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ADCMP604BKSZ-RL Folha de dados(PDF) 10 Page - Analog Devices |
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ADCMP604BKSZ-RL Folha de dados(HTML) 10 Page - Analog Devices |
10 / 16 page ADCMP604/ADCMP605 Rev. A | Page 10 of 16 APPLICATION INFORMATION POWER/GROUND LAYOUT AND BYPASSING The ADCMP604/ADCMP605 comparators are very high speed devices. Despite the low noise output stage, it is essential to use proper high speed design techniques to achieve the specified performance. Because comparators are uncompensated amplifiers, feedback in any phase relationship is likely to cause oscillations or undesired hysteresis. The use of low impedance supply planes is of critical importance particularly the output supply plane (VCCO) and the ground plane (GND). Individual supply planes are recommended as part of a multilayer board. Providing the lowest inductance return path for switching currents ensures the best possible performance in the target application. It is also important to adequately bypass the input and output supplies. Multiple high quality 0.01 μF bypass capacitors should be placed as close as possible to each of the VCCI and VCCO supply pins and should be connected to the GND plane with redundant vias. At least one of these should be placed to provide a physically short return path for output currents flowing back from ground to the VCCI pin and the VCCO pin. High frequency bypass capacitors should be carefully selected for minimum inductance and ESR. Parasitic layout inductance should also be strictly controlled to maximize the effectiveness of the bypass at high frequencies. If the package allows, and the input and output supplies have been connected separately (VCCI ≠ VCCO), be sure to bypass each of these supplies separately to the GND plane. Do not connect a bypass capacitor between these supplies. It is recommended that the GND plane separate the VCCI and VCCO planes when the circuit board layout is designed to minimize coupling between the two supplies to take advantage of the additional bypass capacitance from each respective supply to the ground plane. This enhances the performance when split input/output supplies are used. If the input and output supplies are connected together for single-supply operation (VCCI = VCCO), coupling between the two supplies is unavoidable; however, careful board placement can help keep output return currents away from the inputs. LVDS-COMPATIBLE OUTPUT STAGE Specified propagation delay dispersion performance is only achieved by keeping parasitic capacitive loads at or below the specified minimums. The outputs of the ADCMP604 and ADCMP605 are designed to directly drive any standard LVDS- compatible input. USING/DISABLING THE LATCH FEATURE The latch input is designed for maximum versatility. It can safely be left floating or it can be driven low by any standard TTL/CMOS device as a high speed latch. In addition, the pin can be operated as a hysteresis control pin with a bias voltage of 1.25 V nominal and an input resistance of approximately 70 kΩ. This allows the comparator hysteresis to be easily controlled by either a resistor or an inexpensive CMOS DAC. Driving this pin high or floating the pin disables all hysteresis. Hysteresis control and latch mode can be used together if an open drain, an open collector, or a three-state driver is connected in parallel to the hysteresis control resistor or current source. Due to the programmable hysteresis feature, the logic threshold of the latch pin is approximately 1.1 V, regardless of VCCO. 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. |
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