THS4500 Datasheet, PDF (25/37 Pages) Texas Instruments – WIDEBAND, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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CF
RS
Rg
Rf
5V
VS
RT
5V
10 µF
+−
VOCM
0.1 µF
Riso
IN ^ADS5421
14 Bit/40 MSps
1 µF
−+
THS4501
IN
Riso
CM
Rg
Rf
CF
Using the THS4501 With the ADS5421
Figure 102
0.1 µF
FULLY DIFFERENTIAL LINE DRIVERS
The THS4500 family of amplifiers can be used as
high-frequency, high-swing differential line drivers. Their
high power supply voltage rating (16.5 V absolute
maximum) allows operation on a single 12-V or a single
15-V supply. The high supply voltage, coupled with the
ability to provide differential outputs enables the ability to
drive 26 V_PP into reasonably heavy loads (250 Ω or
greater). The circuit in Figure 103 illustrates the THS4500
family of devices used as high speed line drivers. For line
driver applications, close attention must be paid to thermal
design constraints due to the typically high level of power
dissipation.
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 103
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
THS4500
THS4501
SLOS350D − APRIL 2002 − REVISED JANUARY 2004
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.
CF1
RS
Rg1
Rf1
VS
RT
+
Riso
−
C VO
−+
Rg2
Riso
Rf2
CF2
A Two-Pole, Low-Pass Filter Design Using a Fully
Differential Amplifier With Poles Located at:
P1 = (2πRfCF)−1 in Hz and P2 = (4πRisoC)−1 in Hz
Figure 104
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
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
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