Datasheet

MAX4385E/MAX4386E
Low-Cost, 230MHz, Single/Quad Op Amps with
Rail-to-Rail Outputs and ±15kV ESD Protection
_______________________________________________________________________________________ 9
Rail-to-Rail Outputs,
Ground-Sensing Inputs
The input common-mode range extends from (V
EE
-
200mV) 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 50mV of either power-sup-
ply rail with a 2kΩ load. The input ground sensing and
the rail-to-rail output substantially increase the dynamic
range. The input can swing 2.95V
P-P
and the output
can swing 4.9V
P-P
with minimal distortion.
Output Capacitive Loading and Stability
The MAX4385E/MAX4386E are optimized for AC perfor-
mance and do not drive highly reactive loads, which
decreases phase margin 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
S
) 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 10Ω to 15Ω)
placed before the reactive load prevents ringing and
oscillation. At higher capacitive loads, the interaction of
the load capacitance and the isolation resistor controls
the AC performance. Figure 5 shows the effect of a
15Ω isolation resistor on closed-loop response.
6
-4
100k 10M 100M1M 1G
-2
FREQUENCY (Hz)
GAIN (dB)
0
2
4
5
-3
-1
1
3
C
L
= 10pF
C
L
= 15pF
C
L
= 5pF
Figure 4. Small-Signal Gain vs. Frequency with Load
Capacitance and No Isolation Resistor
Figure 2. Driving a Capacitive Load Through an Isolation Resistor
9
11
10
13
12
15
14
16
0 200100 300 40050 250150 350 450 500
ISOLATION RESISTANCE
vs. CAPACITIVE LOAD
C
LOAD
(pF)
R
ISO
(Ω)
Figure 3. Isolation Resistance vs. Capacitive Load
R
G
R
F
R
ISO
C
L
V
OUT
V
IN
MAX438_E
3
-7
100k 10M 100M1M 1G
-5
FREQUENCY (Hz)
GAIN (dB)
-3
-1
1
2
-6
-4
-2
0
C
L
= 68pF
R
ISO
= 15Ω
C
L
= 120pF
C
L
= 47pF
Figure 5. Small-Signal Gain vs. Frequency with Load
Capacitance and 27
Ω Isolation Resistor