Datasheet

R =50kW
R =50kW
V
S+
V
S-
V
OCM
I
IN
I
IN
=
2 V
OCM
V-
S+
- V
S-
R
V
OCM
=2.5V
5V
V
S
R
S
R
G1
R
G2
R
F1
R
F2
+
R
T
+
R
L
2.5-VDC
2.5-VDC
DCCurrentPathtoGround
DCCurrentPathtoGround
I
2
=
V
OCM
R
F2
+R
G2
I
1
=
V
OCM
R
F1
+R
G1
+R
S
||R
T
-
-
THS4500
THS4501
www.ti.com SLOS350F –APRIL 2002–REVISED OCTOBER 2011
detrimental effect on the distortion performance. The capacitance is a reasonable value for eliminating a
preferred method of filtering is to use the feedback great deal of broadband interference, but additional,
network, as the typically smaller capacitances tuned decoupling capacitors should be considered if a
required at these points in the circuit do not load the specific source of electromagnetic or radio frequency
amplifier nearly as heavily in the passband. 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
SETTING THE OUTPUT COMMON-MODE
Characteristics and Typical Characterisitcs sections.
VOLTAGE WITH THE V
OCM
INPUT
Since the V
OCM
pin provides the ability to set an
The output common-mode voltage pin provides a
output common-mode voltage, the ability for
critical function to the fully differential amplifier; it
increased power dissipation exists. While this
accepts an input voltage and reproduces that input
possibility does not pose a performance problem for
voltage as the output common-mode voltage. In other
the amplifier, it can cause additional power
words, the V
OCM
input provides the ability to level-shift
dissipation of which the system designer should be
the outputs to any voltage inside the output voltage
aware. The circuit shown in Figure 107 demonstrates
swing of the amplifier.
an example of this phenomenon. For a device
A description of the input circuitry of the V
OCM
pin is
operating on a single 5-V supply with an input signal
shown in Figure 106 to facilitate an easier
referenced around ground and an output
understanding of the V
OCM
interface requirements.
common-mode voltage of 2.5 V, a dc potential exists
The V
OCM
pin has two 50-kΩ resistors between the
between the outputs and the inputs of the device. The
power supply rails to set the default output
amplifier sources current into the feedback network in
common-mode voltage to midrail. A voltage applied to
order to provide the circuit with the proper operating
the V
OCM
pin alters the output common-mode voltage
point. While there are no serious effects on the circuit
as long as the source has the ability to provide
performance, the extra power dissipation may need to
enough current to overdrive the two 50-kΩ resistors.
be included in the system power budget.
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.
Figure 106. Equivalent Input Circuit for V
OCM
Figure 107. Depiction of DC Power Dissipation
Caused By Output Level-Shifting in a DC-Coupled
Circuit
By design, the input signal applied to the V
OCM
pin
propagates to the outputs as a common-mode signal.
As shown in Figure 106, 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-frequency noise that could couple into the signal
path through the V
OCM
circuitry. A 0.1-μF or 1-μF
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