LMH6723 Datasheet, PDF (16/28 Pages) National Semiconductor (TI) – Single/Dual/Quad 370 MHz 1 mA Current Feedback Op Amp

LMH6723, LMH6724
SNOSA83I – AUGUST 2003 – REVISED AUGUST 2014
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7.5 Active Filters
When using any current feedback operational amplifier as an active filter it is necessary to be careful using
reactive components in the feedback loop. Reducing the feedback impedance, especially at higher frequencies,
will almost certainly cause stability problems. Likewise capacitance on the inverting input should be avoided. See
Application Notes OA-07 and OA-26 for more information on Active Filter applications for Current Feedback Op
Amps.
When using the LMH6723/LMH6724 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 increased gain at higher
frequencies, which improves attenuation 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.
6.8PF
C2
100nF
R_IN
75:
R_G
1.2k:
C1
X1
++
--
R_F
1.2k:
100nF
C3
6.8PF
R_OUT
75:
C4
Figure 34. Typical Application with Suggested Supply Bypassing
R_IN
RG
51: 1.2k:
X1
++
R_OUT
51:
--
CL
R_L
10pF
1k:
R_F
1.2k:
Figure 35. Decoupling Capacitive Loads
7.6 Driving Capacitive Loads
Capacitive output loading applications will benefit from the use of a series output resistor as shown in Figure 35.
The charts "Suggested R_OUT vs. Cap 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 reduced
slightly from the recommended values.
There will be amplitude lost in the series resistor unless the gain is adjusted to compensate; this effect is most
noticeable with heavy loads (R_L < 150Ω).
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