EL4095CS datasheet(13/24 Pages) ELANTEC

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EL4095C

Video Gain ControlFaderMultiplexer

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 VG and VG

4095 – 28

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

through VG 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 55 K

to over 500 K

X 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 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

Force Inputs

The Force inputs completely override the VGAIN

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

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 bVIN terminals the EL4095

will not give the expected gain range when a

channel is left unconnected

This circuit gives a05 to a20 gain range and

is useful as a signal leveller where a constant

output level is regulated from a range of input

amplitudes

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