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AD8469WBRMZ Datasheet(PDF) 9 Page - Analog Devices |
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AD8469WBRMZ Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page Data Sheet AD8469 Rev. 0 | Page 9 of 12 Q/Q OUTPUT INPUT VOLTAGE 500mV OVERDRIVE 10mV OVERDRIVE DISPERSION VN ± VOS Figure 12. Propagation Delay—Overdrive Dispersion Q/Q OUTPUT INPUT VOLTAGE 10V/ns 1V/ns DISPERSION VN ± VOS Figure 13. Propagation Delay—Slew Rate Dispersion COMPARATOR HYSTERESIS The addition of hysteresis to a comparator is often desirable in noisy environments or when the differential input amplitudes are relatively small or slow moving. The transfer function for a comparator with hysteresis is shown in Figure 14. OUTPUT INPUT 0.0V VOL VOH +VH 2 –VH 2 Figure 14. Comparator Hysteresis Transfer Function As the input voltage approaches the threshold (0.0 V in Figure 14) from below the threshold region in a positive direction, the com- parator 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 threshold, −VH/2, is crossed from below the threshold region in a negative direction. In this way, noise or feedback output signals centered on the 0.0 V input cannot cause the comparator to switch states unless they exceed the region bounded by ±VH/2. 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 level, 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 can even induce oscillation in some cases. The AD8469 comparator offers a programmable hysteresis feature that significantly improves accuracy and stability. By connecting an external pull-down resistor or current source from the HYS pin to ground, the user can vary the amount of hysteresis in a predictable, stable manner. Leaving the HYS pin disconnected or driving it high removes the hysteresis. The maximum hysteresis that can be applied using the HYS pin is approximately 160 mV. Figure 15 illustrates the amount of hysteresis applied as a function of the external resistor value. 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 HYS RESISTOR ( kΩ) VCC = 5.5V VCC = 2.5V Figure 15. Hysteresis vs. HYS Resistor The HYS pin appears as a 1.25 V bias voltage seen through a series resistance of 7 kΩ ± 20% throughout the hysteresis control range. The advantages of applying hysteresis in this manner are improved accuracy, improved stability, reduced component count, and maximum versatility. An external bypass capacitor is not recommended on the HYS pin because it impairs the latch function and often degrades the jitter performance of the device. When the HYS pin is driven low, hysteresis may become large, but in this device, the effect is not reliable or intended as a latch function. CROSSOVER BIAS POINT Rail-to-rail inputs in both op amps and comparators have a dual front-end design. Certain devices are active near the VCC rail, and others are active near the VEE rail. At some predetermined point in the common-mode range, a crossover occurs. At the crossover point (normally VCC/2), the direction of the bias current is reversed and there are changes in measured offset voltages and currents. The AD8469 elaborates slightly on this scheme. The crossover points are at approximately 0.8 V and 1.6 V. |
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