OPA3691 Datasheet, PDF (14/25 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

the DAC, R_T. It is very useful from a distortion standpoint to
scale the characteristic impedance up for the filter. This
reduces the loading at the 1st-stage amplifier output, typi-
cally improving 3rd-order terms directly, as well as some
improvement in 2nd-order terms. Figure 7 assumes a 100Ω
characteristic impedance for the filter. The filter is driven from
a 100Ω source resistor into a 100Ω load that is formed by the
input gain resistor of the inverting amplifier channel. The
other noninverting input is isolated by a series 50Ω resistor—
principally to isolate that input from the out-of-band source
impedance of the filter. In this example, the output stage is
set up for a differential gain of 8. The total gain from the
output of the bandpass filter to the line will be 4 • n, where n
is the turns ratio used in the transformer. Very broad band-
widths at high power levels are possible using the OPA3691
in the circuit of Figure 7. Recognize also, that the output is in
fact bandlimited by the filter. Very high dynamic range is
possible inside the filter bandwidth due to the significant
performance margin provided by the OPA3691.
WIDEBAND DIFFERENTIAL/SINGLE-ENDED AMPLIFIER
The differential amplifier (three amplifier instrumentation to-
pology) on the front page of this data sheet shows a common
application applied to this triple current-feedback op amp.
The two input stage amplifiers are configured for a relatively
high differential gain of 10. Lowering the feedback resistor
values in this input stage provides 120MHz bandwidth, even
at this high gain setting. The signal is applied to the high
impedance, noninverting inputs at the input stage. The differ-
ential gain is set by (1 + 2R_F/R_G) = 10 using the values shown
on the front page. The third amplifier performs the differen-
tial-to-single-ended conversion in a standard single op amp
differential stage. This differential stage, built using the 3rd
wideband current-feedback op amp, in the OPA3691 will give
lower CMRR at DC than using a voltage-feedback part, but
higher CMRR at higher frequencies. Measured performance,
with no resistor value tuning, gave approximately 75dB at DC
and > 55dB CMRR (input referred) through 10MHz. To
maintain good distortion performance for the input stage
amplifiers, the loading at each output has been matched
while achieving the gain of 1 and differential characteristic of
the output stage. To improve DC CMRR, tune the resistor to
ground at the noninverting input of the output stage amplifier.
WIDEBAND PROGRAMMABLE GAIN
By tying all three inputs together from a single source, and all
three outputs together to drive a common load, a very
wideband, programmable gain function may be implemented.
See Figure 8 for an example of this application where the
three channels have been set up for gains of 1, 2, and 4 to
the load. The feedback resistor value has been optimized for
maximum flat bandwidth in each channel. This will give an
almost constant > 200MHz bandwidth at any of the three gain
settings. The desired gain is selected by using the disable
control lines to choose one of the three possible amplifiers as
the active channel. Isolation resistors have been optimized to
match the 50Ω load, and will limit the output current if more
than one output is on during gain-select transition. The
isolation resistors have been adjusted for each amplifier such
that the load impedance sees a matching 50Ω independant
from the operating amplifier. This, in turn, requires gain
matching so that the gains are 1, 2, and 4 to the load.
The 20Ω series resistors on each noninverting input serves
to isolate the input parasitic capacitance from the source.
Also, limit the voltage swing across the inputs of the inactive
channels to < ±1.2Vp-p.
+5V
Supply decoupling
not shown.
DSL
AFE
1/3
100Ω
R_T
OPA3691
50Ω
1/3
OPA3691
400Ω
133Ω
R_{S 1:n}
R_S
400Ω
Bandpass
Filter
100Ω
400Ω
–5V
R_G
1/3
OPA3691
FIGURE 7. Single-to-Differential xDSL Line Driver.
14
OPA3691
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