LMH6714MFX datasheet(10/15 Pages) NSC

10 / 15 page

Application Section (Continued)

to be avoided. See Application Notes OA-7 and OA-26 for

more information on Active Filter applications for Current

Feedback Op Amps.

ENABLE/DISABLE OPERATION USING ±5V SUPPLIES

(LMH6720 ONLY)

The LMH6720 has a TTL logic compatible disable function.

Apply a logic low (<.8V) to the DS pin and the LMH6720 is

disabled. Apply a logic high (>2.0V), or let the pin float and

the LMH6720 is enabled. Voltage, not current, at the Disable

pin determines the enable/disable state. Care must be exer-

cised to prevent the disable pin voltage from going more

than .8V below the midpoint of the supply voltages (0V with

split supplies, V

CC/2 with single supplies) doing so could

cause transistor Q1 to Zener resulting in damage to the

disable circuit. The core amplifier is unaffected by this, but

disable operation could become slower as a result.

Disabled, the LMH6720 inputs and output become high im-

pedances. While disabled the LMH6720 quiescent current is

approximately 500µA. Because of the pull up resistor on the

disable circuit the I

CC and IEE currents are not balanced in

the disabled state. The positive supply current (I

CC)isap-

proximately 500µA while the negative supply current (I

EE)is

only 200µA. The remaining I

current of 300µA flows

through the disable pin.

The disable function can be used to create analog switches

or multiplexers. Implement a single analog switch with one

LMH6720 positioned between an input and output. Create

analog

multiplexer

with

several

LMH6720’s. The

LMH6720 is at it’s best at a gain of 1 for multiplexer appli-

cations because there is no R

G to shunt signals to ground.

DISABLE LIMITATIONS (LMH6720 ONLY)

The feedback Resistor (R

F) limits off isolation in inverting

gain configurations. During shutdown the impedance of the

LMH6720 inputs and output become very high (>1M

Ω),

however R

F and RG are the dominant factor for effective

output impedance.

Do not apply voltages greater than +V

CC or less than 0V

CC/2 single supply) to the disable pin. The input ESD

diodes will also conduct if the signal leakage through the

feedback resistors brings the inverting input near either sup-

ply rail.

LAYOUT CONSIDERATIONS

Whenever questions about layout arise, use the evaluation

board as a guide. The following Evaluation boards are avail-

able with sample parts:

LMH6714

SOT

CLC730216

SOIC

CLC730227

LMH6720

SOT

CLC730216

SOIC

CLC730227

LMH6722

SOIC

CLC730231

To reduce parasitic capacitances, the ground plane should

be removed near the input and output pins. To reduce series

inductance, trace lengths of components in the feedback

loop should be minimized. For long signal paths controlled

impedance lines should be used, along with impedance

matching at both ends.

Bypass capacitors should be placed as close to the device

as possible. Bypass capacitors from each rail to ground are

applied in pairs. The larger electrolytic bypass capacitors

can be located anywhere on the board, the smaller ceramic

capacitors should be placed as close to the device as pos-

sible. In addition Figure 2 shows a capacitor (C1) across the

supplies with no connection to ground. This capacitor is

optional, however it is required for best 2nd Harmonic sup-

pression. If this capacitor is omitted C2 and C3 should be

increased to .1µF each.

VIDEO PERFORMANCE

The LMH6714/6720/6722 has been designed to provide ex-

cellent performance with both PAL and NTSC composite

video signals. Performance degrades as the loading is in-

creased, therefore best performance will be obtained with

back terminated loads. The back termination reduces reflec-

tions from the transmission line and effectively masks ca-

pacitance from the amplifier output stage. While all parts

offer excellent video performance the LMH6714 and

LMH6722 are slightly better than the LMH6720.

20056521

FIGURE 4. Enable/Disable Operation

20056524

FIGURE 5. Typical Application with Suggested Supply

Bypassing

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