LMH6723MASLASHNOPB datasheet(15/28 Pages) TI1

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200

400

600

800

1000

1200

1400

GAIN (-V/V)

500

1000

1500

2000

2500

GAIN (V/V)

LMH6723, LMH6724

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SNOSA83I – AUGUST 2003 – REVISED AUGUST 2014

Feedback Resistor Selection (continued)

For more information see Application Note OA-13 which describes the relationship between RF and closed-loop

frequency response for current feedback operational amplifiers. The value for the inverting input impedance for

the LMH6723/LMH6724 is approximately 500

Ω. The LMH6723/LMH6724 is designed for optimum performance

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 32. RF vs. Non-Inverting Gain

Figure 32 and Figure 33 show the value of RF versus gain. A higher RF is required at higher gains to keep RG

from decreasing too far below the input impedance of the inverting input. This limitation applies to both inverting

and non-inverting configurations. For the LMH6723/LMH6724 the input resistance of the inverting input is

approximately 500

Ω and 100Ω is a practical lower limit for RG. The LMH6723/LMH6724 begins to operate in a

gain bandwidth limited fashion in the region where RF must be increased for higher gains. Note that the amplifier

will operate with RG values well below 100 Ω; however, results will be substantially different than predicted from

ideal models. 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 RG || RT. For most sources this limits the

maximum inverting gain since RF is determined by the desired gain as shown in Figure 33. The value of RG is

then RF/Gain. Thus for 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 50-

Ω or 75-Ω source; however, a 150Ω source cannot be matched

without a severe compromise in RF.

Figure 33. RF vs. Inverting Gain

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