MAX4225 Maxim, MAX4225 Datasheet - Page 13

• Manufacturer Part Number: MAX4225
• Description: The MAX4223-MAX4228 current-feedback amplifiers combine ultra-high-speed performance, low distortion, and excellent video specifications with low-power operation
• Manufacturer: Maxim

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.
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.
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 5Ω to 20Ω)
isolation resistor R
prevents ringing and oscillation at the expense of a
Figure 3. Video Line Driver
75 CABLE
R
75
G
R
T
Current-Feedback Amplifiers with Shutdown
IN+
IN-
______________________________________________________________________________________
ISO
R
F
, placed before the reactive load,
Driving Capacitive Loads
MAX4223
MAX4224
MAX4225
MAX4226
MAX4227
MAX4228
OUT
Video Line Driver
75
R
T
75 CABLE
75
R
T
1GHz, Low-Power, SOT23,
small gain error. At higher capacitive loads, AC perfor-
mance is limited by the interaction of load capacitance
with the isolation resistor.
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 50Ω 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.
Figure 4. Using an Isolation Resistor (R
Capacitive Loads
R
G
IN+
IN-
R
F
Evaluation Board Layout
MAX4223
MAX4224
MAX4225
MAX4226
MAX4227
MAX4228
Maxim’s High-Speed
OUT
ISO
) for High
R
ISO
C
L
R
13
L