THS4502 Datasheet, PDF (23/49 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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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 95.
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 THS4502/3. 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 THS4502/3 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_IN+ V_IN-
(V/V) (V) (V)
1
-2.0 to
2.0
0
2
-1.0 to
1.0
0
4
-0.5 to
0.5
0
8
-0.25
to 0.25
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
THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
Table 2. Negative-Rail Referenced (continued)
Gain
(V/V)
V_IN+
(V)
V_IN- V_IN V_OCM V_OD V_NMIN V_NMAX
(V) (V_PP) (V) (V_PP) (V)
(V)
NOTE: This table assumes a negative-rail referenced,
single-ended input signal on a single 5-V supply as shown in
Figure 93. V_NMIN = V_PMIN and V_NMAX = V_PMAX
Table 3. Midrail Referenced
Gain
(V/V)
1
V_IN+
(V)
0.5 to
4.5
V_IN- V_IN
(V) (V_PP)
2.5 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
2.0 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
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 (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.
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