REV. 0

AD5302/AD5312/AD5322

–15–

Decoding Multiple AD5302/AD5312/AD5322s

The

SYNC pin on the AD5302/AD5312/AD5322 can be used

in applications to decode a number of DACs. In this applica-

tion, all the DACs in the system receive the same serial clock

and serial data, but only the

SYNC to one of the devices will be

active at any one time allowing access to two channels in this

eight-channel system. The 74HC139 is used as a 2-to-4 line

decoder to address any of the DACs in the system. To prevent

timing errors from occurring, the enable input should be brought

to its inactive state while the coded address inputs are changing

state. Figure 40 shows a diagram of a typical setup for decoding

multiple AD5302/AD5312/AD5322 devices in a system.

74HC139

ENABLE

CODED

ADDRESS

1G

1A

1B

DGND

1Y0

1Y1

1Y2

1Y3

SCLK

DIN

AD5302/AD5312/AD5322

SYNC

DIN

SCLK

SYNC

DIN

SCLK

SYNC

DIN

SCLK

SYNC

DIN

SCLK

AD5302/AD5312/AD5322

AD5302/AD5312/AD5322

AD5302/AD5312/AD5322

Figure 40. Decoding Multiple AD5302/AD5312/AD5322

Devices in a System

AD5302/AD5312/AD5322 as a Digitally Programmable

Window Detector

A digitally programmable upper/lower limit detector using the

two DACs in the AD5302/AD5312/AD5322 is shown in Figure

41. The upper and lower limits for the test are loaded to DACs

A and B which, in turn, set the limits on the CMP04. If the

an LED will indicate the fail condition.

AD5302/AD5312/

AD5322

SCLK

DIN

SYNC

GND

+5V

0.1 F

10 F

SCLK

DIN

SYNC

1/2

CMP04

1k

FAIL

PASS/

FAIL

1k

PASS

1/6 74HC05

Figure 41. Window Detector Using AD5302/AD5312/AD5322

Coarse and Fine Adjustment Using the AD5302/AD5312/

AD5322

The DACs in the AD5302/AD5312/AD5322 can be paired

together to form a coarse and fine adjustment function, as

shown in Figure 42. DAC A is used to provide the coarse ad-

justment while DAC B provides the fine adjustment. Varying

the ratio of R1 and R2 will change the relative effect of the

coarse and fine adjustments. With the resistor values and exter-

nal reference shown, the output amplifier has unity gain for the

DAC A output, so the output range is 0 V to 2.5 V – 1 LSB. For

DAC B the amplifier has a gain of 7.6

range equal to 19 mV.

The circuit is shown with a 2.5 V reference, but reference volt-

rail-to-rail output swing.

1 F

GND

0.1 F

10 F

GND

EXT

REF

AD820/

OP295

+5V

R3

51.2k

R4

390

AD780/REF192

R1

390

R2

51.2k

AD5302/AD5312/

AD5322

Figure 42. Coarse/Fine Adjustment

Power Supply Bypassing and Grounding

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board on which the

AD5302/AD5312/AD5322 is mounted should be designed so

that the analog and digital sections are separated, and confined

to certain areas of the board. If the AD5302/AD5312/AD5322

is in a system where multiple devices require an AGND-to-DGND

connection, the connection should be made at one point only.

The star ground point should be established as close as possible

to the AD5302/AD5312/AD5322. The AD5302/AD5312/

AD5322 should have ample supply bypassing of 10

µF in paral-

lel with 0.1

µF on the supply located as close to the package as

possible, ideally right up against the device. The 10

µF capaci-

tors are the tantalum bead type. The 0.1

µF capacitor should

have low Effective Series Resistance (ESR) and Effective Series

Inductance (ESI), like the common ceramic types that provide a

low impedance path to ground at high frequencies to handle

transient currents due to internal logic switching.

The power supply lines of the AD5302/AD5312/AD5322 should

use as large a trace as possible to provide low impedance paths

and reduce the effects of glitches on the power supply line. Fast

switching signals such as clocks should be shielded with digital

ground to avoid radiating noise to other parts of the board, and

should never be run near the reference inputs. Avoid crossover

of digital and analog signals. Traces on opposite sides of the

board should run at right angles to each other. This reduces the

effects of feedthrough through the board. A microstrip tech-

nique is by far the best, but not always possible with a double-

sided board. In this technique, the component side of the board

is dedicated to ground plane while signal traces are placed on

the solder side.