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AD5545/AD5555
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 24 shows this
implementation. VREFAB is in the feedback network, therefore,
VREF AB
VREF
 1
RWB
RWA
  VREF_AB
DA
2N
RWB
RWA

(7)
where:
VREFAB = reference voltage of VREFA and VREFB
VREF = external reference voltage
DA = DAC A digital code in decimal
N = number of bits of DAC
RWB and RWA are digital potentiometer 128-step programmable
resistances and are given by
RWB
DC
128
RAB
RWA
128 DC
128
RAB
RWB DC
RWA 128 DC
where DC = digital potentiometer digital code in decimal
(0 ≤ DC ≤ 127).
(8)
(9)
(10)
Data Sheet
By putting Equations 7 through 10 together, the following
results:
VREF AB VREF
1
DC
128 DC

1
DA
2N
DC
128 DC
(11)
Table 9 shows a few examples of VREFAB of the 14-bit AD5555.
Table 9. VREFAB vs. DB and DC of the AD5555
DC DA
VREFAB
0X
VREF
32 0
1.33 VREF
32 8192
1.6 VREF
64 0
2 VREF
64 8192
4 VREF
96 0
4 VREF
96 8192
–8 VREF
The output of DAC B is, therefore,
VOB
VREF
AB
DB
2N
(12)
where DB is the DAC B digital code in decimal.
The accuracy of VREFAB 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.
+5V
VDD RFBA IOUTA
VREFA U1A
AGNDA
AD5555
RFBB IOUTB
VREFB U1B
AGNDB
C1 +15V
V+
OP4177
A U4 B
V–
AD7376
+15V
W C2
U2A
–15V
2 U3
2.2p
VIN
35
TEMP TRIM 6
VOUT
OP4177
VREF
GND
4
ADR03
U2C
C3
VREF_AB
POT
OP4177
VOB
U2B
02918-0-024
Figure 24. DAC with Programmable Input Reference Range
Rev. I | Page 14 of 23

AD5545 Datasheet PDF



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