LMH6715QML-SP datasheet(10/18 Pages) TI1

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100

200

300

400

500

600

700

800

900

1000

GAIN (V/V)

FREQUENCY (MHz)

100

-8

-7

-6

-5

-4

-3

-2

-1

-225

-180

-135

-90

-45

RF = 200:

GAIN

PHASE

VO = .5VPP

AV = 2

RF = 700:

RF = 500:

RF = 300:

LMH6715QML

SNOSAQ3B – NOVEMBER 2010 – REVISED MAY 2013

www.ti.com

Figure 29. Frequency Response vs. RF

The plot labeled “Frequency Response vs. RF” shows the LMH6715's frequency response as RF is varied (RL =

100

Ω, AV = +2). This plot shows that an RF of 200Ω results in peaking and marginal stability. An RF of 300Ω

gives near maximal bandwidth and gain flatness with good stability, but with very light loads (RL > 300Ω) the

device may show some peaking. An RF of 500Ω gives excellent stability with good bandwidth and is the

recommended value for most applications. Since all applications are slightly different it is worth some

experimentation to find the optimal RF for a given circuit. For more information see Application Note OA-13 which

describes the relationship between RF and closed-loop frequency response for current feedback operational

amplifiers.

When configuring the LMH6715 for gains other than +2V/V, it is usually necessary to adjust the value of the

feedback resistor. The two plots labeled “RF vs. Non-inverting Gain” and “RF vs. Inverting Gain” provide

recommended feedback resistor values for a number of gain selections.

Figure 30. RF vs. Non-Inverting Gain

Both plots show the value of RF approaching a minimum value (dashed line) at high gains. Reducing the

feedback resistor below this value will result in instability and possibly oscillation. The recommended value of RF

is depicted by the solid line, which begins to increase at higher gains. The reason that a higher RF is required at

higher gains is the need to keep RG from decreasing too far below the output impedance of the input buffer. For

the LMH6715 the output resistance of the input buffer is approximately 160

Ω and 50Ω is a practical lower limit for

RG. Due to the limitations on RG the LMH6715 begins to operate in a gain bandwidth limited fashion for gains of

±5V/V or greater.

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