THS4503-EP_17 Datasheet, PDF (21/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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FULLY DIFFERENTIAL AMPLIFIER
TERMINAL FUNCTIONS
Fully differential amplifiers are typically packaged in 8-pin
packages as shown in the diagram. The device pins
include two inputs (V_IN+, V_IN−), two outputs (V_OUT−,
V_OUT+), two power supplies (V_S+, V_S−), an output
common-mode control pin (V_OCM), and an optional
power-down pin (PD).
V_IN− 1
V_{OCM 2}
V_{S+ 3}
V_OUT+ 4
8 V_IN+
7 NC
6 V_S−
5 V_OUT−
Fully Differential Amplifier Pin Diagram
A standard configuration for the device is shown in the
figure. The functionality of a fully differential amplifier can
be imagined as two inverting amplifiers that share a
common noninverting terminal (though the voltage is not
necessarily fixed). For more information on the basic
theory of operation for fully differential amplifiers, see the
Texas Instruments application note titled Fully Differential
Amplifiers (SLOA054).
INPUT COMMON-MODE VOLTAGE RANGE
AND THE THS4500 FAMILY
The key difference between the THS4500/1 and the
THS4502/3 is the input common-mode range for the two
devices. The THS4503 has an input common-mode range
that is centered around midrail, and the THS4500 and
THS4501 have an input common-mode range that is
shifted to include the negative power supply rail. Selection
of one or the other is determined by the nature of the
application. Specifically, the THS4500 and THS4501 are
designed for use in single-supply applications where the
input signal is ground-referenced, as depicted in
Figure 94. The THS4503 is designed for use in
single-supply or split-supply applications where the input
signal is centered between the power supply voltages, as
depicted in Figure 95.
THS4503−EP
SGLS291A − APRIL 2005 − JANUARY 2012
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 94
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 95
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).
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