LMH6703_16 Datasheet, PDF (12/26 Pages) Texas Instruments – 1.2 GHz, Low Distortion Op Amp with Shutdown

LMH6703
SNOSAF2E – FEBRUARY 2005 – REVISED MAY 2016
8 Detailed Description
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8.1 Overview
The LMH6703 has been optimized for exceptionally low harmonic distortion while driving very demanding
resistive or capacitive loads. Generally, when used as the input amplifier to very high speed flash ADCs, the
distortions introduced by the converter will dominate over the low LMH6703 distortions shown in Typical
Characteristics.
8.2 Feature 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 SOT-23-6 package 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 (SNOA366) discusses this in detail along with the occasions where a different R_F might
be advantageous.
8.3 Device Functional Modes
8.3.1 Feedback Resistor Selection
One of the key benefits of a current feedback operational amplifier is the ability to maintain optimum frequency
response independent of gain by using appropriate values for the feedback resistor (R_F). The Electrical
Characteristics and Typical Characteristics plots specify an R_F of 560 Ω (390 Ω for the SOIC package), a gain of
2 V/V, and ±5-V power supplies (unless otherwise specified). Generally, lowering R_F from it’s recommended
value will peak the frequency response 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, oscillation.
800
700
INVERTING (SOT23-6)
600
500
NON-INVERTING _(SOT23-6)
400
300
INVERTING _(SOIC)
200
NON-INVERTING (SOIC)
100
1 2 3 4 5 6 7 8 9 10
GAIN (V/V)
Figure 27. Recommended R_F vs. Gain
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 27, the SOT-23-6 and the SOIC package use different values for the feedback resistor, R_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
maximum 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 (390 Ω for SOIC
package) feedback resistor for stable operation.
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