AD7225LR datasheet(8/12 Pages) AD

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AD7225

REV. B

–8–

SPECIFICATION RANGES

For the AD7225 to operate to rated specifications, its input ref-

erence voltage must be at least 4 V below the VDD power supply

voltage. This voltage differential is the overhead voltage re-

quired by the output amplifiers.

The AD7225 is specified to operate over a VDD range from

+12 V

± 5% to +15 V ±10% (i.e., from +11.4 V to +16.5 V)

with a VSS of –5 V ± 10%. Operation is also specified for a single

+15 V

± 5% V

DD supply. Applying a VSS of –5 V results in im-

proved zero code error, improved output sink capability with

outputs near AGND and improved negative going settling time.

Performance is specified over a wide range of reference voltages

from 2 V to (VDD – 4 V) with dual supplies. This allows a range

of standard reference generators to be used such as the AD580,

a +2.5 V bandgap reference and the AD584, a precision +10 V

reference. Note that an output voltage range of 0 V to +10 V re-

quires a nominal +15 V

± 5% power supply voltage.

UNIPOLAR OUTPUT OPERATION

This is the basic mode of operation for each channel of the

AD7225, with the output voltage having the same positive

polarity as VREF. The AD7225 can be operated single supply

(VSS = AGND) or with positive/negative supplies (see op-amp

section which outlines the advantages of having negative VSS).

Connections for the unipolar output operation are shown in Fig-

ure 13. The voltage at any of the reference inputs must never be

negative with respect to DGND. Failure to observe this precau-

tion may cause parasitic transistor action and possible device de-

struction. The code table for unipolar output operation is shown

in Table III.

Figure 13. Unipolar Output Circuit

Table III. Unipolar Code Table

DAC Latch Contents

MSB

LSB

Analog Output

1 1 1 1

REF

255

256









1 0 0 0

0 0 0 1

REF

129

256









1 0 0 0

0 0 0 0

REF

128

256









V REF

0 1 1 1

1 1 1 1

REF

127

256









0 0 0 0

0 0 0 1

REF

256









0 0 0 0

0 V

Note: 1 LSB

= V

REF

() 2−8

()=V

REF

256









BIPOLAR OUTPUT OPERATION

Each of the DACs of the AD7225 can be individually config-

ured to provide bipolar output operation. This is possible using

one external amplifier and two resistors per channel. Figure 14

shows a circuit used to implement offset binary coding (bipolar

operation) with DAC A of the AD7225. In this case

OUT =

+ R2









⋅ D

AV REF

() – R2









⋅ V

REF

()

With R1 = R2

VOUT = (2 DA – 1) • VREF

where DA is a fractional representation of the digital word in

latch A. (0

≤ D

A ≤ 255/256)

Mismatch between R1 and R2 causes gain and offset errors and,

therefore, these resistors must match and track over tempera-

ture. Once again the AD7225 can be operated in single supply

or from positive/negative supplies. Table IV shows the digital

code versus output voltage relationship for the circuit of Figure

14 with R1 = R2.

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