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ADCMP603BCPZ-R7 Datasheet(PDF) 11 Page - Analog Devices |
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ADCMP603BCPZ-R7 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADCMP603 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 16. 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 17 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 +V COMPARATOR PROPAGATION DELAY DISPERSION The ADCMP603 comparator is designed to reduce propagation delay dispersion over a wide input overdrive range of 5 mV to V H /2, and the new switching threshold becomes −VH/2. The comparator remains in the high state until the new threshold, −V CCI – 1 V. 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). H /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. 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 15 and Figure 16). OUTPUT INPUT 0 VOL VOH +VH 2 –VH 2 ADCMP603 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 for both positive-going and negative-going inputs. Figure 17. Comparator Hysteresis Transfer Function Q/Q OUTPUT INPUT VOLTAGE 500mV OVERDRIVE 10mV OVERDRIVE DISPERSION VN ± VOS 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 15. Propagation Delay—Overdrive Dispersion |
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