Datasheet
CHOOSING THE PROPER VALUE FOR THE
APPLICATION CIRCUITS USING FULLY
Gain
V
OD
V
IN
THS4504
THS4505
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......................................................................................................................................................... SLOS363D – AUGUST 2002 – REVISED MAY 2008
requirements, determine the value of the gain
FEEDBACK AND GAIN RESISTORS resistors, directly impacting the input impedance of
the entire circuit. While there are no strict rules about
The selection of feedback and gain resistors impacts
resistor selection, these trends can provide qualitative
circuit performance in a number of ways. The values
design guidance.
in this section provide the optimum high-frequency
performance (lowest distortion, flat frequency
response). Since the THS4500 family of amplifiers is
DIFFERENTIAL AMPLIFIERS
developed with a voltage-feedback architecture, the
choice of resistor values does not have a dominant
Fully differential amplifiers provide designers with a
effect on bandwidth, unlike a current-feedback
great deal of flexibility in a wide variety of
amplifier. However, resistor choices do have
applications. This section provides an overview of
second-order effects. For optimal performance, the
some common circuit configurations and gives some
following feedback resistor values are recommended.
design guidelines. Designing the interface to an ADC,
In higher gain configurations (gain greater than two),
driving lines differentially, and filtering with fully
the feedback resistor values have much less effect on
differential amplifiers are a few of the circuits that are
the high-frequency performance. Example feedback
covered.
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
Table 2. Midrail Referenced
Gain (V/V) V
IN+
(V) V
IN –
(V) V
IN
(V
PP
) V
OCM
(V) V
OD
(V
PP
) V
NMIN
(V) V
NMAX
(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 in Various Gain Configurations
R2 & R4 ( Ω ) R1 ( Ω ) R3 ( Ω ) R
T
( Ω )
1 392 412 383 54.9
1 499 523 487 53.6
2 392 215 187 60.4
2 1.3 k 665 634 52.3
5 1.3 k 274 249 56.2
5 3.32 k 681 649 52.3
10 1.3 k 147 118 64.9
10 6.81 k 698 681 52.3
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