Application Note
LMV771/LMV772/LMV774
The LMV771/LMV772/LMV774 are a family of precision am-
plifiers with very low noise and ultra low offset voltage.
LMV771/LMV772/LMV774's extended temperature range of
−40°C to 125°C enables the user to design this family of
products into a variety of applications including automotive.
The LMV771 has a maximum offset voltage of 1mV over the
extended temperature range. This makes the LMV771 ideal
for applications where precision is important.
The LMV772/LMV774 have a maximum offset voltage of 1mV
at room temperature and 1.2mV over the extended tempera-
ture range of −40°C to 125°C. Care must be taken when the
LMV772/LMV774 are designed into applications with heavy
loads under extreme temperature conditions. As indicated in
the DC tables, the LMV772/LMV774's gain and output swing
may be reduced at temperatures between 85°C and 125°C
with loads heavier than 2k
Ω.
INSTRUMENTATION AMPLIFIER
Measurement of very small signals with an amplifier requires
close attention to the input impedance of the amplifier, gain
of the overall signal on the inputs, and the gain on each input
since we are only interested in the difference of the two inputs
and the common signal is considered noise. A classic solution
is an instrumentation amplifier. Instrumentation amplifiers
have a finite, accurate, and stable gain. Also they have ex-
tremely high input impedances and very low output
impedances. Finally they have an extremely high CMRR so
that the amplifier can only respond to the differential signal. A
typical instrumentation amplifier is shown in Figure 1.
20039636
FIGURE 1. Instrumentation Amplifier
There are two stages in this amplifier. The last stage, output
stage, is a differential amplifier. In an ideal case the two am-
plifiers of the first stage, input stage, would be set up as
buffers to isolate the inputs. However they cannot be con-
nected as followers because of real amplifier's mismatch.
That is why there is a balancing resistor between the two. The
product of the two stages of gain will give the gain of the in-
strumentation amplifier. Ideally, the CMRR should be infinite.
However the output stage has a small non-zero common
mode gain which results from resistor mismatch.
In the input stage of the circuit, current is the same across all
resistors. This is due to the high input impedance and low
input bias current of the LMV771. With the node equations we
have:
(1)
By Ohm’s Law:
(2)
However:
(3)
So we have:
(4)
Now looking at the output of the instrumentation amplifier:
(5)
(6)
This shows the gain of the instrumentation amplifier to be:
−K(2a+1)
Typical values for this circuit can be obtained by setting: a =
12 and K= 4. This results in an overall gain of −100.
Figure 2 shows typical CMRR characteristics of this Instru-
mentation amplifier over frequency. Three LMV771 amplifiers
are used along with 1% resistors to minimize resistor mis-
match. Resistors used to build the circuit are: R
1
= 21.6k
Ω
,
R
11
= 1.8k
Ω
, R
2
= 2.5k
Ω
with K = 40 and a = 12. This results
in an overall gain of −1000, −K(2a+1) = −1000.
20039673
FIGURE 2. CMRR vs. Frequency
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LMV771/LMV772/LMV774
