AD7303 データシートの表示（PDF） - Analog Devices

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AD7303

+2.7 V to +5.5 V, Serial Input, Dual Voltage Output 8-Bit DAC

Analog Devices 

AD7303

GENERAL DESCRIPTION

D/A Section

The AD7303 is a dual 8-bit voltage output digital-to-analog

converter. The architecture consists of a reference amplifier and

a current source DAC, followed by a current-to-voltage con-

verter capable of generating rail-to-rail voltages on the output of

the DAC. Figure 20 shows a block diagram of the basic DAC

architecture.

VDD

REF

30kΩ

30kΩ

REFERENCE

AMPLIFIER

AD7303

CURRENT

DAC

11.7kΩ

11.7kΩ

OUTPUT

AMPLIFIER

VO A/B

Figure 20. DAC Architecture

Both DAC A and DAC B outputs are internally buffered and

these output buffer amplifiers have rail-to-rail output character-

istics. The output amplifier is capable of driving a load of 10 kΩ

to both VDD and ground and 100 pF to ground. The reference

selection for the DAC can be either internally generated from

VDD or externally applied through the REF pin. Reference

selection is via a bit in the control register. The range on the

external reference input is from 1.0 V to VDD/2. The output

voltage from either DAC is given by:

where:

VO A/B = 2 × VREF × (N/256)

VREF is the voltage applied to the external REF pin or

VDD/2 when the internal reference is selected.

N is the decimal equivalent of the code loaded to the DAC

register and ranges from 0 to 255.

Reference

The AD7303 has the facility to use either an external reference

applied through the REF pin or an internal reference generated

from VDD. Figure 21 shows the reference input arrangement

where the internal VDD/2 has been selected.

VDD

30kΩ

AD7303

INT/EXT

reference appears at the reference pin as an output voltage for

decoupling purposes. When using the internal reference, external

references should not be connected to the REF pin. If external ref-

erence is selected, both switches are open and the externally

applied voltage to the REF pin is applied to the reference amplifier.

Decoupling capacitors applied to the REF pin decouple both

the internal reference and external reference. In noisy environ-

ments it is recommended that a 0.1 µF capacitor be connected

to the REF pin to provide added decoupling even when the in-

ternal reference is selected.

Analog Outputs

The AD7303 contains two independent voltage output DACs

with 8-bit resolution and rail-to-rail operation. The output buffer

provides a gain of two at the output. Figures 3 to 5 show the sink

and source capabilities of the output amplifier. The slew rate of the

output amplifier is typically 8 V/µs and has a full-scale settling to 8

bits with a 100 pF capacitive load in typically 1.2 µs.

The input coding to the DAC is straight binary. Table I shows

the binary transfer function for the AD7303. Figure 22 shows

the DAC transfer function for binary coding. Any DAC output

voltage can ideally be expressed as:

where:

VOUT = 2 × VREF (N/256)

N is the decimal equivalent of the binary input code.

N ranges from 0 to 255.

VREF is the voltage applied to the external REF pin when

the external reference is selected and is VDD/2 if the

internal reference is used.

Table I. Binary Code Table for AD7303 DAC

Digital Input

MSB . . . LSB

1111 1111

1111 1110

1000 0001

1000 0000

0111 1111

0000 0001

0000 0000

Analog Output

2 × 255/256 × VREF V

2 × 254/256 × VREF V

2 × 129/256 × VREF V

VREF V

2 × 127/256 × VREF V

2 × VREF/256 V

0V

2.VREF

REF

0.1µF

30kΩ

REFERENCE

AMPLIFIER

VREF

Figure 21. Reference Input

When the internal reference is selected during the write to the

DAC, both switches are closed and VDD/2 is generated and

applied to the reference amplifier. This internal VDD/2 reference

appears at the reference pin as an output voltage for decoupling

purposes. When using the internal reference, external references

should not be connected to the REF Pin. This internal VDD/2

0

DAC INPUT 00 01

CODE

7F 80 81

FE FF

Figure 22. DAC Transfer Function

–8–

REV. 0

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