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

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.

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.

impedance of the cable) may be placed between the AD848

output and the cable in order to damp any reflected signals

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