Summary

AN4586

14/19

DocID026915 Rev 1

5

Summary

Table 3 summarizes the main features of the three most flexible methods (based on op

amps) which amplify a differential voltage into a single-ended signal.

For the 3-op amp instrumentation amplifier, the offset due to the op amp Vio on the output

voltage is bigger compared to the other configurations. Consequently, it is recommended to

use a high precision op amp such as the TSZ121 (or the TSZ124 for a quad version). The

TSZ121 has a maximum Vio of 5 µV and thus it makes accurate measurements.

Table 3. Main features of the three most flexible methods for amplifying a differential voltage into

a single-ended signal

Differential

configuration

2-op amp

instrumentation

amplifier

3-op amp

instrumentation

amplifier

Differential gain

(Gdiff)

Gdiff including errors

Gcm including

resistor tolerance

CMRR due to resistor

inaccuracy

Example of CMRR

with gain = 100 and

resistor tolerance

= 1 %

68 dB

68 dB

74 dB if Rx = Ry

77.5 dB if Rx = 2Ry

Vicm limitation

must be included in

the op amp Vicm

range

V1 and V2 must ve

included within the

op amps Vicm range

Error on Vout due to

Vio

Zdiff

2Rg

∞(1)

Zcm

∞(1)

∞(1)

1. Only limited by op amp input impedance

Rf

Rg

--------

1

R

α

R

β

-------

+

1

Ry

Rx

-------

+

⎝⎠

⎛⎞ 12 Rf

Rg

--------

+

⎝⎠

⎛⎞

Rf

Rg

--------

1±2

ε

()

1

R

α

R

β

-------

+

⎝⎠

⎛⎞ 1±2ε

()

Ry

Rx

-------

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ 12ε

instru

+

()

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ 2ε

diff

+

Rf

Rg Rf

+

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

4

ε

4

ε

Ry

Rx Ry

+

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

4

1

Rf

Rg

--------

+

4

ε

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

1

Rf

Rg

--------

+

4

ε

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

1

Ry

Rx

-------

+

⎝⎠

⎛⎞ 12 Rf

Rg

--------

+

⎝⎠

⎛⎞

4

ε

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

V2

Rf2

Rf2 Rg2

+

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

⋅

V1

Vcc

1

R

β

R

α

-------

+

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

<

Vccn

V2 V1

–

()

GRx

⋅

2Ry

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

V1 V2

+

2

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

+

Vccp

<<

Vccn

V1 V2

–

()

GRx

⋅

2Ry

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

V1 V2

+

2

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

+

Vccp

<<

Vccn

V2 V1

–

()GVccp

<<

Vio 1

Rf

Rg

--------

+

⎝⎠

⎛⎞

2Vio 1

R

α

R

β

-------

+

⎝⎠

⎛⎞

Vio1 Vio2

–

()

Ry

Rx

-------

12

Rf

Rg

--------

+

⎝⎠

⎛⎞ Vio3 1 Ry

Rx

-------

+

⎝⎠

⎛⎞

–

Rg Rf

+

2

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