Datasheet AD9696, AD9698 (Analog Devices) - 6

AD9696/AD9698 APPLICATIONS R General R
Two characteristics of the AD9696 and AD9698 should be con-
R1 A2
sidered for any application. First is the fact that all TTL com-
A1 R = 10k
Ω parators are prone to oscillate if the inputs are close to equal for
R2 R1 + R2 >5k
Ω any appreciable period of time. One instance of this happening
A1 ,A2 = AD708 or OP– 290 +V
would be slow changes in the unknown signal; the probability of
REF (±5V) (+5V) V
oscillation is reduced when the unknown signal passes through
IN +IN Q 1 AD9698 1 OUT
the threshold at a high slew rate. Another instance is if the un-
–IN1
known signal does not overdrive the comparator logic. Unless
Q1 OUT Q
they are overdriven, TTL comparators have undershoot when
1 OUT +Q V 2 OUT SIGNAL
switching logic states. The smaller the overdrive, the greater the
+IN Q 2 2 OUT
undershoot; when small enough, the comparator will oscillate,
–VREF –IN2
not being able to determine a valid logic state. For the AD9696
Q2 OUT
and AD9698, 20 mV is the smallest overdrive which will assure crisp switching of logic states without significant undershoot. Figure 1. AD9698 Used as Window Detector 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 When configured as shown, the op amps generate reference lev- impedance decreases when the comparator is in its linear region. els for the comparators that are equally spaced above and below the applied VIN. The width of the window is established by the During the time both transistors in the differential pair are con- ratio of R1 and R2. For a given ratio of R1 and R2, +VREF and ducting, the ac input impedance drops by orders of magnitude. –VREF will be fixed percentages above and below VIN. As an ex- Additionally, the input bias current switches from one input to ample, using 2.2 kΩ for R1 and 10 kΩ for R2 creates a ± 10% the other, depending upon whether or not the threshold is ex- window. When VIN equals +3 V, +VREF will be +3.3 V and ceeded. As a result, the input currents follow approximately the –VREF will be +2.7 V. Likewise, for a –2 V input, the thresholds characteristic curves shown below. will be –1.8 V and –2.2 V. Windows of differing percentage
LINEAR
width can be calculated with the equation:
REGION
{
SIGNAL VOLTAGE
(1–X)/2X = R2/R1
AT +INPUT
where: X = % window
+INPUT
Additionally, the low impedance of the op amp outputs assures
CURRENT
that the threshold voltages will remain constant when the input currents change as the signal passes through the threshold volt-
– INPUT
age levels.
CURRENT
The output of the AND gate will be high while the signal is in- side the window. Q1OUT will be high when the signal is above Threshold Input Currents +VREF, and Q2OUT will be high when the signal is below –VREF. This characteristic will not cause problems unless a high imped-
Crystal Oscillator
ance threshold circuit or drive circuit is employed. A circuit Oscillators are used in a wide variety of applications from audio similar to that shown in the window comparator application can circuits to waveform generators, from ATE triggers and tele- eliminate this possible problem. communications transceivers to radar. Figure 2 shows a versatile and inexpensive oscillator. The circuit uses the AD9696, in a
Window Comparator
positive feedback mode, and is capable of generating accurate Many applications require determining when a signal’s voltage and stable oscillations with frequencies ranging from 1 MHz to falls within, above, or below a particular voltage range. A simple more than 40 MHz. tracking window comparator can provide this data. Figure 1 shows such a window comparator featuring high speed, TTL To generate oscillations from 1 to 25 MHz, a fundamental compatibility, and ease of implementation. mode crystal is used without the dc blocking capacitor and choke. The parallel capacitor on the inverting input is selected Two comparators are required to establish a “window” with up- for stability (0.1 per and lower threshold voltages. The circuit shown uses the µF for 1–10 MHz; 220 pF for frequencies above 10 MHz). 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. –6– REV. B