25
Analog Applications Journal
Texas Instruments Incorporated
Amplifiers: Op Amps
4Q 2005
www.ti.com/aaj
Analog and Mixed-Signal Products
Getting the most out of your
instrumentation amplifier design
Many industrial and medical applications
use instrumentation amplifiers (INAs) to
condition small signals in the presence of
large common-mode voltages and DC
potentials. Standard INAs using a unity-
gain difference amplifier in the output
stage, however, can limit the input common-
mode range significantly. Thus, common-
mode signals induced by adjacent equipment,
as well as large differential DC potentials
from differently located signal sources, can
increase the input voltage of the INA,
causing its input stage to saturate. Satura-
tion causes the INA output voltage, although
of wrong value, to appear normal to the
following processing circuitry. This could
lead to disastrous effects with unpredict-
able consequences.
This article reviews some principles of
the classic three-op-amp INA and provides
design hints that extend the input common-
mode range to avoid saturation while pre-
serving overall gain at maximum value. The article also
discusses the removal of large differential DC voltages
through active filtering, avoiding passive RC filters at the
INA input that otherwise would lower its common-mode
rejection ratio (CMRR).
INA principles
Figure 1 shows the block diagram of the classic three-op-
amp INA. The inputs, V
IN+
and V
IN–
, are defined through
the input polarities of the difference amplifier, A3.
By definition, the INA’s input signals are subdivided into
a common-mode voltage, V
CM
, and a differential voltage,
V
D
. While V
CM
, the voltage common to both inputs, is
defined as the average of the sum of V
IN+
and V
IN–
, V
D
represents the net difference between the two.
(1)
Solving both equations for V
IN+
or V
IN–
and equating the
received terms results in a new set of equations, which,
when solved for either input voltage, yields
(2)
VV
V
V
IN
CM
D
IN
CM
D
+−
=+
=−
22
and
.
V
V
CM
IN
IN
DI
N
I
N
=
+
2
and
.
In the nonsaturated mode, the op amp action of A1 and
A2 applies the differential voltage V
D
across the gain resis-
tor, R
G
, generating the input current, I
D
:
(3)
The output voltages of A1 and A2 are therefore
Replacing current I
D
with Equation 3 yields
(4)
where
Equation 4 shows that only the differential component,
V
D
/2, is amplified by the input gain, G
1
, while the common-
mode voltage, V
CM
, passes the input stage with unity gain.
The difference amplifier, A3, subtracts V
1
from V
2
and
amplifies the difference with the gain G
2
:
(5)
V
G
G
R
R
O
()
=
212
2
2
1
,.
where
G
R
R
F
G
1
12
.
V
GV
V
V
G
CM
D
CM
D
11
21
and
,
V
IR
V
V
V
CM
D
DF
C
M
D
−
+
and
.
I
R
V
R
D
IN
IN
G
D
G
=
+
=
.
By Thomas Kugelstadt
(Email: tk@ti.com)
Senior Systems Engineer, Industrial Systems
A3
+
–
+
–
R
1
R
2
R
1
R
F
R
G
I
D
V/
2
D
V
CM
R
2
V
O
V
–
=V
–V /2
IN
CM
D
V = V
– G V /2)
1C
M 1
D
(
A2
G =1+2R /R
1F
G
G=R/
R
1
V
REF
+
–
+
+
–
–
R
F
2
D
A1
V=
V+
V
/
2
IN+
CM
D
V = V
+ G V /2)
2C
D
(
Figure 1. Classic three-op-amp INA and its voltage nodes