THS4505 Datasheet, PDF (24/45 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363D – AUGUST 2002 – REVISED MAY 2008 ......................................................................................................................................................... www.ti.com
Often times, filters like these are used to eliminate
broadband noise and out-of-band distortion products
in signal acquisition systems. It should be noted that
the increased load placed on the output of the
amplifier by the second low-pass filter has a
detrimental effect on the distortion performance. The
preferred method of filtering is using the feedback
network, as the typically smaller capacitances
required at these points in the circuit do not load the
amplifier nearly as heavily in the pass-band.
SETTING THE OUTPUT COMMON-MODE
VOLTAGE WITH THE V_OCM INPUT
The output common-mode voltage pin provides a
critical function to the fully differential amplifier; it
accepts an input voltage and reproduces that input
voltage as the output common-mode voltage. In other
words, the V_OCM input provides the ability to level-shift
the outputs to any voltage inside the output voltage
swing of the amplifier.
A description of the input circuitry of the V_OCM pin is
shown below to facilitate an easier understanding of
the V_OCM interface requirements. The V_OCM pin has
two 50-kΩ resistors between the power supply rails to
set the default output common-mode voltage to
midrail. A voltage applied to the V_OCM pin alters the
output common-mode voltage as long as the source
has the ability to provide enough current to overdrive
the two 50-kΩ resistors. This phenomenon is
depicted in the V_OCM equivalent circuit diagram.
Current drive is especially important when using the
reference voltage of an analog-to-digital converter to
drive V_OCM. Output current drive capabilities differ
from part to part, so a voltage buffer may be
necessary in some applications.
V_OCM
I_IN
V_S+
R = 50 kW
I_IN =
2V_OCM - V_S+ - V_S-
R
R = 50 kW
V_S-
Figure 83. Equivalent Input Circuit for V_OCM
By design, the input signal applied to the V_OCM pin
propagates to the outputs as a common-mode signal.
As shown in the equivalent circuit diagram, the V_OCM
input has a high impedance associated with it,
dictated by the two 50-kΩ resistors. While the high
impedance allows for relaxed drive requirements, it
also allows the pin and any associated printed-circuit
board traces to act as an antenna. For this reason, a
decoupling capacitor is recommended on this node
for the sole purpose of filtering any high frequency
noise that could couple into the signal path through
the V_OCM circuitry. A 0.1-µF or 1-µF capacitance is a
reasonable value for eliminating a great deal of
broadband interference, but additional, tuned
decoupling capacitors should be considered if a
specific source of electromagnetic or radio frequency
interference is present elsewhere in the system.
Information on the ac performance (bandwidth, slew
rate) of the V_OCM circuitry is included in the
specification table and graph section.
Since the V_OCM pin provides the ability to set an
output common-mode voltage, the ability for
increased power dissipation exists. While this does
not pose a performance problem for the amplifier, it
can cause additional power dissipation of which the
system designer should be aware. The circuit shown
in Figure 84 demonstrates an example of this
phenomenon. For a device operating on a single 5-V
supply with an input signal referenced around ground
and an output common-mode voltage of 2.5 V, a dc
potential exists between the outputs and the inputs of
the device. The amplifier sources current into the
feedback network in order to provide the circuit with
the proper operating point. While there are no serious
effects on the circuit performance, the extra power
dissipation may need to be included in the system
power budget.
V_OCM
I₁ =
R_F1 + R_G1 + R_S || R_T
DC Current Path to Ground
R_S
R_G1
R_F1
V_S
R_T
5V
V_OCM = 2.5 V
+_-
+
-
2.5-V DC
R_S
2.5-V DC
R_G2
R_F2
DC Current Path to Ground
V_OCM
I₂ = _{RF2 + RG2}
Figure 84. Depiction of DC Power Dissipation
Caused by Output Level-Shifting in a DC-Coupled
Circuit
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