MAX4395 Maxim, MAX4395 Datasheet - Page 12

• Manufacturer Part Number: MAX4395
• Description: The MAX4389/MAX4390/MAX4392–MAX4396 family of op amps are unity-gain stable devices that combine high-speed performance, rail-to-rail outputs, and disable mode
• Manufacturer: Maxim

The MAX4389/MAX4390/MAX4392–MAX4396 are low-
power, voltage-feedback amplifiers featuring large-sig-
nal (2V
signal gain flatness of 24MHz. They are designed to
minimize differential-gain error and differential-phase
error to 0.015% and 0.015°, respectively. They have a
21ns settling time to 0.1%, 500V/µs slew rates, and out-
put-current-drive capability of up to 50mA making them
ideal for driving video loads.
Select the gain-setting feedback (R
resistor values to fit your application. Large resistor val-
ues increase voltage noise and interact with the amplifi-
er’s input and PC board capacitance. This can
generate undesirable poles and zeros and decrease
bandwidth or cause oscillations. For example, a nonin-
verting gain-of-two configuration (R
resistors, combined with 1pF of amplifier input capaci-
tance and 1pF of PC board capacitance, causes a pole
at 79.6MHz. Since this pole is within the amplifier band-
width, it jeopardizes stability. Reducing the 2kΩ resis-
tors to 100Ω extends the pole frequency to 1.59GHz,
but could limit output swing by adding 200Ω in parallel
with the amplifier’s load resistor (Figures 1a and 1b).
Ultra-Small, Low-Cost, 85MHz Op Amps with
Rail-to-Rail Outputs and Disable
Figure 1a. Noninverting Gain Configuration
Figure 1b. Inverting Gain Configuration
12
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P-P
Inverting and Noninverting Configurations
) bandwidths of 90MHz and 0.1dB large-
V
IN
R
V
G
IN
R
G
MAX43_ _
MAX43_ _
R
F
V
OUT
V
R
OUT
F
= [1 + (R
= -(R
V
F
F
OUT
F
Video Line Driver
/ R
F
/ R
= R
) and input (R
V
G
G
) V
OUT
)] V
IN
G
IN
) using 2kΩ
G
)
The MAX4389/MAX4390/MAX4392–MAX4396 operate
from single 4.5V to 11V or from dual ±2.25V to ±5.5V
supplies. Bypass each supply with a 0.1µF capacitor as
close to the pin as possible.
Maxim recommends using microstrip and stripline tech-
niques to obtain full bandwidth. To ensure that the PC
board does not degrade the amplifier’s performance,
design it for a frequency greater than 1GHz. Pay care-
ful attention to inputs and outputs to avoid large para-
sitic capacitance. Whether or not you use a con-
stant-impedance board, observe the following design
guidelines:
•
•
•
•
•
The MAX4389/MAX4393/MAX4394/MAX4396 feature a
disable function that allows the amplifiers to be placed
in a low-power, high-output-impedance state. When the
disable pin (DISABLE) is active, the amplifier’s output
impedance is 95kΩ. This high resistance and the
low 2pF output capacitance make the MAX4389/
MAX4390/MAX4392–MAX4396 in RF/video multiplexer
or switch applications. For larger arrays, pay careful
attention to capacitive loading (see the Output
Capacitive Loading and Stability section).
The MAX4389/MAX4390/MAX4392–MAX4396 are opti-
mized for AC performance. They are not designed to
drive highly reactive loads, which decrease phase mar-
gin and may produce excessive ringing and oscillation.
Figure 2 shows a circuit that eliminates this problem.
Figure 3 is a graph of the Optimal Isolation Resistor
(R
itive load causes excessive peaking of the amplifier’s
frequency response if the capacitor is not isolated from
the amplifier by a resistor. A small isolation resistor
(usually 10Ω to 15Ω) placed before the reactive load
prevents ringing and oscillation. At higher capacitive
loads, AC performance is controlled by the interaction
of the load capacitance and the isolation resistor.
Figure 5 shows the effect of a 15Ω isolation resistor on
closed-loop response.
S
) vs. Capacitive Load. Figure 4 shows how a capac-
Do not use wire-wrap boards; they are too inductive.
Do not use IC sockets; they increase parasitic cap-
acitance and inductance.
Use surface-mount instead of through-hole compo-
nents for better, high-frequency performance.
Use a PC board with at least two layers; it should be
as free from voids as possible.
Keep signal lines as short and as straight as possi-
ble. Do not make 90° turns; round all corners.
Output Capacitive Loading and Stability
Layout and Power-Supply Bypassing
Low-Power Disable Mode