Datasheet

1
10 100
FREQUENCY (MHz)
-110
-105
-100
-95
-90
-80
-60
HD2 (dBc)
A
V
= +2
R
L
= 100:
V
O
= 2V
PP
HD3, SOT23
HD2, SOIC
HD2, SOT23
HD3, SOIC
-65
-70
-85
-75
1
10 100
FREQUENCY (MHz)
-90
-80
-70
-60
-50
-40
-30
HD2 (dBc)
A
V
= +2
R
L
= 100:
V
O
= 2V
PP
C
POS
& C
NEG
REMOVED
C
POS
& C
NEG
INCLUDED
LMH6702
SNOSA03F –NOVEMBER 2002–REVISED MARCH 2013 www.ti.com
If this lay out technique has not been observed on a particular application board, designer may actually find that
supply decoupling caps could adversely affect HD2 performance by increasing the coupling phenomenon already
mentioned. Figure 28 below shows actual HD2 data on a board where the ground plane is "shared" between the
supply decoupling capacitors and the rest of the circuit. Once these capacitors are removed, the HD2 distortion
levels reduce significantly, especially between 10MHz-20MHz, as shown in Figure 28 below:
Figure 28. Decoupling Current Adverse Effect on a Board with Shared Ground Plane
At these extremely low distortion levels, the high frequency behavior of decoupling capacitors themselves could
be significant. In general, lower value decoupling caps tend to have higher resonance frequencies making them
more effective for higher frequency regions. A particular application board which has been laid out correctly with
ground returns "split" to minimize coupling, would benefit the most by having low value and higher value
capacitors paralleled to take advantage of the effective bandwidth of each and extend low distortion frequency
range.
Another important variable in getting the highest fidelity signal from the LMH6702 is the package itself. As
already noted, coupling between high frequency current transients on supply lines and the device input can lead
to excess harmonic distortion. An important source of this coupling is in fact through the device bonding wires. A
smaller package, in general, will have shorter bonding wires and therefore lower coupling. This is true in the case
of the SOT-23 compared to the SOIC package where a marked improvement in HD can be measured in the
SOT-23 package. Figure 29 below shows the HD comparing SOT-23 to SOIC package:
Figure 29. SOIC and SOT-23 Packages Distortion Terms Compared
The LMH6702 data sheet shows both SOT-23 and SOIC data in the Electrical Characteristic section to aid in
selecting the right package. The Typical Performance Characteristics section shows SOIC package plots only.
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