OPA3693 Datasheet, PDF (18/29 Pages) Burr-Brown (TI) – Triple, Ultra-Wideband, Fixed-Gain, VIDEO BUFFER with Disable

OPA3693
SBOS353 – DECEMBER 2006
HIGH-FREQUENCY ACTIVE FILTERS
The extremely wide bandwidth of the OPA3693
allows a wide range of active filter topologies to be
implemented with minimal amplifier bandwidth
interaction in the filter shape. Sallen-Key filters, for
example, using either a gain of +1V/V or gain of
+2V/V amplifier, may be easily implemented with no
external gain setting elements. In general, given a
desired filter ω_O, the amplifier should have at least
20X that ω_O to minimize filter interaction with the
amplifier frequency response. Figure 49 illustrates an
example gain of +2 line driver using the OPA3693
that incorporates a 40MHz low-pass Butterworth
response with just a few external components. The
filter resistor values have been adjusted slightly here
from an ideal filter analysis to account for parasitic
effects.
+5V
22pF
100W
V_I
0W
Source
226W
22pF _1/3
OPA3693
R_F
300W
50W
V_O
50W
R_G
300W _-5V
Figure 49. Line Driver with 40MHz Low-Pass
Active Filter
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This type of filter depends on a low output
impedance from the amplifier through very high
frequencies to continue to provide an increasing
attenuation with frequency. As the amplifier output
impedance rises with frequency, any input signal or
noise starts to feed directly through to the output via
the feedback capacitor. Because the OPA3693 used
in Figure 49 has a 650MHz bandwidth, the active
filter continues to rolloff through frequencies
exceeding 200MHz. Figure 50 shows the frequency
response for the filter of Figure 49, where the desired
40MHz cutoff is achieved and a 40dB/dec roll-off is
held through very high frequencies.
9
6
3
0
-3
-6
-9
-12
-15
-18
-21
-24
1
10
100
Frequency (MHz)
1000
Figure 50. 40MHz Low-Pass Active Filter
Response
18
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