THS4500 Datasheet, PDF (23/37 Pages) Texas Instruments – WIDEBAND, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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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 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, and
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,
directly impacting 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 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 Figures 100 through 104 are used to
highlight basic design considerations for fully differential
amplifier circuit designs.
THS4500
THS4501
SLOS350D − APRIL 2002 − REVISED JANUARY 2004
Table 3. Resistor Values for Balanced Operation
in Various Gain Configurations
ǒ Ǔ Gain
V_OD
V_IN
1
R2 & R4 (Ω) R1 (Ω) R3 (Ω)
392
412
383
RT (Ω)
54.9
1
499
523
487
53.6
2
392
215
187
60.4
2
1.3k
665
634
52.3
5
1.3k
274
249
56.2
5
3.32k
681
649
52.3
10
1.3k
147
118
64.9
10
6.81k
698
681
52.3
NOTE: Values in the table above assume a 50 Ω source impedance.
R1
R2
RS
VS
R3
RT
Vn _−
+
+−
VP
R4
Vout+
Vout−
VOCM
Figure 100
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
equations 6 through 9.
R_T +
1
1– ^K
1
R_S
–
2(1)K)
R3
K
+
R2
R1
R2 + R4
(6)
R3
+
R1
*
ǒ_Rs
||
R
Ǔ
T
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)
For more detailed information about balance in fully
differential amplifiers, see Fully Differential Amplifiers,
referenced at the end of this data sheet.
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