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GS4882-CKAE3 Datasheet(PDF) 3 Page - Gennum Corporation |
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GS4882-CKAE3 Datasheet(HTML) 3 Page - Gennum Corporation |
3 / 7 page 3 of 7 521 - 61 - 01 CIRCUIT DESCRIPTION The Block Diagrams for the GS4882 and GS4982 are shown in Figure 6 and Figure 7, with timing diagrams for the devices shown in Figure 8. When presented with a composite video input signal, the GS4882 outputs composite sync, vertical sync, back porch and odd/even field information. The GS4982 substitutes the composite sync output with a horizontal sync output, for those applications requiring horizontal sync extraction. An external resistor, R SET , connected to pin 6 is used to set all timing currents in the device. For standard NTSC applications, R SET should be set to 227 k Ω. The value of R SET for a standard NTSC application is different for the GS4882/GS4982 line of sync separators(R SET = 227 k Ω) than it is for the GS1881/ GS4881/GS4981 line of sync separators (R SET = 680 k Ω). This change was made to improve jitter performance of the device. COMPOSITE VIDEO INPUT (pin 2) The GS4882 and GS4982 will operate with input signal amplitudes ranging from 0.5 V to 4.0 V p-p. Composite video is AC coupled into the device via an external coupling capacitor connected to pin 2. Immediately upon entering the GS4882/GS4982 the video signal is passed to the device’s dual mode input clamp in order to clamp the sync tip of the input video waveform to 1.55 Volts. The GS4882/GS4982’s dual mode input clamp, with both Hard Clamp and Soft Clamp capabilities, has been specifically designed for use in high performance sync sepa- ration. The dual mode input clamp aids in maintaining the accuracy of the internally derived 50% sync slicing level to within ±5% by utilizing the Soft Clamp during steady state operation. The device improves system start-up and impulse noise recovery time by utilizing the device’s Hard Clamp and Nosync Recovery Current during initial start-up and when steady state operation has been disturbed by impulse noise. During the clamping operation, the input video signal is passed through the device's internal color burst filter. The internal filter attenuates the color burst by typically >15 dB. Figure 1 shows the typical frequency response of the internal color burst filter. The 50% point of sync is determined by using two identical resistors to divide the voltage between sync tip and back porch. The importance of precision sync tip clamping may be appreciated here, since the sync tip voltage is used in deriving the 50% slicing level. The back porch voltage is derived through an internal integrate and hold circuit that is gated by the Back Porch output signal. By integrating over the entire back porch period, the accuracy and noise immunity of acquired back porch voltage is greatly improved when compared to systems using simple sample and hold techniques. The output of the comparator is a reproduction of the input video signal with the active portion of video removed. This represents the composite sync waveform presented on pin 1 of the GS4882. The video path and composite sync slicing circuitry have been optimized and compensated to achieve superior temperature stability. Variations in composite sync output timing over the commercial temperature range are less than ±5 ns. COMPOSITE SYNC OUTPUT (pin 1 GS4882) The filtered video signal is then fed to a comparator which compares it to an internally derived voltage corresponding to the 50% point of the sync pulse amplitude. By slicing the composite video waveform at 50% of the sync pulse amplitude, variations in output pulse timing due to variations in input signal amplitude are minimized. Figure 1 demonstrates the stability of output pulse timing achieved with 50% sync slicing . 0 -5 -15 -25 -35 0.01 0.1 1 10 FREQUENCY (MHz) Fig. 1 Frequency Response of Internal Color Burst Filter A A/2 50% Slice 50% Slice t o Time Sync Pulse No.1 Sync Pulse No.2 With 50% Sync Slicing no Time Base Errors (TBEs) are introduced due to variations in sync pulse amplitude. Output pulses are always produced at t O+tD , where t D represents the delay through the device. Fig. 2 Stability of Output Pulse Timing with Variations in Sync Pulse Amplitude |
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