REV. E

–10–

AD7846

AD7846

DAC1

DGND

DB0

DB15

–15V

AD588

AD7846

DAC2

DGND

DB0

DB15

AD7846

DAC3

DGND

DB0

DB15

AD7846

DAC4

DGND

DB0

DB15

COMPARE DATA

AND DON'T

CARE DATA

PERIOD

GENERATION

AND DELAY

DUT

FORMATTER

D

D

INH

INH

AD345

COMPARE

REGISTER

AD9687

DC PARAMETRICS

+15V

R1

39k

STORED DATA

AND INHIBIT

PATTERN

Figure 21. Digital Test System with 16-Bit Performance

TEST APPLICATION

Figure 21 shows the AD7846 in an Automatic Test Equipment

application. The readback feature of the AD7846 is very useful

in these systems. It allows the designer to eliminate phantom

memory used for storing DAC contents and increases system

reliability since the phantom memory is now effectively on chip

with the DAC. The readback feature is used in the following

manner to control a data transfer. First, write the desired 16-bit

word to the DAC input latch using the

CS and R/W inputs.

Verify that correct data has been received by reading back the

latch contents. Now, the data transfer can be completed by

bringing the asynchronous

LDAC control line low. The analog

equivalent of the digital word now appears at the DAC output.

In Figure 21, each pin on the Device Under Test can be an

input or output. The AD345 is the pin driver for the digital

inputs, and the AD9687 is the receiver for the digital outputs.

The digital control circuitry determines the signal timing and

format.

DACs 3 and 4 set the receiver voltage levels. The pin drivers

used in ATE systems normally have a nonlinearity between

input and output. The 16-bit resolution of the AD7846 allows

compensation for these input/output nonlinearities. The dc

parametrics shown in Figure 21 measure the voltage at the

device pin and feed this back to the system processor. The pin

voltage can thus be fine-tuned by incrementing or decrementing

DACs 1 and 2 under system processor control.