AD8021 datasheet(14/24 Pages) AD

14 / 24 page

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. VREFAB is in the feedback network, therefore,

































REF_AB

REF

–

(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

128



(8)

128

128 



(9)





128

(10)

where DC = digital potentiometer digital code in decimal

(0 ≤ DC ≤ 127).

By putting Equations 7 through 10 together, the following

results:

REF



























128

(11)

Table 9 shows a few examples of VREFAB of the 14-bit AD5555.

Table 9. VREFAB vs. DB and DC of the AD5555

VREFAB

VREF

1.33 VREF

8192

1.6 VREF

2 VREF

8192

4 VREF

8192

–8 VREF

The output of DAC B is, therefore,

REF





(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.

AD5555

VOUT

VIN

GND

02918- 0- 024

VOB

TRIM

TEMP

POT

U2A

+5V

+15V

V+

–15V

V–

VREFA

VREF

VREF_AB

IOUTA

AGNDA

VDD RFBA

2.2p

OP4177

U2B

VREFB

IOUTB

AGNDB

RFBB

OP4177

U2C

OP4177

ADR03

AD7376

U1A

U1B

Figure 25. DAC with Programmable Input Reference Range

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