LMH6733 Datasheet, PDF (12/16 Pages) Texas Instruments – Single Supply, 1.0 GHz, Triple Operational Amplifier

ACTIVE FILTER
The choice of reactive components requires much attention
when using any current feedback operational amplifier as an
active filter. Reducing the feedback impedance, especially at
higher frequencies, will almost certainly cause stability prob-
lems. Likewise capacitance on the inverting input should be
avoided. See Application Notes OA-7 and OA-26 for more in-
formation on Active Filter applications for Current Feedback
Op Amps.
When using the LMH6733 as a low pass filter the value of
R_F can be substantially reduced from the value recommended
in the R_F vs. Gain charts. The benefit of reducing R_F is in-
creased gain at higher frequencies, which improves attenua-
tion in the stop band. Stability problems are avoided because
in the stop band additional device bandwidth is used to cancel
the input signal rather than amplify it. The benefit of this
change depends on the particulars of the circuit design. With
a high pass filter configuration reducing R_F will likely result in
device instability and is not recommended.
20199108
FIGURE 5. Decoupling Capacitive Loads
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R_OUT. Figure 5 shows the use
of a series output resistor, R_OUT, to stabilize the amplifier out-
put under capacitive loading. Capacitive loads of 5 to 120 pF
are the most critical, causing ringing, frequency response
peaking and possible oscillation. The chart “Frequency Re-
sponse vs. Capacitive Load” give a recommended value for
selecting a series output resistor for mitigating capacitive
loads. The values suggested in the charts are selected for .5
dB or less of peaking in the frequency response. This gives a
good compromise between settling time and bandwidth. For
applications where maximum frequency response is needed
and some peaking is tolerable, the value of R_OUT can be re-
duced slightly from the recommended values.
FIGURE 4. Typical Video Application
20199107
www.national.com
12