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AD842JR-16-REEL Datasheet(PDF) 8 Page - Analog Devices |
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AD842JR-16-REEL Datasheet(HTML) 8 Page - Analog Devices |
8 / 10 page AD842 –8– REV. E GROUNDING AND BYPASSING In designing practical circuits with the AD842, the user must remember that whenever high frequencies are involved, some Figure 24. AD842 Settling Demonstrating No Settling Tails special precautions are in order. Circuits must be built with short interconnect leads. Large ground planes should be used whenever possible to provide a low resistance, low inductance circuit path, as well as minimizing the effects of high frequency coupling. Sockets should be avoided because the increased interlead capacitance can degrade bandwidth. Feedback resistors should be of low enough value to assure that the time constant formed with the circuit capacitances will not limit the amplifier performance. Resistor values of less than 5 k Ω are recommended. If a larger resistor must be used, a small (<10 pF) feedback capacitor connected in parallel with the feed- back resistor, RF, may be used to compensate for these stray capacitances and optimize the dynamic performance of the amplifier in the particular application. Power supply leads should be bypassed to ground as close as possible to the amplifier pins. A 2.2 µF capacitor in parallel with a 0.1 µF ceramic disk capacitor is recommended. CAPACITIVE LOAD DRIVING ABILITY Like all wideband amplifiers, the AD842 is sensitive to capaci- tive loading. The AD842 is designed to drive capacitive loads of up to 20 pF without degradation of its rated performance. Ca- pacitive loads of greater than 20 pF will decrease the dynamic performance of the part although instability should not occur unless the load exceeds 100 pF. USING A HEAT SINK The AD842 draws less quiescent power than most precision high speed amplifiers and is specified for operation without a heat sink. However, when driving low impedance loads, the cur- rent to the load can be 10 times the quiescent current. This will create a noticeable temperature rise. Improved performance can be achieved by using a small heat sink such as the Aavid Engi- neering #602B. TERMINATED LINE DRIVER The AD842 is optimized for high speed line driver applications. Figure 25 shows the AD842 driving a doubly terminated cable in a gain-of-2 follower configuration. The AD842 maintains a typical slew rate of 375 V/ µs, which means it can drive a ±10 V, 6.0 MHz signal or a ±3 V, 19.9 MHz signal. The termination resistor, RT, (when equal to the characteristic impedance of the cable) minimizes reflections from the far end of the cable. A back-termination resistor (RBT, also equal to the characteristic impedance of the cable) may be placed between the AD842 output and the cable in order to damp any stray signals caused by a mismatch between RT and the cable’s char- acteristic impedance. This will result in a “cleaner” signal. With this circuit, the voltage on the line equals VIN because one half of VOUT is dropped across RBT. The AD842 has ±100 mA minimum output current and, there- fore, can drive ±5 V into a 50 Ω cable. The feedback resistors, R1 and R2, must be chosen carefully. Large value resistors are desirable in order to limit the amount of current drawn from the amplifier output. But large resistors can cause amplifier instability because the parallel resistance R1 R2 combines with the input capacitance (typically 2–5 pF) to create an additional pole. Also, the voltage noise of the AD842 is equivalent to a 5 k Ω resistor, so large resistors can signifi- cantly increase the system noise. Resistor values of 1 k Ω or 2 kΩ are recommended. If termination is not used, cables appear as capacitive loads and can be decoupled from the AD842 by a resistor in series with the output. AD842 TERMINATION RESISTOR FOR INPUT SIGNAL VIN +VS 0.1 F 2.2 F –VS 0.1 F 2.2 F R2 R1 RST 50 OR 75 CABLE RT RT = RST = CABLE CHARACTERISTIC IMPEDANCE + – Figure 25. Line Driver Configuration |
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