LMH6702-MIL Datasheet, PDF (11/19 Pages) Texas Instruments – 1.7-GHz Ultra-Low Distortion Wideband Op Amp

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7 Detailed Description
LMH6702-MIL
SNOSD58 – JUNE 2017
7.1 Overview
The LMH6702-MIL 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-MIL distortions shown in Typical
Characteristics.
7.2 Feature Description
7.2.1 Harmonic Distortion
The capacitor C_SS, shown across the supplies in Figure 24 and Figure 25, 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 24 and Figure 25) 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).
If this layout 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 22 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 10 MHz to 20 MHz, as shown in Figure 22:
-30
-40
C_POS & C_NEG
-50
INCLUDED
-60
C_POS & C_NEG
REMOVED
-70
-80
-90
1
10
100
Frequency (MHz)
Figure 22. 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-MIL 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 23 shows the HD comparing SOT-23 to SOIC package:
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