Datasheet
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
_
+
_
+
_
+
THS4012
THS4012
V
IN1
V
IN2
R2
R1
R2
R
(g)
R
(g)
R
f
R
f
THS413x
THS4130
THS4131
www.ti.com
SLOS318H –MAY 2000– REVISED MAY 2011
Fully-differential amplifiers may be viewed as two inverting amplifiers. In this case, the equation of an inverting
amplifier holds true for gain calculations. One advantage of fully-differential amplifiers is that they offer twice as
much dynamic range compared to single-ended amplifiers. For example, a 1-V
PP
ADC can only support an input
signal of 1 V
PP
. If the output of the amplifier is 2 V
PP
, then it is not as practical to feed a 2-V
PP
signal into the
targeted ADC. Using a fully-differential amplifier enables the user to break down the output into two 1-V
PP
signals
with opposite signs and feed them into the differential input nodes of the ADC. In practice, the designer has been
able to feed a 2-V peak-to-peak signal into a 1-V differential ADC with the help of a fully-differential amplifier. The
final result indicates twice as much dynamic range. Figure 39 illustrates the increase in dynamic range. The gain
factor should be considered in this scenario. The THS413x fully-differential amplifier offers an improved CMRR
and PSRR due to its symmetrical input and output. Furthermore, second-harmonic distortion is improved. Second
harmonics tend to cancel because of the symmetrical output.
Figure 39. 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 40 depicts the general format of instrumentation amplifiers.
The general transfer function for this circuit is:
(13)
Figure 40. Instrumentation Amplifier
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