LMV931MFX National Semiconductor Corporation, LMV931MFX Datasheet - Page 14

• Manufacturer Part Number: LMV931MFX
• Description: LMV931 - Single 1.8V, Rrio Operational Amplifiers, Package: SOT-23, Pin Nb=5
• Manufacturer: National Semiconductor Corporation

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Typical Applications
4.0 HALF-WAVE RECTIFIER WITH RAIL-TO-GROUND
OUTPUT SWING
Since the LMV931/LMV932/LMV934 input common mode
range includes both positive and negative supply rails and
the output can also swing to either supply, achieving half-
wave rectifier functions in either direction is an easy task. All
that is needed are two external resistors; there is no need for
diodes or matched resistors. The half wave rectifier can have
either positive or negative going outputs, depending on the
way the circuit is arranged.
5.0 INSTRUMENTATION AMPLIFIER WITH
RAIL-TO-RAIL INPUT AND OUTPUT
Some manufactures make a non-“rail-to-rail”-op amp rail-to-
rail by using a resistive divider on the inputs. The resistors
divide the input voltage to get a rail-to-rail input range. The
problem with this method is that it also divides the signal, so
in order to get the obtained gain, the amplifier must have a
higher closed loop gain. This raises the noise and drift by the
internal gain factor and lowers the input impedance. Any
mismatch in these precision resistors reduces the CMRR as
well. The LMV981/LMV982 is rail-to-rail and therefore
doesn’t have these disadvantages.
Using three of the LMV981/LMV982 amplifiers, an instru-
mentation amplifier with rail-to-rail inputs and outputs can be
made as shown in Figure 5.
In this example, amplifiers on the left side act as buffers to
the differential stage. These buffers assure that the input
impedance is very high and require no precision matched
resistors in the input stage. They also assure that the differ-
ence amp is driven from a voltage source. This is necessary
to maintain the CMRR set by the matching R
The gain is set by the ratio of R
and R
ranges, the input and output are only limited by the supply
4
equal R
FIGURE 3. Half-Wave Rectifier with Rail-To-Ground Output Swing Referenced to Ground
2
. With both rail-to-rail input and output
FIGURE 4. Half-Wave Rectifier with Negative-Going Output Referenced to V
200326C3
200326C0
2
/R
1
and R
(Continued)
3
should equal R
1
-R
2
with R
3
-R
4
1
.
14
In Figure 3 the circuit is referenced to ground, while in Figure
4 the circuit is biased to the positive supply. These configu-
rations implement the half wave rectifier since the LMV931/
LMV932/LMV934 can not respond to one-half of the incom-
ing waveform. It can not respond to one-half of the incoming
because the amplifier can not swing the output beyond either
rail therefore the output disengages during this half cycle.
During the other half cycle, however, the amplifier achieves a
half wave that can have a peak equal to the total supply
voltage. R
LMV931/LMV932/LMV934.
voltages. Remember that even with rail-to-rail outputs, the
output can not swing past the supplies so the combined
common mode voltages plus the signal should not be
greater that the supplies or limiting will occur. For additional
applications, see National Semiconductor application notes
AN–29, AN–31, AN–71, and AN–127.
FIGURE 5. Rail-to-rail Instrumentation Amplifier
I
200326C2
200326B9
should be large enough not to load the
CC
200326C1
200326C4
200326G4