Datasheet

V
OCM
15 V
V
S
R
S
R
g
R
f
R
f
+
-
R
T
-
+
R
g
C
G
0.1 µF
C
G
THS4500/2
V
DD
C
S
C
S
R
L
V
OD
= 26 V
PP
R
iso
R
iso
Fully Differential Line Driver With High Output Swing
R = 50 kΩ
R = 50 kΩ
V
S+
V
S-
V
OCM
I
IN
I
IN
=
2 V
OCM
- V
S+
- V
S-
R
Equivalent Input Circuit for V
OCM
V
S
R
S
R
g1
R
f1
R
f2
+
-
R
T
-
+
V
O
R
iso
C
C
F2
C
F1
R
g2
R
iso
A Two-Pole, Low-Pass Filter Design Using a Fully
Differential Amplifier With Poles Located at:
P1 = (2πR
f
C
F
)
-1
in Hz and P2 = (4πR
iso
C)
-1
in Hz
THS4502
THS4503
SLOS352E –APRIL 2002–REVISED OCTOBER 2011 www.ti.com
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
Figure 99.
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
Filtering With Fully Differential Amplifiers
output common-mode voltage as long as the source
Similar to their single-ended counterparts, fully
has the ability to provide enough current to overdrive
differential amplifiers have the ability to couple
the two 50-kΩ resistors. This phenomenon is
filtering functionality with voltage gain. Numerous filter
depicted in the V
OCM
equivalent circuit diagram. The
topologies can be based on fully differential
table contains some representative examples to aid in
amplifiers. Several of these are outlined in A
determining the current drive requirement for the
Differential Circuit Collection, (literature number
V
OCM
voltage source. This parameter is especially
SLOA064) referenced at the end of this data sheet.
important when using the reference voltage of an
The circuit below depicts a simple two-pole low-pass
analog-to-digital converter to drive V
OCM
. Output
filter applicable to many different types of systems.
current drive capabilities differ from part to part, so a
The first pole is set by the resistors and capacitors in
voltage buffer may be necessary in some
the feedback paths, and the second pole is set by the
applications.
isolation resistors and the capacitor across the
outputs of the isolation resistors.
Figure 101.
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
Figure 100.
impedance allows for relaxed drive requirements, it
also allows the pin and any associated printed-circuit
Often times, filters like these are used to eliminate
board traces to act as an antenna. For this reason, a
broadband noise and out-of-band distortion products
decoupling capacitor is recommended on this node
in signal acquisition systems. It should be noted that
for the sole purpose of filtering any high frequency
the increased load placed on the output of the
noise that could couple into the signal path through
amplifier by the second low-pass filter has a
the V
OCM
circuitry. A 0.1-µF or 1-µF capacitance is a
detrimental effect on the distortion performance. The
reasonable value for eliminating a great deal of
preferred method of filtering is using the feedback
broadband interference, but additional, tuned
network, as the typically smaller capacitances
decoupling capacitors should be considered if a
required at these points in the circuit do not load the
amplifier nearly as heavily in the pass-band.
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