT
D
TO OTHER
COMPONENTS
Figure 37. Programmable DAC Reference