GB4551 Video Buffer with Strobed DC Restore

Size: px

Start display at page:

Download "GB4551 Video Buffer with Strobed DC Restore"

Lillian Dixon
5 years ago
Views:

1 GB Video Buffer with Strobed DC Restore DATA SHEET FEATURES accurate clamping to within ±mv ultra low differential gain (0.0% typ.) and differential phase (0.0 o typ.) wideband unity gain, can typically be flattened to ±db at 0MHz can be configured as a gain stage with reduced bandwidth. drives 00pF loads convenient pin SIP packaging DESCRIPTION The GB is a high performance video buffer with DC restore primarily used for accurate back porch clamping. The circuit features a wideband differential amplifier that can be configured as a gain stage. The output signal voltage is limited to +V above the clamp level in order to prevent damage to crosspoints or other circuits connected to the output. The clamping action occurs when the (STB) input is taken to ground. A typical application would be for DC restoration during the back porch period. In this case the negative-going back porch pulse would be derived from an external sync separator circuit. PIN CONNECTIONS GB The GB will operate from ± 9V to ±V power supplies and will directly interface with Gennum s video crosspoint switches. The pin SIP package is ideally suited for space restricted board layouts. APPLICATIONS Pin No. GB Function +IN non-inverting input V CC positive power supply STB strobe GND ground C X external capacitor V EE negative power supply - IN inverting input output ORDERING INFORMATION Input buffering and DC restoration to crosspoint switches Accurate back porch video signal clamping Inter-circuit buffering and DC restoration. C (+V PUT CLAMP) X +IN 0k k pf - + 0k IN - Part Number Package Type Temperature Range GB-CSA Pin SIP 0 to 0 o C V EE ma NOM. GND V CC SIMPLIFIED CIRCUIT DIAGRAM Document No GENNUM CORPORATION P.O. Box 9, Stn A, Burlington, Ontario, Canada LR Y tel. (90) -99 fax: (90) -0 Japan Branch: B-0 Miyamae Village, -0- Miyamae, Suginami-ku Tokyo, Japan tel. (0) - fax: (0) -9

2 ABSOLUTE MAXIMUM RATINGS PARAMETER Supply Voltage Operating Temperature Range Storage Temperature Range Lead Temperature (Soldering, 0 sec) Differential Video Input Voltage Strobe Input Voltage VALUE/UNITS ±. V 0 C T A 0 C - C T S 0 C 0 C ± V -V s V +V s CAUTION CLASS ESD SENSITIVITY ELECTRICAL CHARACTERISTICS V S = ± 0V, T A = 0 C to 0 C, R L = 0kΩ, C L = 0pF, unless otherwise shown PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS POWER SUPPLIES Supply Voltage V S Operating Range ±9 ±0 ± V + Supply Current I ma Supply Current I ma Frequency Response at ±db (NOTE, see Fig. ) - - MHz Full Power Bandwidth FPBW at -db V IN = V p-p MHz Phase Delay at.mhz (see Fig. ) deg Phase Variation at.mhz - ± ±0. deg Differential Gain g at.mhz (V IN = 0 IRE) % Differential Phase p at.mhz (V IN = 0 IRE) degrees SIGNAL PATH Max. Input Voltage V IN(MAX) A V =+ Clipping occurs at:..0. V Input Resistance R IN 0 - kω Input Capacitance C IN A V = pf Output Resistance R A V =+, ƒ = 0 tomhz -. - Ω A V =+, ƒ = 00MHz - - Ω Clamp Accuracy V IN =+V p-p - ± ± mv Strobe Pulse Width t P ns Strobe Level V INHI V V INLO V Note:. The frequency peaking can be flattened by placing a small value series resistor at the GB output. The value of the resistor will depend on the load capacitance and the amount of flattening required. Figure shows the frequency response for three combinations of R S and C L

