THS4504 Datasheet, PDF (21/45 Pages) National Semiconductor (TI) – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
www.ti.com ......................................................................................................................................................... SLOS363D – AUGUST 2002 – REVISED MAY 2008
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.
Gain (V/V)
1
2
4
8
V_IN+ (V)
0.5 to 4.5
1.5 to 3.5
2.0 to 3.0
2.25 to 2.75
V_IN– (V)
2.5
2.5
2.5
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
V_NMAX (V)
3
2.83
2.7
2.61
Table 3. Resistor Values for Balanced Operation in Various Gain Configurations
Gain
V_OD
V_IN
1
1
2
2
5
5
10
10
R2 & R4 (Ω)
392
499
392
1.3 k
1.3 k
3.32 k
1.3 k
6.81 k
R1 (Ω)
412
523
215
665
274
681
147
698
R3 (Ω)
383
487
187
634
249
649
118
681
R_T (Ω)
54.9
53.6
60.4
52.3
56.2
52.3
64.9
52.3
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