LMV922 NSC [National Semiconductor], LMV922 Datasheet - Page 18

• Manufacturer Part Number: LMV922
• Description: 1.8V, 1MHz, Low Power Operational Amplifiers with Rail-To-Rail Input and Output
• Manufacturer: NSC [National Semiconductor]

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Typical Applications
2.0 Instrumentation Amplifier with Rail-To-Rail Input and
Output
Using three of the LMV924 Amplifiers, an instrumentation
amplifier with rail-to-rail inputs and outputs can be made.
Some manufacturers use a precision voltage divider array of
5 resistors to divide the common mode 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 unity gain, the am-
plifier must be run at high loop gains. This raises the noise
and drift by the internal gain factor and lowers the input im-
pedance. Any mismatch in these precision resistors reduces
the CMRR as well. Using the LMV924 eliminates all of these
problems.
In this example, amplifiers A and B act as buffers to the dif-
ferential stage. These buffers assure that the input imped-
(Continued)
FIGURE 7. Rail-to-rail instrumentation amplifier
18
ance is very high and require no precision matched resistors
in the input stage. They also assure that the difference amp
is driven from a voltage source. This is necessary to main-
tain the CMRR set by the matching R
The gain is set by the ratio of R
and R
With both rail-to-rail input and output ranges, the input and
output are only limited by the supply 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 lim-
iting will occur. For additional applications, see National
Semiconductor application notes AN–29, AN–31, AN–71,
and AN–127.
4
equal R
2
DS100979-G4
.
2
/R
1
and R
1
-R
2
3
with R
should equal R
3
-R
4
.
1