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ADCMP566BCP Datasheet(PDF) 10 Page - Analog Devices |
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ADCMP566BCP Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADCMP566 Rev. 0 | Page 10 of 16 as the variation in propagation delay as the input overdrive conditions are changed (Figure 4). For the ADCMP566, overdrive dispersion is typically 35 ps as the overdrive is changed from 100 mV to 1 V. This specification applies for both positive and negative overdrive since the ADCMP566 has equal delays for positive and negative going inputs. The 35 ps propagation delay overdrive dispersion of the ADCMP566 offers considerable improvement of the 100 ps dispersion of other similar series comparators. Q OUTPUT INPUT VOLTAGE 1.5V OVERDRIVE 20mV OVERDRIVE DISPERSION VREF ± VOS 03633-0-004 Figure 4. Propagation Delay Dispersion COMPARATOR HYSTERESIS The addition of hysteresis to a comparator is often useful in a noisy environment or where it is not desirable for the compara- tor to toggle between states when the input signal is at the switching threshold. The transfer function for a comparator with hysteresis is shown in Figure 5. If the input voltage approaches the threshold from the negative direction, the comparator will switch from a 0 to a 1 when the input crosses +VH/2. The new switching threshold becomes −VH/2. The comparator will remain in a 1 state until the threshold −VH/2 is crossed coming from the positive direction. In this manner, noise centered on 0 V input will not cause the comparator to switch states unless it exceeds the region bounded by ±VH/2. Positive feedback from the output to the input is often used to produce hysteresis in a comparator (Figure 9). The major problem with this approach is that the amount of hysteresis varies with the output logic levels, resulting in a hysteresis that is not symmetrical around zero. Another method to implement hysteresis is generated by introducing a differential voltage between LATCH ENABLE and LATCH ENABLE. inputs (Figure 10). Hysteresis generated in this manner is independent of output swing and is symmetri- cal around zero. The variation of hysteresis with input voltage is shown in Figure 6. OUTPUT INPUT 0 1 0V –VH 2 +VH 2 03633-0-005 Figure 5. Comparator Hysteresis Transfer Function ∆ LATCH = LE – LEB (mV) –20 0 10 20 30 40 50 60 –10 –15 –5 0 10 515 Figure 6. Comparator Hysteresis Transfer Function Using Latch Enable Input MINIMUM INPUT SLEW RATE REQUIREMENT As for all high speed comparators, a minimum slew rate must be met to ensure that the device does not oscillate when the input crosses the threshold. This oscillation is due in part to the high input bandwidth of the comparator and the parasitics of the package. Analog Devices recommends a slew rate of 5 V/µs or faster to ensure a clean output transition. If slew rates less than 5 V/µs are used, then hysteresis should be added to reduce the oscillation. |
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