EL4094C
Video Gain Control/Fader
perature. The EL4094 thus cannot be operated
Applications Information Ð 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.
g
with 15V supplies at 75 C in the surface-mount
package; the supplies should be reduced to 5V
§
g
g
to 12V levels, especially if extra dissipation oc-
curs when driving a load.
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 EL4094 as a Level Adjust
A common use for gain controls is as an input
signal levellerÐa circuit that scales too-large or
too-small signals to a standard amplitude. A typi-
cal situation would be to scale a variable video
a
b
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.
input by 6 dB to 6 dB to obtain a standard
amplitude. The EL4094 cannot provide more
than 0 dB gain, but it can span the range of 0 dB
b
to 12 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
Noise
The EL4094 has a very simple noise characteris-
tic: the output noise is constant (40 nV/ Hz
S
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
b
and 12 dB extremes, and good performance be-
tween. The circuit is shown on the following
noise is 40 nV/
S
Hz/0.5, or 80 nV/
SHz.
page.
Bypassing
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 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
g
supply current, only 510 mW at 15V supplies,
and the circuit has no dissipation problems in
this package.
The SO-8 surface-mount package has a 153 /W
§
4094–14
Differential Gain and Phase of
Linearized Level Control
thermal resistance with the EL4094, and only
490 mW can be dissipated at 75 C ambient tem-
§
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