3 DETAILED DESCRIPTION The GB is intended for video applications requiring precision DC restoration coupled with very flat frequency response. The signal path features a wide band operational amplifier designed to be unity gain stable. While this amplifier is not intended to drive Ω transmission lines, it is ideal for applications where high capacitive loads, up to several hundred picofarads, must be driven. These applications include input and internal system buffering and DC restoration of video signals. Optimal frequency response for the GB occurs with load capacitances in the range of 0 pf. For smaller loads, an external capacitor can be added to extend the bandwidth of the device. As shown in Figure, a small resistor, R, should be included in series with the GB output to obtain optimal response flatness. The value of this resistor should be chosen to match the value of the load capacitance, as shown in Figure. The DC restoration function is achieved through the use of a strobed operational transconductance amplifier (OTA), as shown in the Simplified Block Diagram. When enabled by taking low, the OTA sources or sinks a current depending on whether the output of the GB is below or above ground, respectively. The output current from the OTA is integrated by the hold capacitor connected to pin, CX. The resulting voltage is buffered and fed to the non-inverting input of the op-amp through a 0 kω resistor. The non-inverting input also receives the input video signal through a large capacitor which acts as the DC reservoir for the corrective level shift. A 00 Ω resistor in series with the input provides some phase advance to improve the stability of the DC restoration feedback loop, and provides fast recovery for offset steps as large as ± 00 mv (± IRE). The values of the input and hold capacitors can be reduced to speed up the offset recovery time. The ratio of the capacitors should be maintained at approximately 0,000: to ensure closed loop stability. The OTA receives its inputs from a differential low pass filter. This filter has a corner frequency of 00 khz which attenuates the color burst and any high frequency noise that may be present in the signal. One input to the low pass filter senses the output of the GB while the other input is connected to the ground reference. In the application shown in figure, the input to pin of the GB,, is provided by the back porch output of the GS sync separator. The GS generates back porch pulses which remain at horizontal rate throughout the vertical interval. Constant rate sample pulses eliminate the possibility of introducing additional DC offset due to a change in duty cycle. The method of adjusting the DC level of the video signal used in the GB integrates over the entire back porch period. This reduces sensitivity to noise and improves the accuracy of the DC restoration compared with systems using sample and hold techniques. Typical clamping accuracy of ± mv is achieved by the GB. TEST CIRCUITS +0V -0V COMPOSITE COLOR BURST FILTER 0 NON-POLAR GB 0p CLAMPED VIDEO PUT 0k GS SYNC. SEP. +0V 0V 0V BACK PORCH PULSE Fig. Clamping Operation 0 - -

4 +0V -0V +V NETWORK ANALYSER Sweep: -00MHz Level: 0dBM 0 GB 0p C L R S CLC0 0k VIDEO PUT TO NETWORK ANALYSER I/P +V +V Fig. Differential Gain and Phase, Frequency Response TYPICAL PERFORMANCE CURVES (Unless otherwise shown V S = ±0V) C L = 00pF GAIN (db) VIN = 00mVp-p pf pf 0pF PHASE( o ) C L = 00pF V IN = 00mVp-p R S = 0Ω CL = 0pF Fig. Gain vs Frequency Fig. Phase vs Frequency RS / CL Ω/pF VIN = 0IRE GAIN (db) Ω/0pF Ω/0pF dg (%), dp ( ) dg dp Fig. Flattened Frequency Response Fig. Differential Gain & Phase vs Frequency 0 - -

5 COMPOSITE COLOR BURST FILTER COMPOSITE 0k 0 NON-POLAR GS SYNC. SEP. 0p +V +0V -0V GB V 0V BACK PORCH PULSE R TO OTHER GBs INSERT BORDER INPUT FROM (GB) BACKGROUND VIDEO IN (FROM GB) KEY INPUT GT CONTROL CONTROL CONTROL GT Fig. GB Driving Several GT Video Mixers GT GB VIDEO PUT VIDEO PUT APPLICATION CIRCUIT The above circuit represents a possible use of the GB as an input back-porch clamp for video mixers in a simple Production Switcher. For simplicity, the input is derived from the GS sync separator IC, driven from the composite input signal. The value of R is chosen to roll-off the frequency response of the GB. The value is usually between and ohms depending on the total load capacitance seen by the device. The GB may be used anywhere in the circuit where accurate DC restoration is required as shown by the second IC following the output of the top GT. DOCUMENT IDENTIFICATION PRODUCT PROPOSAL This data has been compiled for market investigation purposes only, and does not constitute an offer for sale. ADVANCE INFORMATION NOTE This product is in development phase and specifications are subject to change without notice. Gennum reserves the right to remove the product at any time. Listing the product does not constitute an offer for sale. PRELIMINARY DATA SHEET The product is in a preproduction phase and specifications are subject to change without notice. DATA SHEET The product is in production. Gennum reserves the right to make changes at any time to improve reliability, function or design, in order to provide the best product possible. Gennum Corporation assumes no responsibility for the use of any circuits described herein and makes no representations that they are free from patent infringement. Copyright June 99 Gennum Corporation. All rights reserved. Printed in Canada

GB4550 & GB4550A Monolithic Video Buffer/Clamp

GB4550 & GB4550A Monolithic Video Buffer/Clamp GB55 & GB55A Monolithic Video Buffer/Clamp DATA SHEET FEATURES adjustable clamp level from 5.5 V to 5.5 V at ±1 V supplies. ultra low differential gain (.2% typ.) and differential phase (.3 typ.) wideband