10

100

1k

10k

FREQUENCY (Hz)

-140

-120

-100

-80

-60

-40

-20

0

V

) I

11

= (2a + 1) V R

11

= (2a + 1)

11

GIVEN: I

= I

LMV771, LMV772, LMV774

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SNOSA04F – MAY 2004 – REVISED SEPTEMBER 2010

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 connected 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 instrumentation 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:

(2)

By Ohm’s Law:

(3)

However:

(4)

So we have:

(5)

Now looking at the output of the instrumentation amplifier:

(6)

Substituting from Equation 5:

(7)

This shows the gain of the instrumentation amplifier to be:

−K(2a+1)

(8)

Typical values for this circuit can be obtained by setting: a = 12 and K= 4. This results in an overall gain of

−100.

Figure 6 shows typical CMRR characteristics of this Instrumentation amplifier over frequency. Three LMV771

amplifiers are used along with 1% resistors to minimize resistor mismatch. Resistors used to build the circuit are:

=

−1000.

Figure 6. CMRR vs. Frequency

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