Datasheet
REV.
AD7226
–11–
STAIRCASE WINDOW COMPARATOR
In many test systems, it is important to be able to determine
whether some parameter lies within defined limits. The staircase
window comparator of Figure 14a is a circuit that can be used,
for example, to measure the V
OH
and V
OL
thresholds of a TTL
device under test. Upper and lower limits on both V
OH
and V
OL
can be programmably set using the AD7226. Each adjacent pair
of comparators forms a window of programmable size. If V
TEST
lies within a window, then the output for that window will be
high. With a reference of 2.56 V applied to the V
REF
input, the
minimum window size is 10 mV.
V
REF
V
DD
AGND
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
V
OH
(HIGH)
V
OL
(HIGH)
V
OH
(LOW)
V
OL
(LOW)
V
TEST
FROM D.U.T.
1/4 CA339
10k
5V
10k
5V
5V
5V
10k
10k
5V
10k
WINDOW 5
WINDOW 4
WINDOW 3
WINDOW 2
WINDOW 1
AD7226
Figure 14a. Logic Level Measurement
V
REF
AGND
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
WINDOW 5
WINDOW 4
WINDOW 3
WINDOW 2
WINDOW 1
Figure 14b. Window Structure
The circuit can easily be adapted to allow for overlapping of
windows as shown in Figure 15a. If the three outputs from this
circuit are decoded then five different nonoverlapping program-
mable windows can again be defined.
V
REF
V
DD
AGND
5V
10k
WINDOW 3
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
V
TEST
FROM D.U.T.
10k
5V
10k
5V
WINDOW 2
WINDOW 1
AD7226
Figure 15a. Overlapping Windows
V
REF
AGND
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
WINDOW 3
WINDOW 2
WINDOW 1
Figure 15b. Window Structure
DAC A
V
REF
V
DD
DGND
AGND
V
SS
V
OUT
A
AD7226
*
*
DIGITAL INPUTS OMITTED
FOR CLARITY
+4V
–4V
+15V
15k
10k
Figure 16. Varying Reference Signal
VARYING REFERENCE SIGNAL
In some applications, it may be desirable to have a varying signal
applied to the reference input of the AD7226. The AD7226 has
multiplying capability within upper and lower limits of reference
voltage when operated with dual supplies. The upper and lower
limits are those required by the AD7226 to achieve its linearity
specification. Figure 16 shows a sine wave signal applied to the
reference input of the AD7226. For input signal frequencies up
to 50 kHz, the output distortion typically remains less than 0.1%.
Typical 3 dB bandwidth figure is 700 kHz.
D