AD9696/AD9698
APPLICATIONS
General
R
R
R
1
A1
R
2
+V
REF
V
IN
+IN
1
–IN
1
V
SIGNAL
+IN
2
–V
REF
–IN
2
Q
2 OUT
Q
1 OUT
Q
2 OUT
Q
1 OUT
+Q
2 OUT
A2
R = 10kΩ
R
1
+ R
2
>5kΩ
A
1
,A
2
= AD708 or OP– 290
(±5V)
Q
1 OUT
(+5V)
Two characteristics of the AD9696 and AD9698 should be con-
sidered for any application. First is the fact that all TTL com-
parators are prone to oscillate if the inputs are close to equal for
any appreciable period of time. One instance of this happening
would be slow changes in the unknown signal; the probability of
oscillation is reduced when the unknown signal passes through
the threshold at a high slew rate. Another instance is if the un-
known signal does not overdrive the comparator logic. Unless
they are overdriven, TTL comparators have undershoot when
switching logic states. The smaller the overdrive, the greater the
undershoot; when small enough, the comparator will oscillate,
not being able to determine a valid logic state. For the AD9696
and AD9698, 20 mV is the smallest overdrive which will assure
crisp switching of logic states without significant undershoot.
The second characteristic to keep in mind when designing
threshold circuits for these comparators is twofold: (1) bias cur-
rents change when the threshold is exceeded; and (2) ac input
impedance decreases when the comparator is in its linear region.
During the time both transistors in the differential pair are con-
ducting, the ac input impedance drops by orders of magnitude.
Additionally, the input bias current switches from one input to
the other, depending upon whether or not the threshold is ex-
ceeded. As a result, the input currents follow approximately the
characteristic curves shown below.
LINEAR
REGION
AD9698
Figure 1. AD9698 Used as Window Detector
SIGNAL
VOLTAGE
AT +INPUT
{
When configured as shown, the op amps generate reference lev-
els for the comparators that are equally spaced above and below
the applied V
IN
. The width of the window is established by the
ratio of R1 and R2. For a given ratio of R1 and R2, +V
REF
and
–V
REF
will be fixed percentages above and below V
IN
. As an ex-
ample, using 2.2 kΩ for R1 and 10 kΩ for R2 creates a
±
10%
window. When V
IN
equals +3 V, +V
REF
will be +3.3 V and
–V
REF
will be +2.7 V. Likewise, for a –2 V input, the thresholds
will be –1.8 V and –2.2 V. Windows of differing percentage
width can be calculated with the equation:
(1–X)/2X = R2/R1
where:
X = % window
+INPUT
CURRENT
– INPUT
CURRENT
Additionally, the low impedance of the op amp outputs assures
that the threshold voltages will remain constant when the input
currents change as the signal passes through the threshold volt-
age levels.
The output of the AND gate will be high while the signal is in-
side the window. Q1
OUT
will be high when the signal is above
+V
REF
, and Q2
OUT
will be high when the signal is below –V
REF
.
Crystal Oscillator
Threshold Input Currents
This characteristic will not cause problems unless a high imped-
ance threshold circuit or drive circuit is employed. A circuit
similar to that shown in the window comparator application can
eliminate this possible problem.
Window Comparator
Many applications require determining when a signal’s voltage
falls within, above, or below a particular voltage range. A simple
tracking window comparator can provide this data. Figure 1
shows such a window comparator featuring high speed, TTL
compatibility, and ease of implementation.
Two comparators are required to establish a “window” with up-
per and lower threshold voltages. The circuit shown uses the
AD9698 dual ultrafast TTL comparator. In addition to the cost
and space savings over a design using two single comparators,
the dual comparator on a single die produces better matching of
both dc and dynamic characteristics.
Oscillators are used in a wide variety of applications from audio
circuits to waveform generators, from ATE triggers and tele-
communications transceivers to radar. Figure 2 shows a versatile
and inexpensive oscillator. The circuit uses the AD9696, in a
positive feedback mode, and is capable of generating accurate
and stable oscillations with frequencies ranging from 1 MHz to
more than 40 MHz.
To generate oscillations from 1 to 25 MHz, a fundamental
mode crystal is used without the dc blocking capacitor and
choke. The parallel capacitor on the inverting input is selected
for stability (0.1
µF
for 1–10 MHz; 220 pF for frequencies
above 10 MHz).
–6–
REV. B
AD [ ANALOG DEVICES ]
