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GT4122 Datasheet(PDF) 3 Page - Gennum Corporation

Part # GT4122
Description  GT4122 Video Multiplier
Download  5 Pages
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Manufacturer  GENNUM [Gennum Corporation]
Direct Link  http://www.gennum.com
Logo GENNUM - Gennum Corporation

GT4122 Datasheet(HTML) 3 Page - Gennum Corporation

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Referring to the block diagram, the input signals are applied
to conventional differential amplifiers (AMP1 and AMP2).
Each amplifier has provisions for individually adjusting the
DC offset (OFFSET).
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 VCA is the sum of a nominal
0.5V source (V
NOM) and a variable source VK while VCB is made
up of the sum of the nominal voltage V
NOM and -VK.
V
K and -VK are themselves proportional to the difference
between an externally applied reference voltage (V
REF) and
an externally applied CONTROL voltage (V
C). The voltages VK
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 RSPAN
according to the following formula:
Note that R
EXT is connected between the REXT pin and ground
and R
SPAN is connected between the pins S1 and S2.
Each of the voltages (+V
K and -VK) 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 = VREF, 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) and presents the mixed signals to
the outside world.
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 -
V
REF)]
(Unity gain configuration)
Where V
A and VB 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.
DETAILED DESCRIPTION
0.85 • R
EXT
A
K
—————
[1k
Ω < R
EXT
< 3k
Ω]
R
SPAN
REXT
COS1
VREF
COS2
VC
+
-
- VS
GND
A OS2
+IN A
- IN A
B OS2
- IN B
VCA=0.5 + VK
+IN B
+VS
B OS1
A OS1
BIAS
AMP 3
S1
S 2
COMP
OUT
VNOM
AMP 1
AMP 2
+
-
+
-
XA
VCB=0.5 - VK
XB
+
+
AMP 4
-
+
AK
0.5V
Σ2
Σ3
+
-
+
-
+
+ VK
- VK
+
+
Σ1
VNOM
Device Function: VOUT = VINA • [VNOM + AK (VC - VREF)] + VINB • [VNOM - AK (VC - VREF)]
The GT4122 is a broadcast quality monolithic inte-
grated circuit specifically designed to linearly mix two
video signals under the control of a third signal.
FUNCTIONAL BLOCK DIAGRAM


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