LMH6654MFX datasheet(13/17 Pages) NSC

13 / 17 page

Application Information (Continued)

The free evaluation board are shipped automatically when a

device sample request is placed with National Semiconduc-

tor.

The CLC730027 datasheet also contains tables of recom-

mended components to evaluate several of National’s high

speed amplifiers. This table for the LMH6654 is illustrated

below. Refer to the evaluation board datasheet for schemat-

ics and further information.

Components Needed to Evaluate the LMH6654 on the

Evaluation Board:

•

fRg use the datasheet to select values.

•

IN,ROUT typically 50Ω (Refer to the Basic Operation

section of the evaluation board datasheet for details)

•

f is an optional resistor for inverting again configurations

(select R

f to yield desired input impedance = Rg||Rf)

•

1,C2 use 0.1µF ceramic capacitors

•

3,C4 use 10µF tantalum capacitors

Components not used:

5,C6,C7,C8

R1 thru R8

The evaluation boards are designed to accommodate dual

supplies. The board can be modified to provide single op-

eration. For best performance;

1) do not connect the unused supply.

2) ground the unused supply pin.

power Dissipation

The package power dissipation should be taken into account

when operating at high ambient temperature and/or high

power dissipative conditions. In determining maximum oper-

able temperature of the device, make sure the total power

dissipation of the device is considered; this power dissipated

in the device with a load connected to the output as well as

the nominal dissipation of the op amp.

Driving Capacitive Loads

Capacitive loads decrease the phase margin of all op amps.

The output impedance of a feedback amplifier becomes

inductive at high frequencies, creating a resonant circuit

when the load is capacitive. This can lead to overshoot,

ringing and oscillation. To eliminate oscillation or reduce

ringing, an isolation resistor can be placed as shown in

Figure 2 below. At frequencies above

the load impedance of the Amplifier approaches R

ISO. The

desired performance depends on the value of the isolation

resistor. The isolation resistance vs. capacitance load graph

in the typical performance characteristics provides the

means for selection of the value of R

S that provides ≤ 3dB

peaking in closed loop A

V = 1 response. In general, the

bigger the isolation resistor, the more damped the pulse

response becomes. For initial evaluation, a 50

Ω isolation

resistor is recommended.

Components Selection and Feedback Resistor

It is important in high-speed applications to keep all compo-

nent leads short since wires are inductive at high frequency.

For discrete components, choose carbon composition axially

leaded resistors and micro type capacitors. Surface mount

components are preferred over discrete components for

minimum inductive effect. Never use wire wound type resis-

tors in high frequency applications.

Large values of feedback resistors can couple with parasitic

capacitance and cause undesired effects such as ringing or

oscillation in high-speed amplifiers. Keep resistors as low as

possible consistent with output loading consideration. For a

gain of 2 and higher, 402

Ω feedback resistor used for the

typical performance plots gives optimal performance. For

unity gain follower, a 25

Ω feedback resistor is recommended

rather than a direct short. This effectively reduces the Q of

what would otherwise be a parasitic inductance (the feed-

back wire) into the parasitic capacitance at the inverting

input.

Bias Current Cancellation

In order to cancel the bias current errors of the non-inverting

configuration, the parallel combination of the gain setting R

and feedback R

resistors should equal the equivalent

source resistance R

seq as defined in Figure 3. Combining

this constraint with the non-inverting gain equation, allows

both R

f and Rg to be determined explicitly from the following

equations:

f =AVRseq and Rg =Rf/(AV−1)

For inverting configuration, bias current cancellation is ac-

complished by placing a resistor R

b on the non-inverting

input equal in value to the resistance seen by the inverting

input (R

f//(Rg+Rs). The additional noise contribution of Rb

can be minimized through the use of a shunt capacitor.

20016540

FIGURE 2.

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