AD5318 datasheet(21/28 Pages) AD

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AD5308/AD5318/AD5328

Rev. F | Page 21 of 28

APPLICATIONS INFORMATION

TYPICAL APPLICATION CIRCUIT

The AD5308/AD5318/AD5328 can be used with a wide range

of reference voltages where the devices offer full, one-quadrant

multiplying capability over a reference range of 0.25 V to VDD.

More typically, these devices are used with a fixed, precision

reference voltage. Suitable references for 5 V operation are the

AD780, ADR381, and REF192 (2.5 V references). For 2.5 V

operation, a suitable external reference is the AD589 or the

AD1580 (1.2 V band gap references). Figure 40 shows a typical

setup for the AD5308/AD5318/AD5328 when using an external

reference.

AD5308/AD5318/

AD5328

GND

DIN

SYNC

SERIAL

INTERFACE

VOUT

EXT

REF

0.1

μF

VREFABCD

VREFEFGH

AD780/ADR3811/REF192

WITH VDD = 5V OR

AD589/AD1580 WITH

VDD = 2.5V

VDD = 2.5V TO 5.5V

VIN

μF

SCL

VOUTA

VOUTG

VOUTB

VOUTH

Figure 40. AD5308/AD5318/AD5328 Using a 2.5 V or 5 V External Reference

DRIVING VDD FROM THE REFERENCE VOLTAGE

If an output range of 0 V to VDD is required when the reference

inputs are configured as unbuffered, the simplest solution is to

connect the reference input to VDD. As this supply can be noisy

and not very accurate, the AD5308/AD5318/AD5328 can be

powered from a voltage reference. For example, using a 5 V

reference, such as the REF195, works because the REF195

outputs a steady supply voltage for the AD5308/AD5318/

AD5328. The typical current required from the REF195 is a

1 μA supply current and ≈ 112 μA into the reference inputs (if

unbuffered); this is with no load on the DAC outputs. When the

DAC outputs are loaded, the REF195 also needs to supply the

current to the loads. The total current required (with a10 kΩ

load on each output) is

1.22 mA + 8(5 V/10 kΩ) = 5.22 mA

The load regulation of the REF195 is typically 2.0 ppm/mA,

which results in an error of 10.4 ppm (52 μV) for the 5.22 mA

current drawn from it. This corresponds to a 0.003 LSB error at

8 bits and 0.043 LSB error at 12 bits.

BIPOLAR OPERATION USING THE

AD5308/AD5318/AD5328

The AD5308/AD5318/AD5328 have been designed for single-

supply operation, but a bipolar output range is also possible

using the circuit in Figure 41. This circuit gives an output

voltage range of ±5 V. Rail-to-rail operation at the amplifier

output is achievable using an AD820, the AD8519, or an OP196

as the output amplifier.

+5V

–5V

AD820/

AD8519/

OP196

μF

+6V TO +16V

0.1

μF

10k

±5V

10k

GND

VOUT

REF192

+5V

SERIAL

INTERFACE

SCLK SYNC

DIN

μF

AD5308/

AD5318/

AD5328

VREFABCD

VREFB

VOUTC

VOUTB

VOUTA

VOUTH

VIN

VDD

Figure 41. Bipolar Operation with the AD5308/AD5318/AD5328

The output voltage for any input code can be calculated as

follows:

(

) ()

()

REFIN

OUT

−

⎥

⎦

⎤

⎢

⎣

⎡

where:

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

is the DAC resolution.

REFIN

is the reference voltage input.

with

REFIN = 5 V , R1 = R2 = 10 kΩ

(

) V

OUT

−

OPTO-ISOLATED INTERFACE FOR PROCESS

CONTROL APPLICATIONS

The AD5308/AD5318/AD5328 have a versatile 3-wire serial

interface, making them ideal for generating accurate voltages in

process control and industrial applications. Due to noise and

safety requirements, or distance, it may be necessary to isolate

the AD5308/AD5318/AD5328 from the controller. This can

easily be achieved by using opto-isolators that provide isolation

in excess of 3 kV. The actual data rate achieved may be limited

by the type of optocouplers chosen. The serial loading structure

of the AD5308/AD5318/AD5328 makes them ideally suited for

use in opto-isolated applications. Figure 42 shows an opto-

isolated interface to the AD5308/AD5318/AD5328 where DIN,

SCLK, and SYNC are driven from optocouplers. The power

supply to the part also needs to be isolated. This is done by

using a transformer. On the DAC side of the transformer, a 5 V

regulator provides the 5 V supply required for the AD5308/

AD5318/AD5328.

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