LMH6609 Datasheet, PDF(12/15 Page) National Semiconductor (TI)

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LMH6609 Datasheet, PDF (12/15 Pages) National Semiconductor (TI) – 900MHz Voltage Feedback Op Amp

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Application Section (Continued)

COMPONENT SELECTION AND FEEDBACK RESISTOR

Surface mount components are highly recommended for the

LMH6609. Leaded components will introduce unpredictable

parasitic loading that will interfere with proper device opera-

tion. Do not use wire wound resistors.

The LMH6609 operates best with a feedback resistor of

approximately 250â¦ for all gains of +2 and greater and for â1

and less. With lower gains in particular, large value feedback

resistors will exaggerate the effects of parasitic capacitances

and may lead to ringing on the pulse response and fre-

quency response peaking. Large value resistors also add

undesirable thermal noise. Feedback resistors that are much

below 100â¦ will load the output stage, which will reduce

voltage output swing, increase device power dissipation,

increase distortion and reduce current available for driving

the load.

In the buffer configuration the output should be shorted

directly to the inverting input. This feedback does not load

the output stage because the inverting input is a high imped-

ance point and there is no gain set resistor to ground.

OPTIMIZING DC ACCURACY

The LMH6609 offers excellent DC accuracy. The well-

matched inputs of this amplifier allows even better perfor-

mance if care is taken to balance the impedances seen by

the two inputs. The parallel combination of the gain setting

RG and feedback RF resistors should be equal to RSEQ, the

resistance of the source driving the op amp in parallel with

any terminating Resistor (See Figure 1). Combining this with

the non inverting gain equation gives the following param-

eters:

RF = AVRSEQ

RG = RF/(AVâ1)

For Inverting gains the bias current cancellation is accom-

plished by placing a resistor RB on the non-inverting input

equal in value to the resistance seen by the inverting input

(See Figure 2). RB = RF || (RG + RS)

The additional noise contribution of RB can be minimized by

the use of a shunt capacitor (not shown).

POWER DISSIPATION

The LMH6609 has the ability to drive large currents into low

impedance loads. Some combinations of ambient tempera-

ture and device loading could result in device overheating.

For most conditions peak power values are not as important

as RMS powers. To determine the maximum allowable

power dissipation for the LMH6609 use the following for-

mula:

PMAX = (150o - TAMB)/Î¸JA

Where TAMB = Ambient temperature (ËC) and Î¸JA = Thermal

resistance, from junction to ambient, for a given package

(ËC/W). For the SOIC package Î¸JA is 148ËC/W, for the SOT

it is 250ËC/W. 150oC is the absolute maximum limit for the

internal temperature of the device.

Either forced air cooling or a heat sink can greatly increase

the power handling capability for the LMH6609.

VIDEO PERFORMANCE

The LMH6609 has been designed to provide good perfor-

mance with both PAL and NTSC composite video signals.

The LMH6609 is specified for PAL signals. NTSC perfor-

mance is typically marginally better due to the lower fre-

quency content of the signal. Performance degrades as the

loading is increased, therefore best performance will be

obtained with back-terminated loads. The back termination

reduces reflections from the transmission line and effectively

masks transmission line and other parasitic capacitances

from the amplifier output stage. This means that the device

should be configured for a gain of 2 in order to have a net

gain of 1 after the terminating resistor. (See Figure 6)

20079034

FIGURE 6. Typical Video Application

ESD PROTECTION

The LMH6609 is protected against electrostatic discharge

(ESD) on all pins. The LMH6609 will survive 2000V Human

Body model or 200V Machine model events.

Under closed loop operation the ESD diodes have no effect

on circuit performance. There are occasions, however, when

the ESD diodes may be evident. For instance, if the amplifier

is powered down and a large input signal is applied the ESD

diodes will conduct.

TRANSIMPEDANCE AMPLIFIER

The low input current noise and unity gain stability of the

LMH6609 make it an excellent choice for transimpedance

applications. Figure 7 illustrates a low noise transimpedance

amplifier that is commonly implemented with photo diodes.

RF sets the transimpedance gain. The photo diode current

multiplied by RF determines the output voltage.

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