AD7249(2000) 查看數據表（PDF） - Analog Devices

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AD7249
(Rev.:2000)

LC2MOS Dual 12-Bit Serial DACPORT

Analog Devices 

AD7249

Internal Reference

The AD7249 has an on-chip temperature compensated buried

Zener reference which is factory trimmed to 5 V ± 50 mV. The

reference voltage is provided at the REFOUT pin. This refer-

ence can be used to provide the reference voltage for the D/A

converter by connecting the REFOUT pin to the REFIN pin.

The reference voltage can also be used as a reference for other

components and is capable of providing up to 500 µA to an

external load. The maximum recommended capacitance on

REFOUT for normal operation is 50 pF. If the reference output

is required to drive a capacitive load greater than 50 pF, then a

200 Ω resistor should be placed in series with the capacitive

load. Figure 3 shows the suggested REF OUT decoupling

scheme, a 200 Ω resistor and the parallel combination of a

10 µF tantalum and a 0.1 µF ceramic capacitor. This decoupling

scheme reduces the noise spectral density of the reference.

Op Amp Section

The output of the voltage-mode D/A converter is buffered by a

noninverting CMOS amplifier. The ROFS input allows three

output voltage ranges to be selected. The buffer amplifier is

capable of developing +10 V across a 2 kΩ load to AGND.

The output amplifier can be operated from a single +15 V sup-

ply by tying VSS = 0 V.

The amplifier can also be operated from dual supplies to allow

an additional bipolar output range of –5 V to +5 V. Dual sup-

plies are necessary for the bipolar output range but can also be

used for the unipolar ranges to give faster settling time to volt-

ages near 0 V, to allow full sink capability of 2.5 mA over the

entire output range and to eliminate the effects of negative offsets

on the transfer characteristic (outlined previously). A plot of the

output sink capability of the amplifier is shown in Figure 5.

3

REFOUT

200⍀

10␮F

0.1␮F

EXT LOAD

VSS = –15V

2

Figure 3. Reference Decoupling Scheme

External Reference

In some applications, the user may require a system reference or

some other external reference to drive the AD7249. References

such as the AD586 provide an ideal external reference source

(See Figure 10). The REFIN voltage is internally buffered by a

unity gain amplifier before being applied to the D/A converter.

The D/A converter is scaled for a 5 V reference and the device is

tested with 5 V applied to REFIN. Other reference voltages may

be used with degraded performance. Figure 4 shows the degra-

dation in linearity vs. REFIN.

1.0

0.9

VDD = +15V

VSS = –15V

0.8

TA = +25؇C

0.7

0.6

0.5

0.4

INL

0.3

0.2

DNL

0.1

0.0

2

3

4

5

6

7

REFIN – Volts

8

9

Figure 4. Linearity vs. REFIN Voltage

VSS = 0V

1

0

0

2

4

6

8

10

OUTPUT VOLTAGE – Volts

Figure 5. Amplifier Sink Current

500

VDD = +15V

VSS = 0V

TA = +25؇C

200

REFERENCE

(NO DECOUPLING)

100

REFERENCE

(DECOUPLED*)

50

20

OUTPUT WITH ALL

0s ON DAC

0

50 100 200

500 1k 2k 5k 10k 20k 50k 100k

FREQUENCY – Hz

* REFERENCE DECOUPLING COMPONENTS ARE

A 200⍀ RESISTOR IN SERIES WITH A PARALLEL

COMBINATION OF 10␮F AND 0.1␮F TO GND.

Figure 6. Noise Spectral Density vs. Frequency

–6–

REV. C

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