DocID026915 Rev 1

11/19

AN4586

3-op amp instrumentation amplifier

19

Equation 13

Based on the above equations, the worst case for each parameter can be calculated by

and

•

the worst case for the differential gain is:

•

the worst case for the common mode gain is:

•

the worst case for offset on the output voltage is:

To know how well this configuration rejects the Vicm, we calculate the CMRR using

Equation 14 and Equation 15.

Equation 14

Equation 15

We can see from

Equation 15 that to have the best CMRR, the maximum gain should be

set at the first stage of the 3-op amp instrumentation amplifier structure without saturating

op amps 1 and 2. To achieve the global required gain, the remaining gain should be set at

the second stage.

===

==

=

Ry

Rx

-------

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ 12ε

instru

+

()

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ 2ε

diff

+

Ry

Rx Ry

+

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

4

2Vio

Ry

Rx

-------

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ 12ε

instru

+

()

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ 2ε

diff

Vio 1

Ry

Rx

-------

12

+

()

+

⎝⎠

⎛⎞

++

Vout

V2 V1

–

()Gdiff

V1 V2

+

2

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

Gcm

+

=

CMRR

20

Gdiff

Gmc

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

⎝⎠

⎛⎞

log

20

Ry

Rx

-------

12

Rf

Rg

--------

+

⎝⎠

⎛⎞

Ry

Rx Ry

+

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

4

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

⎝⎠

⎜⎟

⎜⎟

⎜⎟

⎛⎞

log

==