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GT4124 Datasheet(PDF) 3 Page - Gennum Corporation

Part # GT4124
Description  GT4124 Video Multiplier with Strobed DC Restore
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Manufacturer  GENNUM [Gennum Corporation]
Direct Link  http://www.gennum.com
Logo GENNUM - Gennum Corporation

GT4124 Datasheet(HTML) 3 Page - Gennum Corporation

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DETAILED DESCRIPTION
The GT4124 is a broadcast quality monolithic integrated circuit
specifically designed to linearly mix two video signals under
the control of a third channel.
Referring to the Functional Block Diagram, the input signals
are applied to conventional differential amplifiers (AMP1 and
AMP2) whose offsets are trimmed by on-chip resistors.
Following each input amplifier, the signals are applied to
linear multiplier circuits (XA and XB) whose outputs are the
product of the incoming signals and controlling voltages (V
CA
)
or (V
CB
). The controlling voltage V
CA
is the sum of a nominal
0.5V source (V
NOM
) and a variable source V
K
while V
CB
is
made up of the sum of the nominal voltage V
NOM
and -V
K
.
V
K
and -V
K
are themselves proportional to the difference
between an externally applied reference voltage (V
REF
) and
an externally applied CONTROL voltage (V
C
). The voltages V
K
and -V
K
are produced by a differential amplifier (AMP3) whose
gain is A
K
. This gain can be altered by two external resistors,
R
EXT
and R
SPAN
according to the following formula:
Note that R
EXT
is connected between the R
EXT
pin and ground
and R
SPAN
is connected between the pins S1 and S2.
Each of the voltages (+V
K
and -V
K
) is applied to summing
circuits (
Σ2 and Σ3) whose second inputs are DC voltage
sources that can also be slightly varied. The nominal value of
these voltage sources is 0.5 volts. When they are exactly 0.5V
and when V
C
= V
REF
then the gain of each signal channel of the
mixer is 0.5 (50%).
By connecting the ends of an external potentiometer
(CONTROL OFFSET) between the offset pins COS1 and COS2,
the voltage sources can be altered differentially. If a second
potentiometer (50% GAIN) is connected between the wiper of
the CONTROL OFFSET potentiometer and the supply voltage,
the voltage sources can be varied in a common mode fashion.
In this way not only can the control range of the mixer be
varied but also the point at which 50% of each input signal
appears at the output.
The outputs from the multiplier circuits (XA and XB) are then
applied to a summing circuit (
Σ1) whose output feeds a
wideband amplifier (AMP4) via a second summing circuit (
Σ4)
and presents the mixed signals to the outside world.
0.85 • R
EXT
A
K
—————
[1k
Ω < R
EXT
< 3k
Ω]
R
SPAN
The GT4124 includes the strobed clamp block. This circuit
samples the output signal when CLAMP SIG. is connected to
the OUTPUT, and compares it to a CLAMP REFERENCE
voltage which normally is set to 0V.
During the strobe period, which is usually the back porch
period of the video signal, DC feedback is applied to the
summing circuit
Σ4 such that the DC offset is held to within one
or two millivolts of the clamp REFERENCE.
A holding capacitor C
HOLD
is used to assure effective clamp
operation and filter residual noise.
Although there are two separate differential inputs, the usual
operational amplifier gain-setting methods can be applied to
determine the closed loop gain of the mixer. Usually the mixer
will be configured for unity gain by connecting both inverting
inputs (-IN A , -IN B) to the common output (OUT). In this case,
the general transfer function is:
V
O = VA •[VNOM + AK•(VC - VREF)] + VB•[VNOM - AK•(VC - VREF)]
(Unity gain configuration)
Where V
A
and V
B
are the input analog signals applied to +IN A
and +IN B respectively, and V
C
is the CONTROL voltage.
Note that V
NOM
ranges between 0.45V < V
NOM
< 0.55.
For normal video mixer operation, the control range (SPAN) is
usually 0 to 1V and will occur when A
K
=1, V
REF
= 0.5V and
V
NOM
=0.5 volts. A change in V
C
from 0 to 1V will then produce
an effect such that the output signal contains 100% of Channel
B when V
C
is 0V and 100% of Channel A when V
C
is 1 volt. For
the above conditions, the general unity gain transfer function
reduces to:
V
O = VA•VC + VB•(1-VC)
Since the operation of the mixer is limited to two quadrants, no
signal inversions occur if the control voltage exceeds the
range zero to one volt in either direction.
The topology is designed so that once the control voltage
reaches either end of its range, the channel which is ON
remains fully ON and the OFF channel remains fully OFF.


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