Datasheet AD848, AD849 (Analog Devices) - 9

AD848/AD849 AD848/AD849–Applications GROUNDING AND BYPASSING
Often termination is not used, either because signal integrity In designing practical circuits with the AD848 or AD849, the requirements are low or because too many high frequency user must remember that whenever high frequencies are signals returned to ground contaminate the ground plane. involved, some special precautions are in order. Circuits must Unterminated cables appear as capacitive loads. Since the be built with short interconnect leads. A large ground plane AD848 and AD849 are stable into any capacitive load, the op should be used whenever possible to provide a low resistance, amp will not oscillate if the cable is not terminated; however low inductance circuit path, as well as minimizing the effects of pulse integrity will be degraded. Figure 23 shows the AD848 high frequency coupling. Sockets should be avoided because the driving both 100 pF and 1000 pF loads. increased interlead capacitance can degrade bandwidth. Feedback resistors should be of low enough value to assure that
LOW NOISE PRE-AMP
the time constant formed with the capacitances at the amplifier The input voltage noise spectral densities of the AD848 and the C1261b–5–9/90 summing junction will not limit the amplifier performance. AD849 are shown in Figure 24. The low wideband noise and Resistor values of less than 5 kΩ are recommended. If a larger high gain bandwidths of these devices makes them well suited as resistor must be used, a small (< 10 pF) feedback capacitor in pre-amps for high frequency systems. 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 Figure 24. Input Voltage Noise Spectral Density drive a ± 1 V, 24 MHz signal on the terminated cable. Input voltage noise will be the dominant source of noise at the A back-termination resistor (RBT, also equal to the characteristic output in most applications. Other noise sources can be impedance of the cable) may be placed between the AD848 minimized by keeping resistor values as small as possible. 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
OUTLINE DIMENSIONS
requires that the op amp supply ± 2 V to the output in order to Dimensions shown in inches and (mm). achieve a ± 1 V swing at the line.
Mini-DIP (N) Package Cerdip (Q) Package
Figure 22. Video Line Driver PRINTED IN U.S.A.
100pF LOAD Small Outline (R) Package 1000pF LOAD
Figure 23. AD848 Driving a Capacitive Load –8– REV. B