THS4502 Datasheet, PDF (21/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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R_S
R_g1
R_f1
V_S
R_T
+V_S
+_−
V_OCM
−⁺
−V_S
R_g2
R_f2
Application Circuit for the THS4500 and THS4501,
Featuring Split-Supply Operation With an Input
Signal Referenced at the Midrail
Figure 94
Equations 1−5 allow for calculation of the required input
common-mode range for a given set of input conditions.
The equations allow calculation of the input common-
mode range requirements given information about the
input signal, the output voltage swing, the gain, and the
output common-mode voltage. Calculating the maximum
and minimum voltage required for V_N and V_P (the
amplifier’s input nodes) determines whether or not the
input common-mode range is violated or not. Four
equations are required. Two calculate the output voltages
and two calculate the node voltages at V_N and V_P (note
that only one of these needs calculation, as the amplifier
forces a virtual short between the two nodes).
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−.
THS4502
THS4503
SLOS352D − APRIL 2002 − REVISED JANUARY 2004
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 1. Negative-Rail Referenced
Gain
(V/V)
1
V_IN+ (V)
V_IN−
(V)
−2.0 to
2.0
0
V_IN
(V_PP)
4
V_OCM
(V)
2.5
V_OD
(V_PP)
4
V_NMIN
(V)
0.75
V_NMAX
(V)
1.75
2
−1.0 to
1.0
0
2
2.5
4
0.5
1.167
4
−0.5 to
0.5
0
1
2.5
4
0.3
0.7
8
−0.25 to
0.25
0
0.5
2.5
4
0.167 0.389
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 2. 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
2.0 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.
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