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11 / 16 page ADCMP600/ADCMP601/ADCMP602 Rev. 0 | Page 11 of 16 Q/Q OUTPUT INPUT VOLTAGE 10V/ns 1V/ns DISPERSION VN ± VOS OPTIMIZING PERFORMANCE As with any high speed comparator, proper design and layout techniques are essential for obtaining the specified performance. Stray capacitance, inductance, inductive power and ground impedances, or other layout issues can severely limit performance and often cause oscillation. Large discontinuities along input and output transmission lines can also limit the specified pulse- width dispersion performance. 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. Thermal noise from large resistances can easily cause extra jitter with slowly slewing input signals; higher impedances encourage undesired coupling. Figure 19. 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. Figure 20 shows the transfer function for a comparator with hysteresis. As the input voltage approaches the threshold (0.0 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, and the new switching threshold becomes −VH/2. The comparator remains in the high state until the new threshold, −VH/2, is crossed from below the threshold region in a negative direction. In this manner, noise or feedback output signals centered on 0.0 V input cannot cause the comparator to switch states unless it exceeds the region bounded by ±VH/2. COMPARATOR PROPAGATION DELAY DISPERSION The ADCMP600/ADCMP601/ADCMP602 comparators are designed to reduce propagation delay dispersion over a wide input overdrive range. Propagation delay dispersion is the variation in propagation delay that results from a change in the degree of overdrive or slew rate (that is, how far or how fast the input signal exceeds the switching threshold). 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. Dispersion is defined as the variation in propagation delay as the input overdrive conditions are changed (Figure 18 and Figure 19). OUTPUT INPUT 0 VOL VOH +VH 2 –VH 2 The device dispersion is typically < 2 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 both positive-going and negative-going inputs. Q/Q OUTPUT INPUT VOLTAGE 500mV OVERDRIVE 10mV OVERDRIVE DISPERSION VN ± VOS Figure 20. 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. Figure 18. Propagation Delay—Overdrive Dispersion These ADCMP600 features a fixed hysteresis of approximately 2 mV. The ADCMP601 and ADCMP602 comparators offer a programmable Hysteresis feature that can significantly improve accuracy and stability. Connecting an external pull-down resistor or a current source from the LE/HYS pin to GND, varies the amount of hysteresis in a predictable, stable manner. |
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