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MDP0027

300 MHz DC-Restored Video Amplifier

Elantec -> Intersil 

EL4094C

Video Gain Control Fader

Applications Information Contd

sistor in series with a capacitor 150X and 100 pF

being typical values The advantage of a snubber

is that it does not draw DC load current

Unterminated coaxial line loads can also cause

resonances and they should be terminated either

at the far end or a series back-match resistor in-

stalled between the EL4094 and the cable

The output stage can deliver up to 140 mA into a

short-circuit load but it is only rated for a con-

tinuous 35 mA More continuous current can

cause reliability problems with the on-chip metal

interconnect Video levels and loads cause no

problems at all

Noise

The EL4094 has a very simple noise characteris-

tic the output noise is constant (40 nV SHz

wideband) for all gain settings The input-re-

ferred noise is then the output noise divided by

the gain For instance at a gain of 50% the input

noise is 40 nV SHz 0 5 or 80 nV SHz

Bypassing

The EL4094 is fairly tolerant of power-supply

bypassing but best multiplier performance is ob-

tained with closely connected 0 1 mF ceramic ca-

pacitors The leaded chip capacitors are good but

neither additional tantalums nor chip compo-

nents are necessary The signal inputs can oscil-

late locally when connected to long lines or un-

terminated cables

Power Dissipation

Peak die temperature must not exceed 150 C At

this temperature the epoxy begins to soften and

becomes unstable chemically and mechanically

This allows 75 C internal temperature rise for a

75 C ambient The EL4094 in the 8-pin PDIP

package has a thermal resistance of 87 W and

can thus dissipate 862 mW at a 75 C ambient

temperature The device draws 17 mA maximum

supply current only 510 mW at g15V supplies

and the circuit has no dissipation problems in

this package

perature The EL4094 thus cannot be operated

with g15V supplies at 75 C in the surface-mount

package the supplies should be reduced to g5V

to g12V levels especially if extra dissipation oc-

curs when driving a load

The EL4094 as a Level Adjust

A common use for gain controls is as an input

signal leveller a circuit that scales too-large or

too-small signals to a standard amplitude A typi-

cal situation would be to scale a variable video

input by a6 dB to b6 dB to obtain a standard

amplitude The EL4094 cannot provide more

than 0 dB gain but it can span the range of 0 dB

to b12 dB with another amplifier gaining the

output up by 6 dB The simplest way to obtain

the range is to simply ground the B input and

vary the gain of the signal applied to the A input

The disadvantage of this approach is that lineari-

ty degrades at low gains By connecting the sig-

nal to the A input of the EL4094 and the signal

attenuated by 12 dB to the B input the gain con-

trol offers the highest linearity possible at 0 dB

and b12 dB extremes and good performance be-

tween The circuit is shown on the following

page

The EL4095 can be used to provide the required

gains without the extra amplifier In practice the

gain control is adjusted to set a standard video

level regardless of the input level The EL4583

sync-separator has a recovered amplitude output

that can be used to servo the gain control volt-

age Here is the curve of differential gain and

phase distortion for varying inputs with the out-

put set to standard video level

The SO-8 surface-mount package has a 153 W

thermal resistance with the EL4094 and only

490 mW can be dissipated at 75 C ambient tem-

9

Differential Gain and Phase of

Linearized Level Control

4094 – 14

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