MAX4390EUK-T Maxim, MAX4390EUK-T Datasheet - Page 12

• Manufacturer Part Number: MAX4390EUK-T
• Description: Ultra-Small / Low-Cost / 85MHz Op Amps with Rail-to-Rail Outputs and Disable
• 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 2k
tors to 100
but could limit output swing by adding 200
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
______________________________________________________________________________________
P-P
Inverting and Noninverting Configurations
) bandwidths of 90MHz and 0.1dB large-
V
IN
extends the pole frequency to 1.59GHz,
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
OUT
) V
)] V
IN
G
IN
) using 2k
in parallel
resis-
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 95k . 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 10
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 15
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
to 15 ) placed before the reactive load
Low-Power Disable Mode
isolation resistor on