OPA3681 Datasheet, PDF (14/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

termination for 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 first stage
amplifier output, typically improving 3rd-order terms di-
rectly, 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 non-inverting 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 bandwidths at high power levels are
possible using the OPA3681 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 pro-
vided by the OPA3681.
WIDEBAND DIFFERENTIAL AMPLIFIER
The differential amplifier (three amplifier instrumentation
topology) 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 feed-
back resistor values in this input stage provides > 120MHz
bandwidth, even at this high gain setting. The signal is
applied to the high impedance, non-inverting inputs at the
input stage. The differential gain is set by (1 + 2R_F/R_G) = 10
using the values shown on the front page. The third amplifier
performs the differential-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 OPA3681 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 distor-
tion 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 non-
inverting 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.
Figure 8 shows an example of this application where the
three channels have been set up for gains of 2, 4, and 8 to
their output pins. When driving a doubly-terminated 50Ω
load, this gives a user-selectable gain of 1, 2 and 4 to the
matched load. The feedback resistor value has been opti-
mized 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. An additional 10Ω resistor
was included inside the loop on each output stage to limit
output stage currents if more than one output is on during
gain select transition. This will reduce the maximum avail-
able output voltage swing into the 100Ω total load shown in
Figure 8 to approximately ±3.2V, but will provide surge
current protection during channel switching. The 20Ω series
resistors on each non-inverting input serves to isolate the
input parasitic capacitance from the source.
+5V
Supply De-Coupling
Not Shown
DSL
AFE
1/3
100Ω
R_T
OPA3681
50Ω
1/3
OPA3681
400Ω
133Ω
R_{S 1:n}
R_S
400Ω
Bandpass
Filter
100Ω
400Ω
–5V
R_G
1/3
OPA3681
FIGURE 7. Single-to-Differential xDSL Line Driver.
®
OPA3681
14