THS4502 Datasheet, PDF (26/49 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
R_S
C_{G Rg}
R_f
V_S
R_T
15 V
R_iso ^CS
V_OCM
+-
THS4500/2
V_DD
R_L
0.1 µF
-+
R_{iso CS}
R_f
R_g
C_G
V_OD = 26 V_PP
Fully Differential Line Driver With High Output Swing
Figure 99.
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, (literature number
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
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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. 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 101.
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 100.
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.
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
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