LMH6682 datasheet(17/20 Pages) NSC

17 / 20 page

Applications Section (Continued)

dent of the load resistance, mainly because of the dc current

delivered by the parts output stage into the load. For more

information about differential gain and phase and how to

measure it see National Semiconductors application note

OA-24 which can be found on via Nationals home page

http://www.national.com

OUTPUT PHASE REVERSAL

This is a problem with some operational amplifiers. This

effect is caused by phase reversal in the input stage due to

saturation of one or more of the transistors when the inputs

exceed the normal expected range of voltages. Some appli-

cations, such as servo control loops among others, are

sensitive to this kind of behavior and would need special

safeguards to ensure proper functioning. The LMH6682/

6683 is immune to output phase reversal with input overload.

With inputs exceeded, the LMH6682/6683 output will stay at

the clamped voltage from the supply rail. Exceeding the

input supply voltages beyond the Absolute Maximum Rat-

ings of the device could however damage or otherwise ad-

versely effect the reliability or life of the device.

DRIVING CAPACITIVE LOADS

The LMH6682/6683 can drive moderate values of capaci-

tance by utilizing a series isolation resistor between the

output and the capacitive load. Capacitive load tolerance will

improve with higher closed loop gain values. Applications

such as ADC buffers, among others, present complex and

varying capacitive loads to the Op Amp; best value for this

isolation resistance is often found by experimentation and

actual trial and error for each application.

DISTORTION

Applications with demanding distortion performance require-

ments are best served with the device operating in the

inverting mode. The reason for this is that in the inverting

configuration, the input common mode voltage does not vary

with the signal and there is no subsequent ill effects due to

this shift in operating point and the possibility of additional

non-linearity. Moreover, under low closed loop gain settings

(most suited to low distortion), the non-inverting configura-

tion is at a further disadvantage of having to contend with the

input common voltage range. There is also a strong relation-

ship between output loading and distortion performance (i.e.

Ω vs. 100Ω distortion improves by about 15dB @1MHz)

especially at the lower frequency end where the distortion

tends to be lower. At higher frequency, this dependence

diminishes greatly such that this difference is only about 5dB

at 10MHz. But, in general, lighter output load leads to re-

duced HD3 term and thus improves THD. (see the curve

THD vs. V

OUT over various frequencies).

PRINTED CIRCUIT BOARD LAYOUT AND COMPONENT

VALUES SELECTION

Generally it is a good idea to keep in mind that for a good

high frequency design both the active parts and the passive

ones are suitable for the purpose you are using them for.

Amplifying frequencies of several hundreds of MHz is pos-

sible while using standard resistors but it makes life much

easier when using surface mount ones. These resistors (and

capacitors) are smaller and therefore parasitics have lower

values and will have less influence on the properties of the

amplifier. Another important issue is the PCB, which is no

longer a simple carrier for all the parts and a medium to

interconnect them. The board becomes a real part itself,

adding its own high frequency properties to the overall per-

formance of the circuit. It’s good practice to have at least one

ground plane on a PCB giving a low impedance path for all

decouplings and other ground connections. Care should be

taken especially that on board transmission lines have the

same impedance as the cables they are connected to (i.e.

Ω for most applications and 75Ω in case of video and

cable TV applications). These transmission lines usually re-

quire much wider traces on a standard double sided PCB

than needed for a ’normal’ connection. Another important

issue is that inputs and outputs must not ’see’ each other or

are routed together over the PCB at a small distance. Fur-

thermore it is important that components are placed as flat

as possible on the surface of the PCB. For higher frequen-

cies a long lead can act as a coil, a capacitor or an antenna.

A pair of leads can even form a transformer. Careful design

of the PCB avoids oscillations or other unwanted behavior.

When working with really high frequencies, the only compo-

nents which can be used will be the surface mount ones (for

more information see OA-15).

As an example of how important the component values are

for the behavior of your circuit, look at the following case: On

a board with good high frequency layout, an amplifier is

placed. For the two (equal) resistors in the feedback path, 5

different values are used to set the gain to +2. The resistors

vary from 200

Ω to 3kΩ.

In Figure 4 can be seen that there’s more peaking with

higher resistor values, which can lead to oscillations and bad

pulse responses. On the other hand the low resistor values

will contribute to higher overall power consumption.

NSC suggests the following evaluation boards as a guide for

high frequency layout and as an aid in device testing and

characterization.

Device

Package

Evaluation

Board PN

LMH6682MA

8-Pin SOIC

CLC730036

LMH6682MM

8-Pin MSOP

CLC730123

LMH6683MA

14-Pin SOIC

CLC730031

LMH6683MT

14-Pin TSSOP

CLC730131

These free evaluation boards are shipped when a device

sample request is placed with National Semiconductor.

20059063

FIGURE 4.

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