THS4500-EP Datasheet, PDF (26/38 Pages) National Semiconductor (TI) – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIER

THS4500-EP
SLOS832 – JUNE 2013
detrimental effect on the distortion performance. The
preferred method of filtering is to use 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 passband.
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 in Figure 107 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 =
2 V_OCM - V_S+ - V_S-
R
R = 50 kW
V_S-
Figure 107. 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 Figure 107, 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 PCB 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-
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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
ELECTRICAL CHARACTERISTICS and TYPICAL
CHARACTERISTICS sections.
Since the V_OCM pin provides the ability to set an
output common-mode voltage, the ability for
increased power dissipation exists. While this
possibility 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 108 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.
I₁ =
V_OCM
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_L
2.5-V DC
R_G2
R_F2
DC Current Path to Ground
I₂ = ^VOCM
R_F2 + R_G2
Figure 108. Depiction of DC Power Dissipation
Caused By Output Level-Shifting in a DC-Coupled
Circuit
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