DocID026915 Rev 1

5/19

AN4586

Differential configuration

19

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:

When we talk about differential configuration, it is important to note that a mismatching

between resistors impacts the output voltage. This impact is measured by the CMRR. Vicm

can only be partially rejected if the resistors are not perfectly marched. The following CMRR

calculations highlight the impact of the resistor inaccuracies. Note that for the CMRR

calculation, we do not take into account the Vio variation (caused by the CMRR of the op

amp).

Equation 4

Equation 5

Therefore, the CMRR can be calculated for different gain and resistor inaccuracies (see

Table 1).

Thus, for accurate measurements it is recommended to use high accuracy resistors. It helps

to reject Vicm.

Regarding Vicm, there is a last criterion that has to be considered for the differential

configuration. It is the Vicm range of the op amp. In order to make the application work, the

Vicm should not exceed the capabilities of the op amp (see example in Figure 3).

Table 1. CMRR for differential configuration

Resistor inaccuracy (%)

CMRR (dB)

Gain = 1

Gain = 10

Gain = 100

10

14

29

48

134

49

68

0.1

54

69

88

±Vio 1

Rf

Rg

--------

12

ε

+

()

+

⎝⎠

⎛⎞

Rf

Rg

--------

1±2

ε

()

Rf

Rf Rg

+

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

4

ε

Vout

Gdiff V2 V1

–

() Gcm

V1 V2

+

2

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

+

=

CMRR

20

Gdiff

Gmc

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

⎝⎠

⎛⎞

log

20

1

Rf

Rg

--------

+

4

ε

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

⎝⎠

⎜⎟

⎜⎟

⎛⎞

log

==