THS4505 Datasheet, PDF (21/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

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THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 2004
R_S
R_g1
R_f1
V_S
R_T
+V_S
+−
V_OCM
−+
R_g2
R_f2
Application Circuit for the THS4500 and THS4501,
Featuring Single-Supply Operation With a
Ground-Referenced Input Signal
Figure 75.
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 76.
Equations 1-5 allow calculation of the required input
common-mode range for a given set of input con-
ditions.
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-.
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 77.
Table 1 and Table 2 show 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.
Gain (V/V)
1
2
4
8
V_IN+ (V)
-2.0 to 2.0
-1.0 to 1.0
-0.5 to 0.5
-0.25 to 0.25
Table 1. Negative-Rail Referenced
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
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