2-op amp instrumentation amplifier

AN4586

8/19

DocID026915 Rev 1

Equation 8

Considering all the resistors have the same tolerance, ε:

•

The worst case for the offset on the output voltage is:

•

The worst case for the differential gain is:

•

The worst case for the common mode gain is:

Similar to differential configuration, when there is mismatching between resistors, the

CMRR is impacted. Thus, the output voltage is also impacted and the measurement

accuracy deteriorates. In order to quantify this impact, we calculate the CMRR using

Equation 9.

Equation 9

For high gain, the CMRR of the 2-op amp instrumentation amplifier is similar to the CMRR of

the differential amplifier. However, the main advantage of the current configuration is that it

offers high input impedance. Note also that it cannot be used in unity gain.

Regarding the Vicm range, the following conditions must be met to make the 2-op amp

instrumentation amplifier work:

•

V1 and V2 must be included within the op amp’s Vicm range

•

To avoid that the output of the first op amp is saturated:

±2Vio 1

R

α

R

β

-------

12

ε

+

()

+

⎝⎠

⎛⎞

1

R

α

R

β

-------

+

⎝⎠

⎛⎞ 1±2ε

()

4

ε

CMRR

20

Gdiff

Gmc

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

⎝⎠

⎛⎞

log

20

1

R

α

R

β

-------

+

4

ε

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

⎝⎠

⎜⎟

⎜⎟

⎛⎞

log

≈

=

V1

Vcc

1

Rf1

Rg1

-----------

+

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

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