LMV921 datasheet(22/42 Pages) NSC

22 / 42 page

2-16

2.0 DEVICE INFORMATION

Typical Applications (Continued)

2.0 Instrumentation Amplifier with Rail-To-Rail Input and

Output

Using three LMV921 Amplifiers, an instrumentation amplifier

with rail-to-rail inputs and outputs can be made.

Some manufactures 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 LMV921 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-

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

1-R2 with R3-R4.

The gain is set by the ratio of R

2/R1 and R3 should equal R1

and R

4 equal R2.

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.

DS100979-G4

Figure 7. Rail-to-rail instumentation amplifier using three LMV921 amplifiers

LMV921 Datasheet(PDF) 22 Page - National Semiconductor (TI)