GT4124 View Datasheet(PDF) - Gennum -> Semtech
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GT4124 Datasheet PDF : 5 Pages
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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 (VCA)
or (VCB). The controlling voltage VCA is the sum of a nominal
0.5V source (VNOM) and a variable source VK while VCB is
made up of the sum of the nominal voltage VNOM and -VK.
VK and -VK are themselves proportional to the difference
between an externally applied reference voltage (VREF) and
an externally applied CONTROL voltage (VC). The voltages VK
and -VK are produced by a differential amplifier (AMP3) whose
gain is AK. This gain can be altered by two external resistors,
REXT and RSPAN according to the following formula:
0.85 • REXT
AK ≈ —————
RSPAN
[1kΩ < REXT < 3kΩ]
Note that REXT is connected between the REXT pin and ground
and RSPAN is connected between the pins S1 and S2.
Each of the voltages (+V and -V ) is applied to summing
K
K
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 = V then the gain of each signal channel of the
C REF
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.
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 CHOLD 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:
VO = VA •[VNOM + AK•(VC - VREF)] + VB•[VNOM - AK•(VC - VREF)]
(Unity gain configuration)
Where VA and VB are the input analog signals applied to +IN A
and +IN B respectively, and VC is the CONTROL voltage.
Note that VNOM ranges between 0.45V < VNOM < 0.55.
For normal video mixer operation, the control range (SPAN) is
usually 0 to 1V and will occur when AK=1, VREF= 0.5V and
VNOM=0.5 volts. A change in VC from 0 to 1V will then produce
an effect such that the output signal contains 100% of Channel
B when VC is 0V and 100% of Channel A when VC is 1 volt. For
the above conditions, the general unity gain transfer function
reduces to:
VO = 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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520 - 59 - 2
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