THS4505 Datasheet, PDF (22/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 2004
www.ti.com
Table 2. Midrail Referenced
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
V_IN (V_PP)
4
2
1
0.5
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 re-
sponse). 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 ampli-
fier. 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).
V_OCM (V)
V_OD (V_PP)
V_NMIN (V)
V_NMAX (V)
2.5
4
2
3
2.5
4
2.16
2.83
2.5
4
2.3
2.7
2.5
4
2.389
2.61
gain configurations, and smaller resistor values can
load the amplifier more heavily, resulting in a re-
duction in distortion performance. In addition,
feedback resistor values, coupled with gain require-
ments, 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 appli-
cations. 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.
Amplifier loading, noise, and the flatness of the
frequency response are three design parameters that
should be considered when selecting feedback re-
sistors. Larger resistor values contribute more noise
and can induce peaking in the ac response in low
BASIC DESIGN CONSIDERATIONS
The circuits in Figures 75 through 78 are used to
highlight basic design considerations for fully differen-
tial 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 & R4 (Ω)
392
499
392
1.3k
1.3k
3.32k
1.3k
6.81k
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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