Datasheet AD8611, AD8612 (Analog Devices) - 10

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AD8611/AD8612 Data Sheet APPLICATIONS INFORMATION OPTIMIZING HIGH SPEED PERFORMANCE
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 As with any high speed comparator or amplifier, proper design input voltage range extends down to the negative supply voltage and layout of the AD8611/AD8612 must ensure optimal to within 2 V of V+. If the input common-mode voltage is performance. Excess stray capacitance or improper grounding exceeded, input signals must be shifted or attenuated to bring can limit the maximum performance of high speed circuitry. them into range, keeping in mind the note about source resistance Minimizing resistance from the source to the comparator input in the Optimizing High Speed Performance section. is necessary to minimize the propagation delay of the circuit. For example, an AD8611 powered from a 5 V single supply has Source resistance in combination with the equivalent input its noninverting input connected to a 1 V peak-to-peak, high capacitance of the AD8611/AD8612 creates an R-C filter that frequency signal centered around 2.3 V and its inverting input could cause a lagged voltage rise at the input to the comparator. connected to a fixed 2.5 V reference voltage. The worst-case The input capacitance of the AD8611/AD8612 in combination input common-mode voltage to the AD8611 is 2.65 V. This is with stray capacitance from an input pin to ground results in well below the 3.0 V input common-mode voltage range to the several picofarads of equivalent capacitance. Using a surface-mount comparator. Note that signals much greater than 3.0 V result in package and a minimum of input trace length, this capacitance increased input currents and can cause the comparator to is typically around 3 pF to 5 pF. A combination of 3 kΩ source operate more slowly. 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 The input bias current to the AD8611 is 7 μA maximum over AD8611/AD8612. Source impedances must be less than 1 kΩ temperature (−40°C to +85°C). This is identical to the maximum for best performance. input bias current for the LT1394, and half of the maximum IB for the LT1016. Input bias currents to the AD8611 and LT1394 Another important consideration is the proper use of power- flow out from the comparator inputs, as opposed to the LT1016 supply-bypass capacitors around the comparator. A 1 μF bypass whose input bias current flows into its inputs. Using low value capacitor must be placed within 0.5 inches of the device between resistors around the comparator and low impedance sources each power supply pin and ground. Another 10 nF ceramic will minimize any potential voltage shifts due to bias currents. capacitor must be placed as close as possible to the device in paral el with the 1 μF bypass capacitor. The 1 μF capacitor reduces The AD8611 is able to swing within 200 mV of ground and any potential voltage ripples from the power supply, and the 10 nF within 1.5 V of positive supply voltage. This is slightly more capacitor acts as a charge reservoir for the comparator during output voltage swing than the LT1016. The AD8611 also uses high frequency switching. less current than the LT1016—5 mA as compared to 25 mA of typical supply current. A continuous ground plane on the PC board is also recommended to maximize circuit performance. A ground plane can be created The AD8611 has a typical propagation delay of 4 ns, compared by using a continuous conductive plane over the surface of the with the LT1394 and LT1016, whose propagation delays are circuit board, only allowing breaks in the plane for necessary typically 7 ns and 10 ns, respectively. traces and vias. The ground plane provides a low inductive current
MAXIMUM INPUT FREQUENCY AND OVERDRIVE
return path for the power supply, thus eliminating any potential The AD8611 can accurately compare input signals up to 100 MHz differences at various ground points throughout the circuit board with less than 10 mV of overdrive. The level of overdrive required caused from ground bounce. A proper ground plane can also increases with ambient temperature, with up to 50 mV of minimize the effects of stray capacitance on the circuit board. overdrive recommended for a 100 MHz input signal and an
UPGRADING THE LT1394 AND LT1016
ambient temperature of +85°C. The AD8611 single comparator is pin-for-pin compatible with It is not recommend to use an input signal with a fundamental the LT1394 and LT1016 and offers an improvement in propagation frequency above 100 MHz because the AD8611 could draw up delay over both comparators. These devices can easily be replaced to 20 mA of supply current and the outputs may not settle to a with the higher performance AD8611; however, there are differ- definite state. The device returns to its specified performance ences, so it is useful to ensure that the system stil operates properly. once the fundamental input frequency returns to below 100 MHz. The five major differences between the AD8611 and the LT1016
OUTPUT LOADING CONSIDERATIONS
include input voltage range, input bias currents, propagation delay, The AD8611 can deliver up to 10 mA of output current without output voltage swing, and power consumption. Input common- increasing its propagation delay. The outputs of the device must mode voltage is found by taking the average of the two voltages not be connected to more than 40 TTL input logic gates or drive at the inputs to the comparator. less than 400 Ω of load resistance. Rev. B | Page 10 of 20 Document Outline FEATURES APPLICATIONS PIN CONFIGURATIONS GENERAL DESCRIPTION REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION OPTIMIZING HIGH SPEED PERFORMANCE UPGRADING THE LT1394 AND LT1016 MAXIMUM INPUT FREQUENCY AND OVERDRIVE OUTPUT LOADING CONSIDERATIONS USING THE LATCHTO MAINTAIN A CONSTANT OUTPUT INPUT STAGE AND BIAS CURRENTS USING HYSTERESIS CLOCK TIMING RECOVERY A 5 V, HIGH SPEED WINDOW COMPARATOR OUTLINE DIMENSIONS ORDERING GUIDE