THS4503-EP Datasheet, PDF (22/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4503−EP
SGLS291A − APRIL 2005 − JANUARY 2012
V _OUT)
+
V_IN)(1–β)–V_IN–(1–β) ) 2V_OCMβ
2β
(1)
V _OUT–
+
–V_IN)(1–β) ) V_IN–(1–β) ) 2V_OCMβ
2β
(2)
V_N + V_IN–(1–β) ) V_OUT)β
(3)
Where:
β
+
R_F
R_G
) R_G
(4)
V_P + V_IN)(1–β) ) V_OUT–β
(5)
NOTE:
The equations denote the device inputs as V_N and
V_P and the circuit inputs as V_IN+ and V_IN−.
R_g
R_f
V_IN+
V_p
V_OCM
+_−
−⁺
V_n
V_OUT−
V_OUT+
V_IN−
R_g
R_f
Diagram For Input Common-Mode Range Equations
Figure 96
The two tables below depict the input common-mode
range requirements for two different input scenarios, an
input referenced around the negative rail and an input
referenced around midrail. The tables highlight the
differing requirements on input common-mode range, and
illustrate reasoning for choosing either the THS4500/1 or
the THS4503. For signals referenced around the negative
power supply, the THS4500/1 should be chosen since its
input common-mode range includes the negative supply
rail. For all other situations, the THS4503 offers slightly
improved distortion and noise performance for
applications with input signals centered between the
power supply rails.
Table 2. Negative-Rail Referenced
Gain
(V/V)
1
2
4
8
V_IN+ (V)
−2 to 2
−1 to 1
−0.5 to
0.5
−0.25 to
0.25
V_IN−
(V)
0
0
0
0
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)
0.75
0.5
0.3
0.167
V_NMAX
(V)
1.75
1.167
0.7
0.389
NOTE^: This table assumes a negative-rail referenced, single-ended
input signal on a single 5-V supply as shown in Figure 94.
V_NMIN = V_PMIN and V_NMAX = V_PMAX.
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Table 3. Midrail Referenced
Gain
(V/V)
1
V_IN+ (V)
V_IN−
(V)
0.5 to
4.5
2.5
V_IN
(V_PP)
4
V_OCM
(V)
2.5
V_OD
(V_PP)
4
V_NMIN
(V)
2
V_NMAX
(V)
3
2
1.5 to
3.5
2.5
2
2.5
4
2.16
2.83
4
2 to 3 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
NOTE^: This table assumes a midrail referenced, single-ended input
signal on a single 5-V supply.
V_NMIN = V_PMIN and V_NMAX = V_PMAX.
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 (see Table 4).
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 Figure 96 through Figure 100 are used to
highlight basic design considerations for fully differential
amplifier circuit designs.