1GHz, Low-Power, SOT23,

Current-Feedback Amplifiers with Shutdown

______________________________________________________________________________________

13

very low output impedance at high frequencies to main-

tain measurement accuracy. The combination of high

speed, fast slew rate, low noise, and low distortion

makes the MAX4223–MAX4228 ideally suited for use as

buffer amplifiers in high-speed ADC applications.

Video Line Driver

The MAX4223–MAX4228 are optimized to drive coaxial

transmission lines when the cable is terminated at both

ends, as shown in Figure 3. Note that cable frequency

response may cause variations in the signal’s flatness.

Driving Capacitive Loads

A correctly terminated transmission line is purely resis-

tive and presents no capacitive load to the amplifier.

Although the MAX4223–MAX4228 are optimized for AC

performance and are not designed to drive highly

capacitive loads, they are capable of driving up to

25pF without excessive ringing. Reactive loads

decrease phase margin and may produce excessive

ringing and oscillation (see

Typical Operating

Characteristics). Figure 4’s circuit reduces the effect of

large capacitive loads. The small (usually 5

Ω to 20Ω)

prevents ringing and oscillation at the expense of a

small gain error. At higher capacitive loads, AC perfor-

mance is limited by the interaction of load capacitance

with the isolation resistor.

Maxim’s High-Speed

Evaluation Board Layout

Figures 7 and 8 show a suggested layout for Maxim’s

high-speed, single-amplifier evaluation boards. These

boards were developed using the techniques described

above. The smallest available surface-mount resistors

were used for the feedback and back-termination resis-

tors to minimize the distance from the IC to these resis-

tors, thus reducing the capacitance associated with

longer lead lengths.

SMA connectors were used for best high-frequency

performance. Because distances are extremely short,

performance is unaffected by the fact that inputs and

outputs do not match a 50

Ω line. However, in applica-

tions that require lead lengths greater than 1/4 of the

wavelength of the highest frequency of interest, con-

stant-impedance traces should be used.

Fully assembled evaluation boards are available for the

MAX4223 in an SO-8 package.

MAX4223

MAX4224

MAX4225

MAX4226

MAX4227

MAX4228

IN-

IN+

OUT

75

Ω

75

Ω

75

Ω

75

Ω CABLE

75

Ω CABLE

Figure 3. Video Line Driver

MAX4223

MAX4224

MAX4225

MAX4226

MAX4227

MAX4228

IN-

IN+

OUT

Capacitive Loads