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AD8612ARUZ-REEL Arkusz danych(PDF) 10 Page - Analog Devices |
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AD8612ARUZ-REEL Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 20 page AD8611/AD8612 Rev. A | Page 10 of 20 APPLICATIONS OPTIMIZING HIGH SPEED PERFORMANCE As with any high speed comparator or amplifier, proper design and layout of the AD8611/AD8612 should be used to ensure optimal performance. Excess stray capacitance or improper grounding can limit the maximum performance of high speed circuitry. Minimizing resistance from the source to the comparator’s input is necessary to minimize the propagation delay of the circuit. Source resistance in combination with the equivalent input capacitance of the AD8611/AD8612 creates an R-C filter that could cause a lagged voltage rise at the input to the comparator. The input capacitance of the AD8611/AD8612 in combination with stray capacitance from an input pin to ground results in several picofarads of equivalent capacitance. Using a surface-mount package and a minimum of input trace length, this capacitance is typically around 3 pF to 5 pF. A combination of 3 kΩ source resistance and 3 pF of input capacitance yields a time constant of 9 ns, which is slower than the 4 ns propagation delay of the AD8611/AD8612. Source impedances should be less than 1 kΩ for best performance. Another important consideration is the proper use of power- supply-bypass capacitors around the comparator. A 1 μF bypass capacitor should be placed within 0.5 inches of the device between each power supply pin and ground. Another 10 nF ceramic capacitor should be placed as close as possible to the device in parallel with the 1 μF bypass capacitor. The 1 μF capacitor reduces any potential voltage ripples from the power supply, and the 10 nF capacitor acts as a charge reservoir for the comparator during high frequency switching. A continuous ground plane on the PC board is also recommended to maximize circuit performance. A ground plane can be created by using a continuous conductive plane over the surface of the circuit board, only allowing breaks in the plane for necessary traces and vias. The ground plane provides a low inductive current return path for the power supply, thus eliminating any potential differences at various ground points throughout the circuit board caused from ground bounce. A proper ground plane can also minimize the effects of stray capacitance on the circuit board. UPGRADING THE LT1394 AND LT1016 The AD8611 single comparator is pin-for-pin compatible with the LT1394 and LT1016 and offers an improvement in propagation delay over both comparators. These devices can easily be replaced with the higher performance AD8611; however, there are differ- ences, so it is useful to ensure that the system still operates properly. The five major differences between the AD8611 and the LT1016 include input voltage range, input bias currents, propagation delay, output voltage swing, and power consumption. Input common-mode voltage is found by taking the average of the two voltages at the inputs to the comparator. The LT1016 has an input voltage range from 1.25 V above the negative supply to 1.5 V below the positive supply. The AD8611 input voltage range extends down to the negative supply voltage to within 2 V of V+. If the input common-mode voltage is exceeded, input signals should be shifted or attenuated to bring them into range, keeping in mind the note about source resistance in the Optimizing High Speed Performance section. For example, an AD8611 powered from a 5 V single supply has its noninverting input connected to a 1 V peak-to-peak, high frequency signal centered around 2.3 V and its inverting input connected to a fixed 2.5 V reference voltage. The worst-case input common-mode voltage to the AD8611 is 2.65 V. This is well below the 3.0 V input common-mode voltage range to the comparator. Note that signals much greater than 3.0 V result in increased input currents and may cause the comparator to operate more slowly. The input bias current to the AD8611 is 7 μA maximum over temperature (−40°C to +85°C). This is identical to the maximum input bias current for the LT1394, and half of the maximum IB for the LT1016. Input bias currents to the AD8611 and LT1394 flow out from the comparator’s inputs, as opposed to the LT1016 whose input bias current flows into its inputs. Using low value resistors around the comparator and low impedance sources will minimize any potential voltage shifts due to bias currents. The AD8611 is able to swing within 200 mV of ground and within 1.5 V of positive supply voltage. This is slightly more output voltage swing than the LT1016. The AD8611 also uses less current than the LT1016—5 mA as compared to 25 mA of typical supply current. The AD8611 has a typical propagation delay of 4 ns, compared with the LT1394 and LT1016, whose propagation delays are typically 7 ns and 10 ns, respectively. MAXIMUM INPUT FREQUENCY AND OVERDRIVE The AD8611 can accurately compare input signals up to 100 MHz with less than 10 mV of overdrive. The level of overdrive required increases with ambient temperature, with up to 50 mV of overdrive recommended for a 100 MHz input signal and an ambient temperature of +85°C. It is not recommend to use an input signal with a fundamental frequency above 100 MHz because the AD8611 could draw up to 20 mA of supply current and the outputs may not settle to a definite state. The device returns to its specified performance once the fundamental input frequency returns to below 100 MHz. |
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