Datasheet

ful attention to inputs and outputs to avoid large para-
sitic capacitance. Whether or not you use a constant-
impedance board, observe the following design guide-
lines:
• Don’t use wire-wrap boards; they are too inductive.
• Don’t use IC sockets; they increase parasitic capaci-
tance 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.
Rail-to-Rail Outputs,
Ground-Sensing Input
The input common-mode range extends from
V
EE
to (V
CC
- 2.25V) with excellent common-mode
rejection. Beyond this range, the amplifier output is a
nonlinear function of the input, but does not undergo
phase reversal or latchup. The output swings to within
125mV of either power-supply rail with a 2kΩ load.
Output Capacitive Load and Stability
The MAX4350/MAX4351 are optimized for AC perfor-
mance. They are not designed to drive highly reactive
loads, which decrease phase margin and may produce
excessive ringing and oscillation. Figure 2 shows a cir-
cuit that eliminates this problem. Figure 3 is a graph of
the Isolation Resistance (R
ISO
) vs. Capacitive Load.
Figure 4 shows how a capacitive load causes exces-
sive peaking of the amplifier’s frequency response if
the capacitor is not isolated from the amplifier by a
resistor. A small isolation resistor (usually 20Ω to 30Ω)
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
27Ω isolation resistor on closed-loop response.
Coaxial cable and other transmission lines are easily
driven when properly terminated at both ends with their
characteristic impedance. Driving back-terminated
transmission lines essentially eliminates the line’s
capacitance.
MAX4350/MAX4351
Ultra-Small, Low-Cost, 210MHz, Dual-Supply
Op Amps with Rail-to-Rail Outputs
8 _______________________________________________________________________________________
R
G
R
F
R
ISO
50Ω
C
L
V
OUT
V
IN
R
TIN
MAX435 _
Figure 2. Driving a Capacitive Load Through an Isolation Resistor
30
25
20
5
10
15
0
CAPACITIVE LOAD
(pF)
500 100 200150 250
ISOLATION RESISTANCE (Ω)
Figure 3. Isolation Resistance vs. Capacitive Load