Application Section

FEEDBACK RESISTOR

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

237

Ω 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 distor-

tions shown in the Typical Performance Characteristics sec-

tion. The capacitor C

1 and Figure 2, is critical to achieving the lowest 2

monic distortion. For absolute minimum distortion levels, it is

also advisable to keep the supply decoupling currents

(ground connections to C

Figure 2) separate from the ground connections to sensitive

input circuitry (such as R

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 re-

sponse (especially 2

If this lay out technique has not been observed on a particu-

lar application board, designer may actually find that supply

decoupling caps could adversely affect HD2 performance by

increasing the coupling phenomenon already mentioned.

Figure 3 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 3

below:

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 ef-

fective 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 sig-

nal 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 SOT23-5 compared

to the SOIC package where a marked improvement in HD

can be measured in the SOT23-5 package. Figure 4 below

shows the HD comparing SOT23-5 to SOIC package:

20039028

FIGURE 1. Recommended Non-Inverting Gain Circuit

20039027

FIGURE 2. Recommended Inverting Gain Circuit

20039022

FIGURE 3. Decoupling Current Adverse Effect on a

Board with Shared Ground Plane

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