OPA3695_14 Datasheet, PDF (15/39 Pages) Texas Instruments – Triple, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3695
www.ti.com ............................................................................................................................................... SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008
Figure 38 shows the ac-coupled, G = +2V/V,
single-supply specification and test circuit. Once
again, the noninverting input is dc-biased at
midsupply to put that same V_S/2 at the output pin.
+V_S +5V
50W Source
0.1mF
V_I
60.4W
604W
604W
0.1mF
6.8mF
DIS
1/3
V_O 100W
OPA3695
V_S/2
R_F
499W
R_G
499W
0.1mF
Figure 38. AC-Coupled, G = +2V/V, Single-Supply
Specification and Test Circuit
HIGH-FREQUENCY ACTIVE FILTERS
The extremely wide bandwidth of the OPA3695
allows an extensive range of active filter topologies to
be implemented with minimal amplifier bandwidth
interaction in the filter shape. While Sallen-Key filters
work very well with current-feedback amplifiers, the
use of multiple feedback (MFB) filters is not
recommended because an MFB filter places a
capacitor in the feedback path which in turn
eliminates compensation and results in an oscillator.
In general, given a desired filter ω_O, the amplifier
should have a minimum of 10X ω_O to minimize filter
interaction with the amplifier frequency response.
Figure 39 illustrates an example gain of +2 line driver
using the OPA3695 that incorporates a 40MHz
low-pass Butterworth response with only 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
OPA3695
R_F
604W
R_G
604W _-5V
50W
V_O
50W
Figure 39. Line Driver with 40MHz Low-Pass
Active Filter
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 OPA3695 used in Figure 39
has a 900MHz bandwidth, the active filter continues
to roll-off through frequencies that exceed 200MHz.
Figure 40 shows the frequency response for the filter
of Figure 39, 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 40. 40MHz Low-Pass Active Filter
Response
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