AD7846
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 1 and 2 set the pin driver voltage levels (V
H
and V
L
), and
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.
DC PARAMETRICS
V
H
STORED DATA
AND INHIBIT
PATTERN
PERIOD
GENERATION
AND DELAY
COMPARE DATA
AND DON'T
CARE DATA
+15V
R1
39k
DAC1
DAC2
DAC3
DAC4
COMPARE
REGISTER
D
D
INH
INH
V
L
FORMATTER
AD345
DUT
AD9687
AD7846
V
OUT
V
REF+
AD7846
V
OUT
V
REF+
AD7846
V
OUT
V
REF+
AD7846
V
OUT
V
REF+
AD588
R
IN
V
REF–
DGND
DB15 DB0
R
IN
V
REF–
DGND
DB15 DB0
R
IN
V
REF–
DGND
DB15 DB0
R
IN
V
REF–
DGND
DB15 DB0
–15V
Figure 21. Digital Test System with 16-Bit Performance
–10–
REV. E
AD [ ANALOG DEVICES ]
