LMH6723 datasheet(12/17 Pages) NSC

12 / 17 page

Application Section (Continued)

Inverting Gain" and "R

F vs. Inverting Gain". These charts

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

F and closed-loop fre-

quency response for current feedback operational amplifiers.

The value for the inverting input impedance for the

LMH6723/LMH6724/LMH6725 is approximately 500

Ω. 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

F versus gain. A

higher R

F is required at higher gains to keep RG from de-

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 500

Ω 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

F must

be increased for higher gains. Note that the amplifier will

operate with R

G values well below 100

Ω; however, results

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

G || RT. For most sources this limits the maximum

inverting gain since R

F is determined by the desired gain as

shown in Figure 3. The value of R

G is then RF/Gain. Thus for

an inverting gain of −4 V/V the input impedance is equal to

100

Ω. Using a termination resistor, this can be brought down

to match a 50

Ω or 75Ω source; however, a 150Ω source

cannot be matched without a severe compromise in R

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

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 frequen-

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

F will likely result in device instability

and is not recommended.

20078905

FIGURE 2. RF vs. Non-Inverting Gain

20078906

FIGURE 3. R

F vs. Inverting Gain

20078933

FIGURE 4. Typical Application with Suggested Supply

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