Datasheet

Gain
ǒ
V
OD
V
IN
Ǔ
R
S
R3
R1
R4
+
R
T
+
V
P
V
OCM
-
-
V
S
V
out+
V
out-
V
n
R2
R
T
+
1
1
R
S
–
1–
K
2(1)K)
R3
K +
R2
R1
R2 + R4
R3 + R1 *
ǒ
R
s
|| R
T
Ǔ
β
1
+
R1
R1 ) R2
β
2
+
R3 ) R
T
|| R
S
R3 ) R
T
|| R
S
) R4
V
OD
V
S
+ 2
ǒ
1–β
2
β
1
) β
2
Ǔǒ
R
T
R
T
) R
S
Ǔ
V
OD
V
IN
+ 2
ǒ
1–β
2
β
1
) β
2
Ǔ
THS4500
THS4501
SLOS350F –APRIL 2002–REVISED OCTOBER 2011 www.ti.com
Table 2. Midrail Referenced
Gain V
IN+
V
IN–
V
IN
V
OCM
V
OD
V
NMIN
V
NMAX
(V/V) (V) (V) (V
PP
) (V) (V
PP
) (V) (V)
1 0.5 to 4.5 2.5 4 2.5 4 2 3
2 1.5 to 3.5 2.5 2 2.5 4 2.16 2.83
4 2.0 to 3.0 2.5 1 2.5 4 2.3 2.7
8 2.25 to 2.75 2.5 0.5 2.5 4 2.389 2.61
Table 3. Resistor Values for Balanced Operation
CHOOSING THE PROPER VALUE FOR THE
in Various Gain Configurations
FEEDBACK AND GAIN RESISTORS
The selection of feedback and gain resistors impacts
R2 and R4
R1 (Ω) R3 (Ω) R
T
(Ω)
circuit performance in a number of ways. The values
(Ω)
presented in this section provide the optimum
high-frequency performance (lowest distortion, flat
1 392 412 383 54.9
frequency response). Since the THS4500 family of
1 499 523 487 53.6
amplifiers is developed with a voltage feedback
2 392 215 187 60.4
architecture, the choice of resistor values does not
2 1.3 k 665 634 52.3
have a dominant effect on bandwidth, unlike a
current-feedback amplifier. However, resistor choices
5 1.3 k 274 249 56.2
do have second-order effects. For optimal
5 3.32 k 681 649 52.3
performance, the following feedback resistor values
10 1.3 k 147 118 64.9
are recommended. In higher gain configurations (gain
10 6.81 k 698 681 52.3
greater than two), the feedback resistor values have
much less effect on the high-frequency performance.
Example feedback and gain resistor values are given
in the section on basic design considerations
(Table 3).
Amplifier loading, noise, and the flatness of the
frequency response are three design parameters that
should be considered when selecting feedback
resistors. Larger resistor values contribute more noise
and can induce peaking in the ac response in low
gain configurations; smaller resistor values can load
the amplifier more heavily, resulting in a reduction in
distortion performance. In addition, feedback resistor
Figure 101. Diagram for Design Calculations
values, coupled with gain requirements, determine
the value of the gain resistors and directly impact the
Equations for calculating fully differential amplifier
input impedance of the entire circuit. While there are
resistor values in order to obtain balanced operation
no strict rules about resistor selection, these trends
in the presence of a 50-Ω source impedance are
can provide qualitative design guidance.
given in Equation 6 through Equation 9.
APPLICATION CIRCUITS USING FULLY
DIFFERENTIAL AMPLIFIERS
Fully differential amplifiers provide designers with a
(6)
great deal of flexibility in a wide variety of
applications. This section provides an overview of
some common circuit configurations and gives some
(7)
design guidelines. Designing the interface to an
analog-to-digital converter (ADC), driving lines
differentially, and filtering with fully differential
(8)
amplifiers are a few of the circuits that are covered.
(9)
BASIC DESIGN CONSIDERATIONS
The circuits in Figure 98 through Figure 101 are used
to highlight basic design considerations for fully
differential amplifier circuit designs.
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