3-op amp instrumentation amplifier

AN4586

12/19

DocID026915 Rev 1

In Table 2 we can see that the 3-op amp instrumentation amplifier offers a better CMRR.

Moreover, it has a high input impedance thanks to the op amps in the first stage.

However, to amplify the signal properly, it is mandatory that the first stage is not saturated.

To achieve this, the following two conditions must be met:

with

Figure 7 highlights the consequences if these two conditions are not met. If the voltage

intersection of V1 and V2 is within the green area, the output voltage of the first op amp

stage is not saturated.

Figure 7 also specifies the area which is in linear configuration at the output of the

instrumentation amplifier. As well as the two conditions listed above, the input voltages have

to be compliant with the following condition:

Table 2. CMRR for 3-op amp instrumentation amplifier configuration

Resistor

inaccuracy (%)

CMRR (dB)

Rf/Rg = 2

Ry/Rx = 0.5

Gain = 2.5

Rf/Rg = 15

Ry/Rx = 0.5

Gain = 15.5

Rf/Rg = 100

Ry/Rx = 0.5

Gain = 100.5

10

25.5

41.3

57.5

1

45.5

61.3

77.5

0.1

65.5

81.3

97.5

Vccn

V2 V1

–

()

GRx

⋅

2Ry

-------------------

V1 V2

+

2

---------------------

Vccp

<

+

<

Vccn

V1 V2

–

()

GRx

⋅

2Ry

-------------------

V1 V2

+

2

---------------------

Vccp

<

+

<

G

Ry

Rx

-------

12

Rf

Rg

--------

+

⎝⎠

⎛⎞

=

Vccn

V2 V1

–

()GVccp

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