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AD848JR2 Datenblatt(PDF) 8 Page - Analog Devices |
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AD848JR2 Datenblatt(HTML) 8 Page - Analog Devices |
8 / 8 page AD848/AD849 REV. B –8– GROUNDING AND BYPASSING In designing practical circuits with the AD848 or AD849, the user must remember that whenever high frequencies are involved, some special precautions are in order. Circuits must be built with short interconnect leads. A large ground plane 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 capacitances at the amplifier summing junction 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 in parallel with the feedback resistor, RF, may be used to compen- sate for the input capacitances and optimize the dynamic per- formance of the amplifier. Power supply leads should be bypassed to ground as close as possible to the amplifier pins. 0.1 µF ceramic disc capacitors are recommended. VIDEO LINE DRIVER The AD848 functions very well as a low cost, high speed line driver of either terminated or unterminated cables. Figure 22 shows the AD848 driving a doubly terminated cable. The termination resistor, RT, (when equal to the characteristic impedance of the cable) minimizes reflections from the far end of the cable. While operating off ±5 V supplies, the AD848 maintains a typical slew rate of 200 V/ µs, which means it can drive a ±1 V, 24 MHz signal on the terminated cable. A back-termination resistor (RBT, also equal to the characteristic impedance of the cable) may be placed between the AD848 output and the cable in order to damp any reflected signals caused by a mismatch between RT and the cable’s characteristic impedance. This will result in a “cleaner” signal, although it requires that the op amp supply ±2 V to the output in order to achieve a ±1 V swing at the line. Figure 22. Video Line Driver 100pF LOAD 1000pF LOAD Figure 23. AD848 Driving a Capacitive Load AD848/AD849–Applications Often termination is not used, either because signal integrity requirements are low or because too many high frequency signals returned to ground contaminate the ground plane. Unterminated cables appear as capacitive loads. Since the AD848 and AD849 are stable into any capacitive load, the op amp will not oscillate if the cable is not terminated; however pulse integrity will be degraded. Figure 23 shows the AD848 driving both 100 pF and 1000 pF loads. LOW NOISE PRE-AMP The input voltage noise spectral densities of the AD848 and the AD849 are shown in Figure 24. The low wideband noise and high gain bandwidths of these devices makes them well suited as pre-amps for high frequency systems. Figure 24. Input Voltage Noise Spectral Density Input voltage noise will be the dominant source of noise at the output in most applications. Other noise sources can be minimized by keeping resistor values as small as possible. OUTLINE DIMENSIONS Dimensions shown in inches and (mm). Cerdip (Q) Package Mini-DIP (N) Package Small Outline (R) Package |
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Ähnliche Beschreibung - AD848JR2 |
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