EL4094CN ELANTEC [Elantec Semiconductor], EL4094CN Datasheet - Page 8

• Manufacturer Part Number: EL4094CN
• Description: Video Gain Control/Fader
• Manufacturer: ELANTEC [Elantec Semiconductor]

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
16 k
and linearity the inputs should be driven from
source impedances of 500
Linearity
The EL4094 is designed to work linearly with
levels and below Errors are closer to those of a
good current-feedback amplifier above 90% gain
Low-frequency linearity is 0 1% or better for
gains 25% to 100% and inputs up to 1V NTSC
differential gain and phase errors are better than
0 3% and 0 3 for the 25% to 100% gain range
These distortions are not strongly affected by
supply voltage nor output loading at least down
to 150
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 V
than 2 5V below V
ferential input impedance is 5 5 k
mon-mode impedance is more than 500 k
zero differential voltage on the gain inputs both
signal inputs have a 50% gain factor Nominal
calibration sets the 100% gain of V
of gain control V
that of V
at V
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
g
a
EL4094C
Video Gain Control Fader
2V inputs but lowest distortion occurs at
0 5V of gain control voltage and 0% at
INB
at zero gain To maximize gain accuracy
and
INA
For settling to 0 1% however it is best
b
a
0 5V gain control sets 100% V
0 5V of gain control sets 0% gain
INB
a
’s gain is complementary to
and 3V above V
at 100% gain down to
or less
and the com-
INA
GAIN
b
input at
The dif-
b
is less
With
g
0 5V
INB
1V
8
it is necessary to overdrive the gain control input
by 30 mV or more This would set the gain con-
trol voltage range as
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
For
(
and all of the current into Vg flows back out
through V
this amount the bridge becomes a high imped-
ance as some of the diodes shut off and the V
impedance rises sharply from the nominal 5 5K
to about 500K
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
and 17 ns risetime for inputs to
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
more If a series resistance cannot be used then
adding a 300
‘‘snubber’’ network may help A snubber is a re-
g
90
gain
A) the diode bridge is a low impedance
Representation of Gain Control
g
control
When gain control inputs exceed
Inputs V
or less load resistor to ground or a
This is the condition of gain
inputs
b
G
0 565V to
and V
b
between
G
3 dB bandwidth
g
0 45V When
a
0 565V or
g
4094 – 12
0 5V
or
g