OPA3691 Datasheet, PDF (13/25 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

to match the typical video source impedance. The disable
control is used to switch between channels by feeding a logic
control line directly to all three V_DIS inputs on one package,
and its complement to the three V_DIS inputs on the other.
Since the disable feature is intentionally make-before-break
(to ensure that the output does not float in transition), each of
the two possible outputs for the three RGB lines are combined
through a limiting resistor. This 82.5Ω resistor limits the current
between the two outputs during switching. The feedback and
output network connected on the output slightly attenuates the
signal going out onto the 75Ω cable. The gain and output
matching resistors (82.5Ω) have been slightly increased to get
a signal gain of +1 to the matched load and provide a 75Ω
output impedance to the cable. The section on Disable Opera-
tion shows the turn-on and turn-off switching glitches, using a
grounded input for the single channel, is typically less than
±50mV. Where two outputs are switched (see Figure 4), the
output line is always under the control of one amplifier or the
other due to the “make-before-break” disable timing. In this
case, the switching glitches for 0V inputs drops to < 20mV.
Large output swing can cause the inactive inverting inputs to
turn on degrading distortion. Keep the voltages across the
inactive channel inputs < ±1.2Vp-p.
VIDEO DAC RECONSTRUCTION FILTER
Wideband current-feedback op amps make ideal elements
for implementing high-speed active filters where the amplifier
is used as fixed gain block inside a passive RC circuit
network. Their relatively constant bandwidth versus gain
provides low interaction between the actual filter poles and
the required gain for the amplifier. Figure 5 shows an ex-
ample of a video Digital-to-Analog Converter (DAC) recon-
struction filter.
The delay-equalized filter in Figure 5 compensates for the
DAC’s sin(x)/x response, and minimizes aliasing artifacts. It
is designed for single +5V operation, with a 13.5MSPS DAC
sampling rate, and a 5.5MHz cutoff frequency.
The first op amp buffers the video DAC output and the first
filter section from each other. This first filter section provides
group delay equalization. The second and third filter sections
provide a 6th-order low-pass filter response that also com-
pensates for the DAC’s sin(x)/x response. The filter response
can be seen in Figure 6.
20
0
–20
–40
–60
–80
–100
0
f_–3dB
1
10
100
Frequency (MHz)
FIGURE 6. DAC Reconstruction Filter Response.
HIGH-POWER XDSL LINE DRIVER
Emerging broadband access technologies are making sig-
nificant demands on the output stage drivers. Some of the
higher frequency versions, particularly in VDSL, require pas-
sive bandpass filters to spectrally isolate the upstream from
downstream frequency bands. See Figure 7 for one possible
implementation of this using single-ended filters and giving
differential push/pull drive into a transformer. The DAC out-
put from the Analog Front End (AFE) typically requires
isolation from the complex filter impedance. The first stage
provides a tunable gain (using R_G) with a fixed termination for
Video
In ^{100µF 402Ω}
402Ω
+5V
120pF
1/3
OPA3691
953Ω
953Ω
100pF
97.6Ω 237Ω 402Ω
220pF
+5V
1/3
56pF OPA3691
100pF
82.5Ω 243Ω 412Ω
220pF
+5V
1/3
56pF OPA3691
75.5Ω
V_O
402Ω
402Ω
402Ω
+5V
100µF
FIGURE 5. Filter Schematic.
OPA3691
13
SBOS227A
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