THS4505 Datasheet, PDF (20/45 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363D – AUGUST 2002 – REVISED MAY 2008 ......................................................................................................................................................... www.ti.com
R_S
R_G1
R_F1
V_S
R_T
+V_S
V_OCM
+_-
-⁺
R_G2
R_F2
Figure 76. Application Circuit for the THS4500 and
THS4501, Featuring Single-Supply Operation with
a Ground-Referenced Input Signal
R_S
R_G1
R_F1
V_S
R_T
+V_S
V_OCM
+_-
-⁺
-V_S
R_G2
R_F2
Figure 77. Application Circuit for the THS4500 and
THS4501, Featuring Split-Supply Operation with
an Input Signal Referenced at the Midrail
Equation 1 to Equation 5 allow 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 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
= ^{VIN+(1 - b) - VIN-(1 - b) + 2VOCMb}
OUT+
2b
(1)
V = ^{-VIN+(1 - b) + VIN-(1 - b) + 2VOCMb}
OUT-
2b
(2)
V
N
=
V (1
IN-
-
b)
+
V_OUT+b
(3)
Where:
R
b=
G
R_F + R_G
(4)
V_P = V_IN+(1 - b) + V_OUT-b
(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_IN-
V_P
V_OCM
V_N
+_-
-⁺
R_G
R_F
V_OUT-
V_OUT+
Figure 78. Diagram for Input Common-Mode
Range Equations
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
20
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