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AD8465 Datasheet(PDF) 11 Page - Analog Devices |
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AD8465 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD8465 Rev. 0 | Page 11 of 16 COMPARATOR PROPAGATION DELAY DISPERSION The AD8465 comparator is designed to reduce propagation delay dispersion over a wide input overdrive range of 5 mV to VCCI − 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 is driven past the switching threshold). Propagation delay dispersion is a specification that becomes important in high speed, time-critical applications, such as data communications, automatic test and measurement, and instru- mentation. It is also important in event-driven applications, such as pulse spectroscopy, nuclear instrumentation, and medical imaging. Dispersion is defined as the variation in propagation delay as the input overdrive conditions are changed (see Figure 16 and Figure 17). The AD8465 dispersion is typically <1.6 ns as the overdrive varies from 10 mV to 125 mV. This specification applies to both positive and negative signals because the AD8465 has substantially equal delays for positive-going and negative- going inputs and very low output skews. Q/Q OUTPUT INPUT VOLTAGE 500mV OVERDRIVE 10mV OVERDRIVE DISPERSION VN ± VOS Figure 16. Propagation Delay—Overdrive Dispersion Q/Q OUTPUT INPUT VOLTAGE 10V/ns 1V/ns DISPERSION VN ± VOS Figure 17. Propagation Delay—Slew Rate Dispersion COMPARATOR HYSTERESIS The addition of hysteresis to a comparator is often desirable in a noisy environment, or when the differential input amplitudes are relatively small or slow moving. The transfer function for a comparator with hysteresis is shown in Figure 18. As the input voltage approaches the threshold (0 V, in this example) from below the threshold region in a positive direction, the comparator switches from low to high when the input crosses +VH/2. The new switching threshold becomes −VH/2. The comparator remains in the high state until the −VH/2 threshold is crossed from below the threshold region in a negative direction. In this manner, noise or feedback output signals centered on 0 V input cannot cause the comparator to switch states unless it exceeds the region bounded by ±VH/2. OUTPUT INPUT 0V VOL VOH +VH 2 –VH 2 Figure 18. Comparator Hysteresis Transfer Function The customary technique for introducing hysteresis into a comparator uses positive feedback from the output back to the input. One limitation of this approach is that the amount of hysteresis varies with the output logic levels, resulting in hysteresis that is not symmetric about the threshold. The external feedback network can also introduce significant parasitics that reduce high-speed performance and induce oscillation in some cases. |
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