lmh6723max National Semiconductor Corporation, lmh6723max Datasheet - Page 12

• Manufacturer Part Number: lmh6723max
• Description: Single/dual/quad 370 Mhz 1 Ma Current Feedback Operational Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Section
Inverting Gain" and "R
provide a good place to start when selecting the best feed-
back resistor value for a variety of gain settings.
For more information see Application Note OA-13 which
describes the relationship between R
quency response for current feedback operational amplifiers.
The value for the inverting input impedance for the
LMH6723/LMH6724/LMH6725 is approximately 500Ω. The
LMH6723/LMH6724/LMH6725 is designed for optimum per-
formance at gains of +1 to +5V/V and −1 to −4V/V. Higher
gain configurations are still useful; however, the bandwidth
will fall as gain is increased, much like a typical voltage
feedback amplifier.
Figure 2 and Figure 3 show the value of R
higher R
creasing too far below the input impedance of the inverting
input. This limitation applies to both inverting and non-
inverting configurations. For the LMH6723/LMH6724/
LMH6725 the input resistance of the inverting input is ap-
proximately 500Ω and 100Ω is a practical lower limit for R
The LMH6723/LMH6724/LMH6725 begins to operate in a
gain bandwidth limited fashion in the region where R
be increased for higher gains. Note that the amplifier will
operate with R
will be substantially different than predicted from ideal mod-
els. In particular, the voltage potential between the Inverting
and Non-Inverting inputs cannot be expected to remain
small.
For inverting configurations the impedance seen by the
source is R
inverting gain since R
shown in Figure 3. The value of R
an inverting gain of −4 V/V the input impedance is equal to
100Ω. Using a termination resistor, this can be brought down
to match a 50Ω or 75Ω source; however, a 150Ω source
cannot be matched without a severe compromise in R
F
FIGURE 2. RF vs. Non-Inverting Gain
is required at higher gains to keep R
G
|| R
G
T
values well below 100Ω; however, results
. For most sources this limits the maximum
F
is determined by the desired gain as
F
vs. Inverting Gain". These charts
G
is then R
(Continued)
F
and closed-loop fre-
F
F
/Gain. Thus for
versus gain. A
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G
from de-
F
F
must
.
G
.
12
ACTIVE FILTERS
When using any current feedback operational amplifier as an
active filter it is necessary to be careful using reactive com-
ponents in the feedback loop. Reducing the feedback imped-
ance, especially at higher frequencies, will almost certainly
cause stability problems. Likewise capacitance on the invert-
ing input should be avoided. See Application Notes OA-7
and OA-26 for more information on Active Filter applications
for Current Feedback Op Amps.
When using the LMH6723/LMH6724/LMH6725 as a low-
pass filter the value of R
the value recommended in the R
benefit of reducing R
cies, 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 con-
figuration reducing R
and is not recommended.
FIGURE 4. Typical Application with Suggested Supply
FIGURE 3. R
F
F
is increased gain at higher frequen-
will likely result in device instability
F
Bypassing
can be substantially reduced from
F
vs. Inverting Gain
F
vs. Gain charts. The
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