MAX4092 Maxim, MAX4092 Datasheet - Page 8

• Manufacturer Part Number: MAX4092
• Description: The single MAX4091, dual MAX4092, and quad MAX4094 operational amplifiers combine excellent DC accuracy with rail-to-rail operation at the input and output
• Manufacturer: Maxim

Single/Dual/Quad, Micropower, Single-Supply,
Rail-to-Rail Op Amps
The single MAX4091, dual MAX4092 and quad
MAX4094 op amps combine excellent DC accuracy
with rail-to-rail operation at both input and output. With
their precision performance, wide dynamic range at low
supply voltages, and very low supply current, these op
amps are ideal for battery-operated equipment, indus-
trial, and data acquisition and control applications.
The MAX4091/MAX4092/MAX4094’s input common-
mode range extends 50mV beyond the positive and
negative supply rails, with excellent common-mode
rejection. Beyond the specified common-mode range,
the outputs are guaranteed not to undergo phase
reversal or latchup. Therefore, the MAX4091/MAX4092/
MAX4094 can be used in applications with common-
mode signals, at or even beyond the supplies, without
the problems associated with typical op amps.
The MAX4091/MAX4092/MAX4094’s output voltage
swings to within 15mV of the supplies with a 100kΩ
load. This rail-to-rail swing at the input and the output
substantially increases the dynamic range, especially
in low-supply-voltage applications. Figure 1 shows the
input and output waveforms for the MAX4092, config-
ured as a unity-gain noninverting buffer operating from
a single 3V supply. The input signal is 3.0V
sinusoid centered at 1.5V. The output amplitude is
approximately 2.98V
Rail-to-rail common-mode swing at the input is obtained
by two complementary input stages in parallel, which
feed a folded cascaded stage. The PNP stage is active
for input voltages close to the negative rail, and the NPN
stage is active for input voltages close to the positive rail.
The offsets of the two pairs are trimmed. However,
there is some residual mismatch between them. This
mismatch results in a two-level input offset characteris-
tic, with a transition region between the levels occurring
at a common-mode voltage of approximately 1.3V
above V
tion region has been widened to approximately 600mV
in order to minimize the slight degradation in CMRR
caused by this mismatch.
The input bias currents of the MAX4091/MAX4092/
MAX4094 are typically less than 20nA. The bias current
flows into the device when the NPN input stage is
active, and it flows out when the PNP input stage is
active. To reduce the offset error caused by input bias
current flowing through external source resistances,
8
_______________________________________________________________________________________
EE
. Unlike other rail-to-rail op amps, the transi-
Applications Information
Rail-to-Rail Inputs and Outputs
P-P
Detailed Description
.
Input Offset Voltage
P-P
, a 1kHz
match the effective resistance seen at each input.
Connect resistor R3 between the noninverting input and
ground when using the op amp in an inverting configu-
ration (Figure 2a); connect resistor R3 between the
noninverting input and the input signal when using the
op amp in a noninverting configuration (Figure 2b).
Select R3 to equal the parallel combination of R1 and
R2. High source resistances will degrade noise perfor-
mance, due to the the input current noise (which is mul-
tiplied by the source resistance).
The MAX4091/MAX4092/MAX4094 include internal pro-
tection circuitry that prevents damage to the precision
input stage from large differential input voltages. This
protection circuitry consists of back-to-back diodes
between IN+ and IN- with two 1.7kΩ resistors in series
(Figure 3). The diodes limit the differential voltage
applied to the amplifiers’ internal circuitry to no more
than V
(about 0.7V at +25°C).
Input bias current for the ICs (±20nA) is specified for
small differential input voltages. For large differential
input voltages (exceeding V
increases the input current at IN+ and IN-:
Even with their low quiescent current of less than
130µA per op amp, the MAX4091/MAX4092/MAX4094
are well suited for driving loads up to 1kΩ while main-
taining DC accuracy. Stability while driving heavy
capacitive loads is another key advantage over compa-
rable CMOS rail-to-rail op amps.
In op amp circuits, driving large capacitive loads
increases the likelihood of oscillation. This is especially
true for circuits with high-loop gains, such as a unity-
gain voltage follower. The output impedance and a
capacitive load form an RC network that adds a pole to
the loop response and induces phase lag. If the pole
frequency is low enough—as when driving a large
capacitive load––the circuit phase margin is degraded,
leading to either an under-damped pulse response or
oscillation.
The MAX4091/MAX4092/MAX4094 can drive capacitive
loads in excess of 2000pF under certain conditions
(Figure 4). When driving capacitive loads, the greatest
potential for instability occurs when the op amp is
sourcing approximately 200µA. Even in this case, sta-
bility is maintained with up to 400pF of output capaci-
F
INPUT CURRENT
, where V
Input Stage Protection Circuitry
F
Output Loading and Stability
is the diodes’ forward-voltage drop
=
[(
F
V
IN
), this protection circuitry
+
2
)
−
✕
(
1 7
V
IN
.
k
−
Ω
)]
−
V
F