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ADCMP582 Datasheet(PDF) 10 Page - Analog Devices |
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ADCMP582 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page Preliminary Technical Data ADCMP580/ADCMP581/ADCMP582 Rev. PrA | Page 10 of 16 OPTIMIZING HIGH SPEED PERFORMANCE As with any high speed comparator, proper design and layout techniques are essential to obtaining the specified performance. Stray capacitance, inductance, inductive power and ground impedances, or other layout issues can severely limit performance and can cause oscillation. Discontinuities along input and output transmission lines can also severely limit the specified pulse width dispersion performance. For applications working in a 50 Ω environment, input and output matching have a significant impact on data-dependant (or deterministic) jitter (DJ) and pulse width dispersion performance. The ADCMP58x family of comparators provides internal 50 Ω termination resistors for both VP and VN inputs. The return side for each termination is pinned out separately with the VTP and VTN pins, respectively. If the a 50 Ω termination is desired at one or both of the VP/VN inputs, the VTP and VTN pins can be connected (or disconnected) to (from) the desired termination potential as appropriate. The termination potential should be carefully bypassed using ceramic capacitors as discussed previously to prevent undesired aberrations on the input signal due to parasitic inductance in the termination return path. If a 50 Ω termination is not desired, either one or both of the VTP/VTN termination pins can be left disconnected. In this case, the open pins should be left floating with no external pull downs or bypassing capacitors. For applications that require high speed operation, but do not have on-chip 50 Ω termination resistors, some reflections should be expected because the comparator inputs can no longer provide matched impedance to the input trace leading up to the device. It then becomes important to back-match the drive source impedance to the input transmission path leading to the input to minimize multiple reflections. For applications in which the comparator is very close to the driving signal source, the source impedance should be minimized. High source impedance in combination with parasitic input capacitance of the comparator could cause undesirable degradation in bandwidth at the input, thus degrading the overall response. Although the ADCMP58x family of comparators has been designed to minimize input capacitance, some parasitic capacitance is inevitable. It is therefore recommended that the drive source impedance should be no more than 50 Ω for best high speed performance. COMPARATOR PROPAGATION DELAY DISPERSION The ADCMP58x family of comparators has been specifically designed to reduce propagation delay dispersion over a wide input overdrive range of 5 mV to 500 mV. Propagation delay dispersion is a change in propagation delays, which results from a change in the degree of overdrive or slew rate (how far or fast the input signal exceeds the switching threshold). The overall result is a higher degree of timing accuracy. Propagation delay dispersion is a specification which becomes important in critical timing applications such as data communication, automatic test and measurement, instrumentation, and event driven applications such as pulse spectroscopy, nuclear instrumentation, and medical imaging. Dispersion is defined as the variation in the overall propagation delay as the input overdrive conditions are changed (Figure 16 and Figure 17). For the ADCMP58x family of comparators, dispersion is typically < 25 ps since the overdrive varies from 5 mV to 500 mV, and the input slew rate varies from 1 V/ns to 10 V/ns. This specification applies for both positive and negative signals since the ADCMP58x family of comparators has almost equal delays for positive- and negative-going inputs. Q/Q OUTPUT INPUT VOLTAGE 500mV OVERDRIVE 5mV 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 |
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