THS3092_14 Datasheet, PDF (17/45 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

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WIDEBAND, INVERTING OPERATION
Figure 53 shows the THS3092 in a typical inverting
gain configuration where the input and output
impedances and signal gain from Figure 52 are
retained in an inverting circuit configuration.
15 V _+VS
50 Ω Source _RG
V_I
402 Ω
R_M
57.6 Ω
0.1 µF
+
6.8 µF
+
49.9 Ω
_
50 Ω LOAD
R_F
806 Ω
0.1 µF 6.8 µF
+
−15 V ^−VS
Figure 53. Wideband, Inverting Gain
Configuration
THS3092
THS3096
SLOS428B – DECEMBER 2003 – REVISED FEBRUARY 2006
+V_S
50 Ω Source
+
V_I
R_T ^{49.9 Ω} _{_}
+V_S
2
R_F
R_G
909 Ω
909 Ω
49.9 Ω
50 Ω LOAD
+V_S
2
50 Ω Source
R_G
V_I
402 Ω
57.6 Ω R_T
+V_S +V_S
2
2
R_F
806 Ω
V_S
_
+
49.9 Ω
50 Ω LOAD
Figure 54. DC-Coupled, Single-Supply Operation
SINGLE SUPPLY OPERATION
The THS3092/6 have the capability to operate from a
single supply voltage ranging from
10 V to 30 V. When operating from a single power
supply, biasing the input and output at mid-supply
allows for the maximum output voltage swing. The
circuits shown in Figure 54 shows inverting and
noninverting amplifiers configured for single supply
operations.
VDSL Driver Circuit
The THS3092 and THS3096 have the ability to drive
over 200 mA of current with very high voltage swings.
Using these amplifiers coupled with the very high
slew rate, low distortion, and low noise required in
VDSL applications, makes for a perfect match. In
VDSL systems where the receive signal is critical, the
use of a low transformer ratio is necessary. With this
low ratio, the output swing required from the line
driver amplifier must increase, especially when
driving the VDSL’s full 14.5-dBm power onto the line.
The line driver's low distortion and noise is critical for
the VDSL as the receive bands are intertwined with
the transmit frequency bands up to the 12-MHz VDSL
limit.
Figure 55 shows a traditional hybrid connection
approach for achieving the 14.5-dBm line power
utilizing a 1:1 transformer. Looking at the input to the
amplifiers shows a low-pass filter consisting of two
separate capacitors to ground. There is an argument
that since the signal coming out of the DAC is
fully-differential then a single capacitor (10 pF in this
case) is perfectly acceptable. The problem with this
idea is that many DACs have common-mode energy
due to images around the sampling frequency which
must be filtered before reaching the amplifier. An
amplifier simply amplifies its input–including the
DAC’s images at high frequencies–and pass it
through to the transformer and ultimately to the line,
possibly causing the system to fail EMC compliance.
A single capacitor does not remove these
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