LMV981MG National Semiconductor Corporation, LMV981MG Datasheet - Page 14

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

www.national.com
Typical Applications
5.0 HALF-WAVE RECTIFIER WITH RAIL-TO-GROUND
OUTPUT SWING
Since the LMV981/LMV982 input common mode range in-
cludes 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.
6.0 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
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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 LMV981/
LMV982 can not respond to one-half of the incoming wave-
form. 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. Dur-
ing the other half cycle, however, the amplifier achieves a
half wave that can have a peak equal to the total supply
voltage. R
LMV981/LMV982.
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
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I
should be large enough not to load the
CC
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