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V
OCM
_
+
_
+
V
CC+
V
IN-
V
IN+
V
O+
V
O-
V
OD
= 1-0 = 1
V
OD
= 0-1 = -1
a
b
+1
0
+1
0
V
CC-
V
OD
V
IN1
– V
IN2
+
R
f
R
(g)
ǒ
1 )
2R2
R1
Ǔ
(12)
_
+
_
+
_
+
THS4012
THS4012
V
IN1
V
IN2
R2
R1
R2
R
(g)
R
(g)
R
f
R
f
THS414x
THS4140
THS4141
SLOS320F – MAY 2000 – REVISED JANUARY 2006
PRINCIPLES OF OPERATION (continued)
Figure 31. Fully Differential Amplifier With Two 1-V
PP
Signals
Similar to the standard inverting amplifier configuration, input impedance of a fully differential amplifier is
selected by the input resistor, R
(g)
. If input impedance is a constraint in design, the designer may choose to
implement the differential amplifier as an instrumentation amplifier. This configuration improves the input
impedance of the fully differential amplifier. Figure 32 depicts the general format of instrumentation amplifiers.
The general transfer function for this circuit is:
Figure 32. Instrumentation Amplifier
17
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