LMV981MG National Semiconductor Corporation, LMV981MG Datasheet - Page 13

• Manufacturer Part Number: LMV981MG
• Description: LMV981 - Single 1.8V, Rrio Operational Amplifiers With Shutdown, Package: Microsmd, Pin Nb=6
• Manufacturer: National Semiconductor Corporation

Application Note
1.0 INPUT AND OUTPUT STAGE
The rail-to-rail input stage of this family provides more flex-
ibility for the designer. The LMV981/LMV982 use a compli-
mentary PNP and NPN input stage in which the PNP stage
senses common mode voltage near V
senses common mode voltage near V
the PNP stage to NPN stage occurs 1V below V
input stages have their own offset voltage, the offset of the
amplifier becomes a function of the input common mode
voltage and has a crossover point at 1V below V
This V
and AC coupled signals if proper care is not taken. Large
input signals that include the V
distortion in the output signal. One way to avoid such distor-
tion is to keep the signal away from the crossover. For
example, in a unity gain buffer configuration and with V
5V, a 5V peak-to-peak signal will contain input-crossover
distortion while a 3V peak-to-peak signal centered at 1.5V
will not contain input-crossover distortion as it avoids the
crossover point. Another way to avoid large signal distortion
is to use a gain of −1 circuit which avoids any voltage
excursions at the input terminals of the amplifier. In that
circuit, the common mode DC voltage can be set at a level
away from the V
transition in V
signal in series with the input signal and can effectively
degrade small signal parameters such as gain and common
mode rejection ratio. To resolve this problem, the small
signal should be placed such that it avoids the V
over point. In addition to the rail-to-rail performance, the
output stage can provide enough output current to drive
600Ω loads. Because of the high current capability, care
should be taken not to exceed the 150˚C maximum junction
temperature specification.
2.0 SHUTDOWN MODE
The LMV981/LMV982 have a shutdown pin. To conserve
battery life in portable applications, the LMV981/LMV982
can be disabled when the shutdown pin voltage is pulled low.
The shutdown pin can’t be left unconnected. In case shut-
down operation is not needed, the shutdown pin should be
connected to V
ing the shutdown pin floating will result in an undefined
operation mode, either shutdown or active, or even oscillat-
ing between the two modes.
3.0 INPUT BIAS CURRENT CONSIDERATION
The LMV981/LMV982 family has a complementary bipolar
input stage. The typical input bias current (I
input bias current can develop a significant offset voltage.
This offset is primarily due to I
feedback resistor, R
100kΩ, then an offset voltage of 5mV will develop (V
x R
Figure 1, cancels this effect. But the input offset current (I
will still contribute to an offset voltage in the same manner.
F
). Using a compensation resistor (R
OS
crossover point can create problems for both DC
OS
+
when the LMV981/LMV982 are used. Leav-
OS
shows up as a V
cross-over point. For small signals, this
F
. For example, if I
B
OS
flowing through the negative
crossover point will cause
CM
+
−
. The transition from
dependent spurious
B
and the NPN stage
is 50nA and R
C
B
), as shown in
) is 15nA. The
+
. Since both
+
OS
.
OS
cross-
= I
S
F
OS
is
=
B
)
13
Typical Applications
4.0 HIGH SIDE CURRENT SENSING
The high side current sensing circuit (Figure 2) is commonly
used in a battery charger to monitor charging current to
prevent over charging. A sense resistor R
to the battery directly. This system requires an op amp with
rail-to-rail input. The LMV981/LMV982 are ideal for this ap-
plication because the common mode input range goes up to
the rail.
FIGURE 1. Canceling the Offset Voltage due to Input
FIGURE 2. High Side Current Sensing
Bias Current
SENSE
is connected
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