Datasheet

capacitance) creates a parasitic pole that produces an
underdamped signal response. Reducing input capaci-
tance or placing a small capacitor across the feedback
resistor improves response in this case.
The MAX4291/MAX4292/MAX4294 family’s inputs are
protected from large differential input voltages by inter-
nal 10.6kΩ series resistors and back-to-back triple-
diode stacks across the inputs (Figure 2). For
differential input voltages (much less than 1.8V), input
resistance is typically 0.75MΩ. For differential input
voltages greater than 1.8V, input resistance is around
21.2kΩ, and the input bias current can be approximat-
ed by the following equation:
In the region where the differential input voltage
approaches 1.8V, the input resistance decreases expo-
nentially from 0.75MΩ to 21.2kΩ as the diode block
begins to conduct. Conversely, the bias current
increases with the same curve.
In unity-gain configuration, high slew-rate input signals
may capacitively couple to the output through the triple-
diode stacks.
Rail-to-Rail Output Stage
The MAX4291/MAX4292/MAX4294 output stage can
drive up to a 2kΩ load and still swing to within 46mV of
the rails. Figure 3 shows the output-voltage swing of a
MAX4291 configured as a unity-gain buffer, powered
from a ±2.5V supply. The output for this setup typically
swings from (V
EE
+ 25mV) to (V
CC
- 2mV) with a 100kΩ
load.
Applications Information
Power-Supply Considerations
The MAX4291/MAX4292/MAX4294 operate from a sin-
gle 1.8V to 5.5V supply (or dual ±0.9V to ±2.75V sup-
plies) and consume only 100µA of supply current per
amplifier. A high power-supply rejection ratio of 100dB
allows the amplifiers to be powered directly off a
decaying battery voltage, simplifying design and
extending battery life.
The MAX4291/MAX4292/MAX4294 are ideally suited for
use with most battery-powered systems. Table 1 lists a
I
(V -1.8V)
21.2k
BIAS
DIFF
=
Ω
MAX4291/MAX4292/MAX4294
Ultra-Small, 1.8V, µPower,
Rail-to-Rail I/O Op Amps
_______________________________________________________________________________________ 9
Table 1. MAX4291 Characteristics with Typical Battery Systems
750Yes
Nickel-Cadmium
(2 cells)
1000Yes
Lithium-Ion
(1 cell)
1000Yes
Nickel-Metal-
Hydride (2 cells)
2000NoAlkaline (2 cells)
BATTERY TYPE
CAPACITY, AA SIZE
(mA-h)
2.4
3.5
2.4
3.0
1.8
2.7
1.8
1.8
V
END-OF-LIFE
(V)
V
FRESH
(V)
RECHARGE-
ABLE
7500
10,000
10,000
20,000
MAX4291
OPERATING TIME IN
NORMAL MODE
(h)
OUT
2.5V/div
IN
2.5V/div
0
0
20µs/div
V
CC
= 2.5V, V
EE
= -2.5V
Figure 3. Rail-to-Rail Input/Output Voltage Range
10.6kΩ
10.6kΩ
IN-
IN+
Figure 2. Input Protection Circuit