LMH6703MAX datasheet(9/13 Pages) NSC

9 / 13 page

Application Section

GENERAL DESCRIPTION

The LMH6703 is a high speed current feedback amplifier,

optimized for excellent bandwidth, gain flatness, and low

distortion. The loop gain for a current feedback op amp, and

hence the frequency response, is predominantly set by the

feedback resistor value. The LMH6703 in the SOT23-6 pack-

age is optimized for use with a 560

Ω feedback resistor. The

LMH6703 in the SOIC package is optimized for use with a

390

Ω feedback resistor. Using lower values can lead to

excessive ringing in the pulse response while a higher value

will limit the bandwidth. Application Note OA-13 discusses

this in detail along with the occasions where a different R

might be advantageous.

EVALUATION BOARDS

Device

Package

Evaluation Board

Part Number

LMH6703MF

SOT23-6

CLC730216

LMH6703MA

SOIC

CLC730227

An Evaluation Board is shipped upon request when a

sample order is placed with National Semiconductor.

FEEDBACK RESISTOR SELECTION

One of the key benefits of a current feedback operational

amplifier is the ability to maintain optimum frequency re-

sponse independent of gain by using appropriate values for

the feedback resistor (R

F). The Electrical Characteristics and

Typical Performance plots specify an R

F of 560

Ω (390Ω for

the SOIC package), a gain of +2 V/V and ±5V power sup-

plies (unless otherwise specified). Generally, lowering R

from it’s recommended value will peak the frequency re-

sponse and extend the bandwidth while increasing the value

of R

F will cause the frequency response to roll off faster.

Reducing the value of R

F too far below it’s recommended

value will cause overshoot, ringing and, eventually, oscilla-

tion.

Since a current feedback amplifier is dependant on the value

of R

F to provide frequency compensation and since the

value of R

F can be used to optimize the frequency response,

different packages use different R

F values. As shown in

Figure 3, Recommended R

F vs. Gain, the SOT23-6 and the

SOIC package use different values for the feedback resistor,

F. Since each application is slightly different, it is worth

some experimentation to find the optimal R

F for a given

circuit. In general, a value of R

F that produces

≈0.1 dB of

peaking is the best compromise between stability and maxi-

mum bandwidth. Note that it is not possible to use a current

feedback amplifier with the output shorted directly to the

inverting input. The buffer configuration of the LMH6703

requires a 560

Ω (390Ω for SOIC package) feedback resistor

for stable operation.

The LMH6703 was optimized for high speed operation. As

shown in Figure 3, the suggested value for R

F decreases for

higher gains. Due to the output impedance of the input

buffer, there is a practical limit for how small R

F can go,

based on the lowest practical value of R

G. This limitation

applies to both inverting and non inverting configurations.

For the LMH6703 the input resistance of the inverting input is

approximately 30

Ω and 20Ω is a practical (but not hard and

fast) lower limit for R

G. The LMH6703 begins to operate in a

gain bandwidth limited fashion in the region when R

G is

nearly equal to the input buffer impedance. Note that the

20110603

FIGURE 1. Recommended Non-Inverting Gain Circuit

(SOIC Pinout Shown)

20110604

FIGURE 2. Recommended Inverting Gain Circuit

(SOIC Pinout Shown)

20110639

FIGURE 3. Recommended R

F vs. Gain

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