11

Choice of Feedback Resistor and Gain Bandwidth

Product

For applications that require a gain of +1, no feedback

resistor is required. Just short the output pin to the inverting

input pin. For gains greater than +1, the feedback resistor

forms a pole with the parasitic capacitance at the inverting

input. As this pole becomes smaller, the amplifier’s phase

margin is reduced. This causes ringing in the time domain

some maximum value that should not be exceeded for

help to reduce the ringing and peaking at the expense of

reducing the bandwidth.

As far as the output stage of the amplifier is concerned, the

combination gets smaller, the bandwidth falls off.

is optimum. For the gains other than +1, optimum response

The EL8202, EL8203 and EL8403 have a gain bandwidth

product of 200MHz. For gains

≥5, its bandwidth can be

predicted by the following equation:

Video Performance

For good video performance, an amplifier is required to

maintain the same output impedance and the same

frequency response as DC levels are changed at the output.

This is especially difficult when driving a standard video load

of 150

Ω, because the change in output current with DC level.

Special circuitry has been incorporated in the EL8202,

EL8203 and EL8403 to reduce the variation of the output

impedance with the current output. This results in dG and dP

specifications of 0.01% and 0.01

°, while driving 150Ω at a

gain of 2. Driving high impedance loads would give a similar

or better dG and dP performance.

Driving Capacitive Loads and Cables

The EL8202, EL8203 and EL8403 can drive 5pF loads in

parallel with 1k

Ω with less than 5dB of peaking at gain of +1.

If less peaking is desired in applications, a small series

resistor (usually between 5

Ω to 50Ω) can be placed in series

with the output to eliminate most peaking. However, this will

reduce the gain slightly. If the gain setting is greater than 1,

gain loss which may be created by the additional series

resistor at the output.

When used as a cable driver, double termination is always

recommended for reflection-free performance. For those

applications, a back-termination series resistor at the

amplifier’s output will isolate the amplifier from the cable and

allow extensive capacitive drive. However, other applications

may have high capacitive loads without a back-termination

resistor. Again, a small series resistor at the output can help

to reduce peaking.

Disable/Power-Down

The EL8202 can be disabled and placed its output in a high

impedance state. The turn off time is about 25ns and the turn

on time is about 200ns. When disabled, the amplifier’s

supply current is reduced to 40µA typically, thereby

effectively eliminating the power consumption. The

amplifier’s power down can be controlled by standard TTL or

CMOS signal levels at the ENABLE pin. The applied logic

the amplifier. The amplifier will be disabled when the signal

Output Drive Capability

The EL8202, EL8203 and EL8403 do not have internal short

circuit protection circuitry. They have a typical short circuit

current of 80mA sourcing and 150mA sinking for the output

is connected to half way between the rails with a 10

Ω

resistor. If the output is shorted indefinitely, the power

dissipation could easily increase such that the part will be

destroyed. Maximum reliability is maintained if the output

current never exceeds ±40mA. This limit is set by the design

of the internal metal interconnections.

Power Dissipation

With the high output drive capability of the EL8202, EL8203

and EL8403. It is possible to exceed the 125

°C absolute

maximum junction temperature under certain load current

conditions. Therefore, it is important to calculate the

maximum junction temperature for the application to

determine if the load conditions or package types need to be

modified for the amplifier to remain in the safe operating

area.

The maximum power dissipation allowed in a package is

determined according to:

Where:

The maximum power dissipation actually produced by an IC

is the total quiescent supply current times the total power

supply voltage, plus the power in the IC due to the load, or:

Gain BW

×

200MHz

=

–

θ

JA

---------------------------------------------

=

EL8202, EL8203, EL8403