EL4094CS datasheet(8/12 Pages) ELANTEC

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EL4094C

Video Gain ControlFader

Applications Information

The EL4094 is a self-contained and calibrated

fader subsystem

When a given channel has

100% gain the circuit behaves as a current-feed-

back amplifier in unity-gain connection As such

video and transfer distortions are very low As

the gain of the input is reduced a 2-quadrant

multiplier is gradually introduced into the signal

path and distortions increase with reducing gain

The input impedance also changes with gain set-

ting from about 1 M

X at 100% gain down to

16 k

X at zero gain To maximize gain accuracy

and linearity the inputs should be driven from

source impedances of 500

X or less

Linearity

The EL4094 is designed to work linearly with

2V inputs but lowest distortion occurs at g1V

levels and below Errors are closer to those of a

good current-feedback amplifier above 90% gain

Low-frequency linearity is 01% or better for

gains 25% to 100% and inputs up to 1V NTSC

differential gain and phase errors are better than

03% and 03 for the 25% to 100% gain range

These distortions are not strongly affected by

supply voltage nor output loading at least down

to 150

X For settling to 01% however it is best

to not load the output heavily and to run the

EL4094 on the lowest practical supply voltages

so that thermal effects are minimized

Gain Control Inputs

The gain control inputs are differential and may

be biased at any voltage as long as VGAIN is less

than 25V below Va and 3V above Vb The dif-

ferential input impedance is 55 k

X and the com-

mon-mode impedance is more than 500 k

X With

zero differential voltage on the gain inputs both

signal inputs have a 50% gain factor Nominal

calibration sets the 100% gain of VINA input at

05V of gain control voltage and 0% at b05V

of gain control VINB’s gain is complementary to

that of VINA a05V of gain control sets 0% gain

at VINB and b05V gain control sets 100% VINB

gain The gain control does not have a complete-

ly abrupt transition at the 0% and 100% points

There is about 10 mV of ‘‘soft’’ transfer at the

gain endpoints

To obtain the most accurate

100% gain factor or best attenuation at 0% gain

it is necessary to overdrive the gain control input

by 30 mV or more This would set the gain con-

trol voltage range as b0565V to a0565V or

30 mV beyond the maximum guaranteed 0% to

100% range In fact the gain control inputs are

very complex Here is a representation of the ter-

minals

4094 – 12

Representation of Gain Control

Inputs VG and VG

For

gain

control

inputs

between

05V

(g90

mA) the diode bridge is a low impedance

and all of the current into Vg flows back out

throughVg When gain control inputs exceed

this amount the bridge becomes a high imped-

ance as some of the diodes shut off and the Vg

impedance rises sharply from the nominal 55K

to about 500K

X This is the condition of gain

control overdrive The actual circuit produces a

much sharper overdrive characteristic than does

the simple diode bridge of this representation

The gain input has a 20 MHz b3 dB bandwidth

and 17 ns risetime for inputs to g045V When

the gain control voltage exceeds the 0% or 100%

values a 70 ns overdrive recovery transient will

occur when it is brought back to linear range If

quicker gain overdrive response is required the

Force control inputs of the EL4095 can be used

Output Loading

The EL4094 does not work well with heavy ca-

pacitive loads Like all amplifier outputs the out-

put impedance becomes inductive over frequency

resonating with a capacitive load The effective

output inductance of the EL4094 is about 350

nH More than 50 pF will cause excessive fre-

quency response peaking and transient ringing

The problem can be solved by inserting a low-

value resistor in series with the load 22

X or

more If a series resistance cannot be used then

adding a 300

X or less load resistor to ground or a

‘‘snubber’’ network may help A snubber is a re-

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