LMH6702QML Datasheet, PDF (9/14 Pages) Texas Instruments – 1.7 GHz, Ultra Low Distortion, Wideband Op Amp

Application Section
FEEDBACK RESISTOR
20151628
FIGURE 1. Recommended Non-Inverting Gain Circuit
arate 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 re-
sults in best harmonic distortion response (especially 2^nd
order distortion).
If this lay out technique has not been observed on a particular
application board, designer may actually find that supply de-
coupling caps could adversely affect HD2 performance by
increasing the coupling phenomenon already mentioned. Fig-
ure 3 below shows actual HD2 data on a board where the
ground plane is "shared" between the supply decoupling ca-
pacitors and the rest of the circuit. Once these capacitors are
removed, the HD2 distortion levels reduce significantly, es-
pecially between 10MHz-20MHz, as shown in Figure 3 below:
20151627
FIGURE 2. Recommended Inverting Gain Circuit
The LMH6702 achieves its excellent pulse and distortion per-
formance 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 re-
sistor. 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 har-
monic 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 1 and
Figure 2, is critical to achieving the lowest 2^nd harmonic dis-
tortion. For absolute minimum distortion levels, it is also ad-
visable to keep the supply decoupling currents (ground
connections to C_POS, and C_NEG in Figure 1 and Figure 2) sep-
20151622
FIGURE 3. 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 sig-
nificant. 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 advan-
tage of the effective bandwidth of each and extend low dis-
tortion frequency range.
CAPACITIVE LOAD DRIVE
Figure 4 shows a typical application using the LMH6702 to
drive an ADC.
www.national.com
8