Differential configuration

AN4586

6/19

DocID026915 Rev 1

Figure 3. Differential configuration example

In Figure 3, the Vicm of the op amp is 9 V. Thus, if the op amp is supplied by 5 V, we cannot

measure the voltage difference. We may even damage the op amp.

The op amp Vicm range is given as follows:

If Vref is applied on Rf2, the op amp Vicm range becomes:

A drawback of the differential configuration is its input impedance. This is defined by the

resistors and consequently it may not be high enough. Below are given the differential and

common mode impedances respectively:

A high input impedance may be mandatory in certain cases so as not to impact the signal

being measured. For this reason the instrumentation amplifier configuration has been

designed (see Section 3).

V2

Rf2

Rf2 Rg2

+

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

⋅

V2

1

Rg2

Rf2

-----------

+

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

Vref

1

Rf2

Rg2

-----------

+

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

+

Zdiff

2Rg

=

Zcm

Rg Rf

+

2

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

=