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AD8468 датащи(PDF) 7 Page - Analog Devices |
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AD8468 датащи(HTML) 7 Page - Analog Devices |
7 / 12 page AD8468 Rev. 0 | Page 7 of 12 APPLICATIONS INFORMATION POWER/GROUND LAYOUT AND BYPASSING The AD8468 comparator is a high speed device. Despite the low noise output stage, it is essential to use proper high speed design techniques to achieve the specified performance. Because comparators are uncompensated amplifiers, feedback in any phase relationship is likely to cause oscillations or undesired hysteresis. Of critical importance is the use of low impedance supply planes, particularly the output supply plane (VCC) and the ground plane (GND). Individual supply planes are recommended as part of a multilayer board. Providing the lowest inductance return path for switching currents ensures the best possible performance in the target application. It is also important to adequately bypass the input and output supplies. A 0.1 µF bypass capacitor should be placed as close as possible to the VCC supply pin. The capacitor should be connected to the GND plane with redundant vias placed to provide a physically short return path for output currents flowing back from ground to the VCC pin. High frequency bypass capacitors should be carefully selected for minimum inductance and ESR. Parasitic layout inductance should also be strictly controlled to maximize the effectiveness of the bypass at high frequencies. TTL-/CMOS-COMPATIBLE OUTPUT STAGE Specified propagation delay performance can be achieved only by keeping the capacitive load at or below the specified minimums. The output of the AD8468 is designed to directly drive one Schottky TTL, three low power Schottky TTL loads, or the equivalent. For large fanouts, buses, or transmission lines, use an appropriate buffer to maintain the excellent speed and stability of the comparator. With the rated 15 pF load capacitance applied, more than half of the total device propagation delay is output stage slew time. Because of this, the total propagation delay decreases as VCC decreases, and instability in the power supply may appear as excess delay dispersion. Delay is measured to the 50% point for whatever supply is in use; thus, the fastest times are observed with the VCC supply at 2.5 V, and larger values are observed when driving loads that switch at other levels. Overdrive and input slew rate dispersions are not significantly affected by output loading and VCC variations. The TTL-/CMOS-compatible output stage is shown in the simplified schematic diagram (see Figure 9). Because of its inherent symmetry and generally good behavior, this output stage is readily adaptable for driving various filters and other unusual loads. OUTPUT Q2 Q1 +IN –IN OUTPUT STAGE VLOGIC GAIN STAGE A2 A1 AV Figure 9. Simplified Schematic Diagram of the TTL-/CMOS-Compatible Output Stage OPTIMIZING PERFORMANCE As with any high speed comparator, proper design and layout techniques are essential for obtaining the specified performance. Stray capacitance, inductance, common power and ground impedances, or other layout issues can severely limit performance and can often cause oscillation. The source impedance should be minimized as much as is practicable. High source impedance, in combination with the parasitic input capacitance of the comparator, causes an undesirable degradation in bandwidth at the input, thus degrading the overall response. Higher impedances encourage undesired coupling. COMPARATOR PROPAGATION DELAY DISPERSION The AD8468 comparator is designed to reduce propagation delay dispersion over a wide input overdrive range of 10 mV to VCC – 1 V. Propagation delay dispersion is the variation in propagation delay that results from a change in the degree of overdrive or slew rate (how far or how fast the input signal exceeds the switching threshold). See Figure 10 and Figure 11. Propagation delay dispersion is a specification that becomes important in high speed, time-critical applications, such as data communication, automatic test and measurement, and instru- mentation. It is also important in event-driven applications, such as pulse spectroscopy, nuclear instrumentation, and medical imaging. AD8468 overdrive dispersion is typically <12 ns as the overdrive varies from 10 mV to 125 mV. This specification applies to both positive and negative signals because the device has very closely matched delays for both positive-going and negative-going inputs and very low output skews. Remember to add the actual device offset to the overdrive for repeatable dispersion measurements. |
Аналогичный номер детали - AD8468 |
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Аналогичное описание - AD8468 |
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