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AD8598ARU 데이터시트(PDF) 7 Page - Analog Devices |
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AD8598ARU 데이터시트(HTML) 7 Page - Analog Devices |
7 / 11 page AD8598 –7– REV. A APPLICATIONS Optimizing High Speed Performance As with any high speed comparator or amplifier, proper design and layout techniques should be used to ensure optimal perfor- mance from the AD8598. The performance limits of high speed circuitry can easily be a result of stray capacitance, improper ground impedance or other layout issues. Minimizing resistance from source to the input is an important consideration in maximizing the high speed operation of the AD8598. Source resistance in combination with equivalent input capacitance could cause a lagged response at the input, thus delaying the output. The input capacitance of the AD8598, in combination with stray capacitance from an input pin to ground could result in several picofarads of equivalent capaci- tance. A combination of 3 k Ω source resistance and 5 pF of input capacitance yields a time constant of 15 ns, which is slower than the 5 ns capability of the AD8598. Source imped- ances should be less than 1 k Ω for the best performance. It is also important to provide bypass capacitors for the power supply in a high speed application. A 1 µF electrolytic bypass capacitor should be placed within 0.5 inches of each power supply pin to ground. These capacitors will reduce any potential voltage ripples from the power supply. In addition, a 10 nF ceramic capacitor should be placed as close as possible from the power supply pins to ground. These capacitors act as a charge reservoir for the device during high frequency switching. A ground plane is recommended for proper high speed perfor- mance. This can be created by using a continuous conductive plane over the surface of the circuit board, only allowing breaks in the plane for necessary current paths. The ground plane provides a low inductance ground, eliminating any potential differences at different ground points throughout the circuit board caused from “ground bounce.” A proper ground plane also minimizes the effects of stray capacitance on the circuit board. Replacing the MAX912 The AD8598 is pin compatible with the MAX912 comparator. While it is easy to replace the MAX912 with the higher perfor- mance AD8598, please note that there are differences, and it is useful to check these to ensure proper operation. There are five major differences between the AD8598 and the MAX912; input voltage range, input bias currents, speed, out- put swing and power consumption. When operated on a +5 V single supply, the MAX912 has an input voltage range from –0.2 V to +3.5 V. The AD8598 has an input range from 0 V to +3.0 V. Signals above +3.0 V may result in slower response times (see Figure 8). If both signals exceed +3.0 V, the signals may be shifted or attenuated to bring them into range, keeping in mind the note about source resis- tance in Optimizing High Speed Performance. If only one of the signals exceeds +3.0 V only slightly, and the other signal is always well within the 0 V to +3 V range, the comparator may operate without changes to the circuit. Example: A comparator compares a fast moving signal to a fixed +2.5 V reference. Since the comparator only needs to operate when the signal is near +2.5 V, both signals will be within the input range (near +2.5 V and well under +3.0 V) when the comparator needs to change output. Note that signals much greater than +3.0 V will result in increased input currents and may cause the device to operate more slowly. The input bias current of the AD8598 is the same magnitude (–3 µA typical) as the MAX912 (+3 µA typical), and the cur- rent flows out of the AD8598 and into MAX912. If relatively low value resistors and/or low impedance sources are used on the inputs, the voltage shift due to bias current should be small. The AD8598 (6.75 ns typical) is faster than the MAX912 (10 ns typical). While this is beneficial to many systems, timing may need to be adjusted to take advantage of the higher speed. The AD8598 has slightly more output voltage swing when the output is lightly loaded. The AD8598 uses less current (typically 10 mA) than the MAX912 (typically 12 mA). Increasing Output Swing Although not required for normal operation, the output voltage swing of the AD8598 can be increased by connecting a 5 k Ω resistor from the output of the device to the V+ power supply. This configuration can be useful in low voltage power supply applications where maximizing output voltage swing is impor- tant. Adding a 5 k Ω pull-up resistor to the device’s output will not adversely affect the specifications of the AD8598. Output Loading Considerations The AD8598 output can deliver up to 40 mA of output current without any significant increase in propagation delay. The output of the device should not be connected to more than twenty (20) TTL input logic gates, nor drive a load resistance less than 100 Ω. To ensure the best performance from the AD8598 it is impor- tant to minimize capacitive loading of the output of the device. Capacitive loads greater than 50 pF will cause ringing on the output waveform and will reduce the operating bandwidth of the comparator. Setup and Hold Times for Latching the Output The latch inputs can be used to retain data at the outputs of the AD8598. When the voltage at the latch input goes high, the output of the device will remain constant regardless of the input voltages. The setup time for the latch is 2 ns–3 ns and the hold time is 3 ns. This means that to ensure data retention at the output, the input signal must be valid at least 5 ns before the latch pin goes high and must remain valid at least 3 ns after the latch pin goes high. Once the latch input voltage goes low, new output data will appear in approximately 8 ns. A logic high for the latch input is a minimum of +2.0 V and a logic low is a maximum of +0.8 V. This makes the latch input easily interface with TTL or CMOS logic gates. The latch circuitry in the AD8598 has no built-in hysteresis. Input Stage and Bias Currents The AD8598 uses a PNP differential input stage that enables the input common-mode range to extend all the way from the negative supply rail to within +2.2 V of the positive supply rail. The input common-mode voltage can be found as the average of the voltage at the two inputs of the device. To ensure the fastest response time, care should be taken not to allow the input common-mode voltage to exceed either of these voltages. |
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