THS4501 Datasheet, PDF (24/48 Pages) National Semiconductor (TI) – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4500
THS4501
SLOS350F – APRIL 2002 – REVISED OCTOBER 2011
Gain
(V/V)
1
2
4
8
V_IN+
(V)
V_IN–
(V)
0.5 to 4.5
2.5
1.5 to 3.5
2.5
2.0 to 3.0
2.5
2.25 to 2.75
2.5
Table 2. Midrail Referenced
V_IN
(V_PP)
4
2
1
0.5
V_OCM
(V)
2.5
2.5
2.5
2.5
V_OD
(V_PP)
4
4
4
4
V_NMIN
(V)
2
2.16
2.3
2.389
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V_NMAX
(V)
3
2.83
2.7
2.61
CHOOSING THE PROPER VALUE FOR THE
FEEDBACK AND GAIN RESISTORS
The selection of feedback and gain resistors impacts
circuit performance in a number of ways. The values
presented in this section provide the optimum
high-frequency performance (lowest distortion, flat
frequency response). Since the THS4500 family of
amplifiers is developed with a voltage feedback
architecture, the choice of resistor values does not
have a dominant effect on bandwidth, unlike a
current-feedback amplifier. However, resistor choices
do have second-order effects. For optimal
performance, the following feedback resistor values
are recommended. In higher gain configurations (gain
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
values, coupled with gain requirements, determine
the value of the gain resistors and directly impact the
input impedance of the entire circuit. While there are
no strict rules about resistor selection, these trends
can provide qualitative design guidance.
APPLICATION CIRCUITS USING FULLY
DIFFERENTIAL AMPLIFIERS
Fully differential amplifiers provide designers with a
great deal of flexibility in a wide variety of
applications. This section provides an overview of
some common circuit configurations and gives some
design guidelines. Designing the interface to an
analog-to-digital converter (ADC), driving lines
differentially, and filtering with fully differential
amplifiers are a few of the circuits that are covered.
BASIC DESIGN CONSIDERATIONS
The circuits in Figure 98 through Figure 101 are used
to highlight basic design considerations for fully
differential amplifier circuit designs.
Table 3. Resistor Values for Balanced Operation
in Various Gain Configurations
ǒ Ǔ Gain
V_OD
V_IN
1
1
2
2
5
5
10
10
R2 and R4
(Ω)
R1 (Ω) R3 (Ω)
392
499
392
1.3 k
1.3 k
3.32 k
1.3 k
6.81 k
412
383
523
487
215
187
665
634
274
249
681
649
147
118
698
681
R_T (Ω)
54.9
53.6
60.4
52.3
56.2
52.3
64.9
52.3
R1
R2
R_S
V_S
R3
R_T
V_n
-
+
+^-
V_P
R4
Vout+
Vout-
VOCM
Figure 101. Diagram for Design Calculations
Equations for calculating fully differential amplifier
resistor values in order to obtain balanced operation
in the presence of a 50-Ω source impedance are
given in Equation 6 through Equation 9.
R_T +
1
1– ^K
1
R_S
–
2(1)K)
R3
K
+
R2
R1
R2 + R4
R3 + R1 * ^ǒR_s || R_T^Ǔ
(6)
β₁
+
R1
R1 ) R2
β₂
+
R3 )
R3 ) R_T
R_T ||
|| R_S
R_S
) R4
(7)
ǒ Ǔ ǒ Ǔ ^VOD
V_S
+
2
1–β₂
β₁ ) β₂
R_T
R_T ) R_S
(8)
ǒ Ǔ ^VOD
V _IN
+
2
1–β₂
β₁ ) β₂
(9)
24
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