THS4503-EP_17 Datasheet, PDF (25/40 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 differential
amplifiers have the ability to couple filtering functionality
with voltage gain. Numerous filter topologies 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
V_S
R_T
+
R_iso
−
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 101
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
THS4503−EP
SGLS291A − APRIL 2005 − JANUARY 2012
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. 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
current drive capabilities differ from part to part, so a
voltage buffer may be necessary in some applications.
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 102
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 103 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’s power budget.
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