Datasheet

AD5545/AD5555 Data Sheet
Rev. G | Page 14 of 24
DAC WITH PROGRAMMABLE INPUT
REFERENCE RANGE
Because high voltage references can be costly, users may
consider using one of the DACs, a digital potentiometer, and a
low voltage reference to form a single-channel DAC with a
programmable input reference range. This approach optimizes
the programmable range as well as facilitates future system
upgrades with just software changes. Figure 25 shows this
implementation. V
REF
AB is in the feedback network, therefore,


















WA
WB
N
A
REF_AB
WA
WB
REFREF
R
R
2
D
V
R
R
VABV ––1
(7)
where:
V
REF
AB = reference voltage of V
REF
A and V
REF
B
V
REF
= external reference voltage
D
A
= DAC A digital code in decimal
N = number of bits of DAC
R
WB
and R
WA
are digital potentiometer 128-step programmable
resistances and are given by
AB
C
WB
R
D
R
128
 (8)
AB
C
WA
R
D
R
128
128

 (9)
C
C
WA
WB
D
D
R
R


128
(10)
where D
C
= digital potentiometer digital code in decimal
(0 ≤ D
C
≤ 127).
By putting Equations 7 through 10 together, the following
results:
C
C
N
A
C
C
REFREF
D
D
D
D
D
VABV













128
2
1
128
1
(11)
Table 9 shows a few examples of V
REF
AB of the 14-bit AD5555.
Table 9. V
REF
AB vs. D
B
and D
C
of the AD5555
D
C
D
A
V
REF
AB
0 X V
REF
32 0 1.33 V
REF
32 8192 1.6 V
REF
64 0 2 V
REF
64 8192 4 V
REF
96 0 4 V
REF
96 8192 –8 V
REF
The output of DAC B is, therefore,
N
B
REF
OB
D
ABVV
2
 (12)
where D
B
is the DAC B digital code in decimal.
The accuracy of V
REF
AB is affected by the matching of the input
and feedback resistors and, therefore, a digital potentiometer is
used for U4 because of its inherent resistance matching. The
AD7376 is a 30 V or ±15 V, 128-step digital potentiometer. If
15 V or ±7.5 V is adequate for the application, a 256-step
AD5260 digital potentiometer can be used instead.
AD5555
V
OUT
V
IN
GND
02918-0-024
V
O
B
TRIMTEMP
POT
U2A
U4
W
AB
U3
53
2
4
6
+5V
+15V
+15V
V+
–15V
V–
C1
C3
V
REF
A
V
REF
V
REF_AB
I
OUT
A
A
GND
A
V
DD
R
FB
A
C2
2.2p
OP4177
U2B
V
REF
B
I
OUT
B
A
GND
B
R
FB
B
OP4177
U2C
OP4177
ADR03
AD7376
U1A
U1B
Figure 25. DAC with Programmable Input Reference Range