AD7303

–13–

REV. 0

SCLK

10k

Ω

AD7303

DIN

SYNC

SCLK

REF

POWER

+5V

REGULATOR

AGND

10µF

0.1µF

0.1µF

10k

Ω

DATA

10k

Ω

SYNC

Figure 32. AD7303 in Opto-Isolated Interface

Decoding Multiple AD7303

The SYNC pin on the AD7303 can be used in applications to

decode a number of DACs. In this application, all DACs in the

system receive the same serial clock and serial data, but only the

SYNC

to one of the DACs will be active at any one time allow-

ing 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 33 shows a dia-

gram of a typical setup for decoding multiple AD7303 devices in

a system.

ENABLE

74HC139

AD7303

SYNC

DIN

SCLK

DIN

SCLK

DGND

CODED

ADDRESS

1A

1B

1Y0

1Y1

1Y2

1Y3

1

G

AD7303

SYNC

DIN

SCLK

AD7303

SYNC

DIN

SCLK

AD7303

SYNC

DIN

SCLK

Figure 33. Decoding Multiple AD7303 Devices in a System

AD7303 as a Digitally Programmable Window Detector

A digitally programmable upper/lower limit detector using the

two DACs in the AD7303 is shown in Figure 34. The upper

and lower limits for the test are loaded to DACs A and B which,

is not within the programmed window, a led will indicate the fail

condition.

AD7303

+5V

GND

0.1µF

REF

PASS/

FAIL

1/2

CMP04

1/6 74HC05

FAIL

PASS

1k

Ω

0.1µF

10µF

SCLK

DIN

SYNC

SCLK

DIN

SYNC

1k

Ω

Figure 34. Window Detector Using AD7303

Programmable Current Source

Figure 35 shows the AD7303 used as the control element of a

programmable current source. In this circuit, the full-scale cur-

rent is set to 1 mA. The output voltage from the DAC is applied

across the current setting resistor of 4.7 k

Ω in series with the

full-scale setting resistor of 470

Ω. Suitable transistors to place

in the feedback loop of the amplifier include the BC107 and the

2N3904, which enable the current source to operate from a min

ating characteristics of the transistor. Suitable amplifiers in-

clude the AD820 and the OP295, both having rail-to-rail

operation on their outputs. The current for any digital input

code can be calculated as follows:

I = 2

× V

REF

× D/(5E + 3 × 256) mA

4.7k

Ω

470

Ω

+5V

LOAD

AD7303

10µF

0.1µF

GND

AD780/ REF192

REF

SCLK

DIN

SYNC

GND

0.1µF

SERIAL

INTERFACE

EXT

REF

AD820/

OP295

Figure 35. Programmable Current Source