LMH6702QML_17 Datasheet, PDF (8/17 Pages) Texas Instruments – 1.7 GHz, Ultra Low Distortion, Wideband Op Amp

LMH6702QML
SNOSAQ2E – JULY 2005 – REVISED MARCH 2013
FEEDBACK RESISTOR
APPLICATION SECTION
+5V _6.8µF
V_IN
R_IN
.01µF
A_V = 1 +R_F/R_G = V_OUT/V_IN
C_SS
0.1µF
3
7 ^CPOS
+
6
LMH6702
-
2 ₄ C_NEG
.01µF ^RF
V_OUT
R_G
6.8µF
-5V
Figure 21. Recommended Non-Inverting Gain Circuit
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+5V
6.8µF
C_SS
25: _0.1µF
V_IN
R_G
.01µF _{AV =}
3
7 ^CPOS
+
6
LMH6702
-
2 ₄ C_NEG
R_F V_OUT
R_G ⁼ V_IN
V_OUT
.01µF ^RF
R_T
6.8µF
SELECT R_T TO
YIELD DESIRED
-5V
R_IN = R_T||R_G
Figure 22. Recommended Inverting Gain Circuit
The LMH6702 achieves its excellent pulse and distortion performance by using the current feedback topology.
The loop gain for a current feedback op amp, and hence the frequency response, is predominantly set by the
feedback resistor value. The LMH6702 is optimized for use with a 237Ω 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.
HARMONIC DISTORTION
The LMH6702 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 LMH6702 distortions. The capacitor C_SS,
shown across the supplies in Figure 21 and Figure 22, is critical to achieving the lowest 2^nd harmonic distortion.
For absolute minimum distortion levels, it is also advisable to keep the supply decoupling currents (ground
connections to C_POS, and C_NEG in Figure 21 and Figure 22) separate from the ground connections to sensitive
input circuitry (such as R_G, R_T, and R_IN ground connections). Splitting the ground plane in this fashion and
separately routing the high frequency current spikes on the decoupling caps back to the power supply (similar to
"Star Connection" layout technique) ensures minimum coupling back to the input circuitry and results in best
harmonic distortion response (especially 2^nd order distortion).
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