THS4505 Datasheet, PDF (25/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

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FILTERING WITH FULLY DIFFERENTIAL
AMPLIFIERS
Similar to their single-ended counterparts, fully differ-
ential amplifiers have the ability to couple filtering
functionality with voltage gain. Numerous filter top-
ologies can be based on fully differential amplifiers.
Several of these are outlined in A Differential Circuit
Collection, (SLOA064) referenced at the end of this
data sheet. The circuit below depicts a simple
two-pole low-pass filter applicable to many different
types of systems. The first pole is set by the resistors
and capacitors in the feedback paths, and the second
pole is set by the isolation resistors and the capacitor
across the outputs of the isolation resistors.
C_F1
R_S
R_g1
R_f1
R_T
R_iso
V_S
+
−
C VO
−+
R_g2
R_iso
R_f2
C_F2
A Two-Pole, Low-Pass Filter Design Using a Fully
Differential Amplifier With Poles Located at:
P1 = (2πR_fC_F)^−1 in Hz and P2 = (4πR_isoC)^−1 in Hz
Figure 82.
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 detri-
mental effect on the distortion performance. The
preferred method of filtering is using the feedback
network, as the typically smaller capacitances re-
quired 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
THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 2004
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 de-
picted in the V_OCM equivalent circuit diagram. The
table contains some representative examples to aid in
determining the current drive requirement for the
V_OCM voltage source. This parameter is especially
important when using the reference voltage of an
analog-to-digital converter to drive V_OCM. Output cur-
rent drive capabilities differ from part to part, so a
voltage buffer may be necessary in some appli-
cations.
V_S+
V_OCM
I_IN
R = 50 kΩ
I_IN =
2 V_OCM − V_S+ − V_S−
R
R = 50 kΩ
V_S−
Equivalent Input Circuit for V_OCM
Figure 83.
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 decoup-
ling capacitors should be considered if a specific
source of electromagnetic or radio frequency inter-
ference is present elsewhere in the system. Infor-
mation 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 in-
creased power dissipation exists. While this does not
pose a performance problem for the amplifier, it can
cause additional power dissipation of which the sys-
tem designer should be aware. The circuit shown in
Figure 84 demonstrates an example of this phenom-
enon. 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
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