Application Note

LMV771/LMV772/LMV774

The LMV771/LMV772/LMV774 is 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 in a variety of applications including automotive.

LMV771 has a maximum offset voltage of 1mV over the

extended temperature range. This makes LMV771 ideal for

applications where precision is of importance.

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 given when

LMV772/LMV774 are designed in 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 am-

plifiers have a finite, accurate, and stable gain. Also they

have extremely 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.

There are two stages in this amplifier. The last stage, output

stage, is a differential amplifier. In an ideal case the two

amplifiers 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 amplifiers mismatch.

That is why there is a balancing resistor between the two.

The product of the two stages of the gain will give the gain of

the instrumentation amplifier. Ideally, the CMRR should be

infinity. 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)

Substituting from equation 4:

(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 amplifi-

ers are used along with 1%resistors to minimize resistor

mismatch. Resistors used to build the circuit are: R

21.6k

Ω,R

Ω,R

Ω with K = 40 and a = 12.

This results in an overall gain of −1000, −K(2a+1) = −1000.

20039636

FIGURE 1.

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