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GT4124 Datasheet(PDF) 3 Page - Gennum Corporation |
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GT4124 Datasheet(HTML) 3 Page - Gennum Corporation |
3 / 5 page ![]() 3 520 - 59 - 2 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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