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AD9698TQ Datasheet(PDF) 6 Page - Analog Devices |
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AD9698TQ Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page AD9696/AD9698 –6– REV. B R R A2 A1 AD9698 +IN1 –IN1 +VREF R1 Q1 OUT R = 10k Ω R1 + R2 >5kΩ A1 ,A2 = AD708 or OP– 290 (±5V) (+5V) R2 Q1 OUT Q1 OUT Q2 OUT Q2 OUT +Q2 OUT –VREF VSIGNAL VIN +IN2 –IN2 Figure 1. AD9698 Used as Window Detector When configured as shown, the op amps generate reference lev- els for the comparators that are equally spaced above and below the applied VIN. The width of the window is established by the ratio of R1 and R2. For a given ratio of R1 and R2, +VREF and –VREF will be fixed percentages above and below VIN. As an ex- ample, using 2.2 k Ω for R1 and 10 kΩ for R2 creates a ±10% window. When VIN equals +3 V, +VREF will be +3.3 V and –VREF 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 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. Q1OUT will be high when the signal is above +VREF, and Q2OUT will be high when the signal is below –VREF. Crystal Oscillator 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). APPLICATIONS General 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. { SIGNAL VOLTAGE AT +INPUT +INPUT CURRENT – INPUT CURRENT LINEAR REGION 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. |
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