Datasheet

4
3
2
7
6
LMH6702
V
OUT
+
-
V
IN
+5V
-5V
C
POS
6.8µF
R
G
R
F
.01µF
6.8µF
C
NEG
.01µF
R
T
25:
0.1µF
C
SS
SELECT R
T
TO
YIELD DESIRED
R
IN
= R
T
||R
G
A
V
=
V
OUT
V
IN
R
F
R
G
=
4
3
2
7
6
LMH6702
V
OUT
+
-
V
IN
+5V
-5V
C
POS
6.8µF
R
G
R
F
.01µF
6.8µF
C
NEG
.01µF
R
IN
0.1µF
C
SS
A
V
= 1 +R
F
/R
G
= V
OUT
/V
IN
LMH6702
www.ti.com SNOSA03F –NOVEMBER 2002–REVISED MARCH 2013
APPLICATION SECTION
FEEDBACK RESISTOR
Figure 26. Recommended Non-Inverting Gain Circuit
Figure 27. 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 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 shown in the Typical
Performance Characteristics section. The capacitor C
SS
, shown across the supplies in Figure 26 and Figure 27,
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 26 and
Figure 27) 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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