EL4095C
Video Gain Control Fader Multiplexer
Applications Information
Contd
In fact the gain control internal circuitry is very
complex Here is a representation of the termi-
nals
Representation of Gain Control
Inputs V
G
and V
G
Force Inputs
The Force inputs completely override the V
GAIN
setting and establish maximum attainable 0%
and 100% gains for the two input channels They
are activated by a TTL logic low on either of the
FORCE pins and perform the analog switching
very quickly and cleanly FORCEA causes 100%
gain on the A channel and 0% on the B channel
FORCEB does the reverse but there is no de-
fined output state when FORCEA and FORCEB
are simultaneously asserted
The Force inputs do not incur recovery time pen-
alties and make ideal multiplexing controls A
typical use would be text overlay where the A
channel is a video input and the B channel is
digitally created text data The FORCEA input
is set low normally to pass the video signal but
released to display overlay data The gain control
can be used to set the intensity of the digital
overlay
4095 – 28
For gain control inputs between
g
0 5V
(
g
90
mA)
the diode bridge is a low impedance
and all of the current into V
G
flows back out
through V
G
When gain control inputs exceed
this amount the bridge becomes a high imped-
ance as some of the diodes shut off and the V
G
impedance rises sharply from the nominal 5 5 KX
to over 500 KX This is the condition of gain con-
trol overdrive The actual circuit produces a
much sharper overdrive characteristics than does
the simple diode bridge of this representation
The gain input has a 20 MHz
b
3 dB bandwidth
and 17 ns risetime for inputs to
g
0 45V 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
Other Applications Circuits
The EL4095 can also be used as a variable-gain
single input amplifier If a 0% lower gain ex-
treme is required one channel’s input should
simply be grounded Feedback resistors must be
connected to both
b
V
IN
terminals the EL4095
will not give the expected gain range when a
channel is left unconnected
This circuit gives
a
0 5 to
a
2 0 gain range and
is useful as a signal leveller where a constant
output level is regulated from a range of input
amplitudes
13
ELANTEC [ ELANTEC SEMICONDUCTOR ]
