GB4571 Video Buffer with Precision Strobed DC Restore

Transcription

1 GB Video Buffer with Precision Strobed DC Restore DATA SHEET FEATURES DESCRIPTION accurate clamping to within ± mv less than mv clamping distortion variable clamping range ( V to -. V) ultra low differential gain and differential phase fast recovery from sudden changes in DC offsets less than ± 0. phase delay variation at 3. MHz ± 0. db flatness to 0 MHz ±. V to ±3. V supply voltage range Pb-free and Green PIN CONNECTIONS GB The GB is a high performance video buffer with DC restore used primarily for accurate back porch clamping. Key features include operation from ±. V to ±3. V supply voltages, fast recovery from sudden changes in DC offset, ±0. db flatness to 0 MHz and availability of industry standard DIP and SOIC packages. In the typical back porch clamping application, a negative going back porch pulse from an external sync separator (such as the GS or GS) is applied to the GB's input. While is low, the GB determines the correct DC level shift that should be applied to the input signal by integrating over the entire back porch period. This reduces sensitivity to noise and improves the accuracy of the DC restoration compared to systems using sample and hold techniques. Typical clamping accuracy of ± mv is achieved by the GB. APPLICATIONS 3 V OUT Clamping of NTSC, PAL, SECAM & RGB Video Signals Multimedia Graphics Overlay Production Switchers Linear Keyers GND SIMPLIFIED CIRCUIT DIAGRAM AVAILABLE PACKAGING VCC pin PDIP pin SOIC COUPLING CAP V OUT ORDERING INFORMATION Part Package Temperature Pb-free Number Range and Green NON-LNEAR V TO I CONVERTER HIGH Z BUFFER GB-CDA pin PDIP 0 to 0 C No GB-CKA pin SOIC 0 to 0 C No GB-CTA pin SOIC 0 to 0 C No Tape GND GB-CKAE3 pin SOIC 0 to 0 C Yes Revision date: July 00 Document No GENNUM CORPORATION P.O. Box 9, Stn A, Burlington, Ontario, Canada LR 3Y3 tel. (90) 3-99 fax: (90) 3-9 Gennum Japan: Shinjuku Green Tower Building F --, Nishi Shinjuku Shinjuku-ku, Tokyo Japan Tel: + (03) Fax: + (03) 339-0

2 ABSOLUTE MAXIMUM RATINGS PARAMETER VALUE/UNITS Supply Voltage ±3. V Operating Temperature Range 0 C T A 0 C Storage Temperature Range - C T S 0 C Lead Temperature (soldering, 0 seconds) 0 C Differential Video Input Voltage ± V Strobe Input Voltage -V S V +V S CAUTION ELECTROSTATIC SENSITIVE DEVICES DO NOT OPEN PACKAGES OR HANDLE EXCEPT AT A STATIC-FREE WORKSTATION ELECTRICAL CHARACTERISTICS V S = ± V, T A = 0 to 0 C, R L = 0kΩ, = 0 pf, unless otherwise shown. PARAMETEYMBOL CONDITIONS MIN TYP MAX UNITS POWER SUPPLIES Supply Voltage ± V S ±. ±.0 ±3. V + Supply Current I+ - 0 ma - Supply Current I- - 0 ma Small Signal B.W. B.W. ±0. db after insertion loss, MHz (flattened response) =00 mv p-p Phase Delay ø D at 3. MHz and.3 MHz deg Differential Gain δg at 3. MHz and.3 MHz % Differential Phase δp at 3. MHz and.3 MHz deg Input Voltage V Input Resistance R IN - - kω Input Capacitance C IN pf Output Resistance R OUT ƒ= 0 to MHz -. 0 Ω ƒ= 00 MHz Ω SIGNAL PATH Insertion Loss I.L. ƒ= 00 khz db Power Supply Rejection Ratio PSRR ƒ= khz db ƒ= 0 MHz db PP Signal / RMS Noise S/N V SIG = Vp-p - - db Strobe Pulse Width t PWL Active low.0. - µs Accuracy = 0.V to Vp-p, - ± ± mv pulses at H rate NOTE: Recovery Time Scan rate.khz & µs strobe pulses 0.V offset - 3 ms 0.V offset µs Clamping Distortion Distortion amplitude - - ± mv Distortion duration (amp. within 0. mv) µs Clamp Voltage Range VCLAMP = Vp-p V Input Logic High V IH. - - V Input Logic Low V IL V. V A or B = + Vp-p output taken from OUTPUT. V C = + Vp-p output taken from V A or V B of

3 DETAILED DESCRIPTION The GB is intended for video applications requiring precision DC restoration. The GB's signal path consists of a simple Darlington emitter follower for maximum bandwidth performance. With this configuration, the GB s small signal frequency response remains with ± 0. db of 0 db out to 0 MHz. Optimal frequency response for the GB occurs with load capacitance in the range of pf. For smaller loads, an external capacitor can be added to maintain the bandwidth of the device. As shown in Figure, a small resistor, R OUT, should be included in series with the GB output to obtain optimal response flatness. For a nominal load of pf, R OUT should be chosen to be approximately 30 Ω. The DC restoration function is achieved through the use of a strobed operational transconductance amplifier (OTA). The OTA receives its input 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 clamp voltage reference. When enabled by taking low, the OTA sources or sinks current depending on whether the output of the GB is below or above respectively. The output current from the OTA is integrated by the hold capacitor connected to pin,. develops a corrective voltage during the period when the clamp is enabled and holds the corrective voltage while the clamp is disabled. The resulting voltage is buffered and fed back to the GB input through a non-linear V to I converter. This feedback current transfers charge to the GB's input coupling capacitor which act as a DC reservoir for corrective level shifts. The non - linear V to I converter feeds back larger corrective currents to the GB's input for larger differences between and than it does for proportionally smaller differences between and voltages. The non-linear V to I converter thus provides for substantially faster recovery from large changes in input signal DC offset while maintaining the overall stability of the device during near-steady-state operation. A 00 Ω resistor in series with the input provides some phase advance to improve the stability of the DC restoration feedback loop. The ratio of the capacitors C IN : should be maintained at approximately 0,000: to ensure closed loop stability. Additional reductions in recovery time from changes in DC offset may be obtained by reducing the values of C IN and. In the application shown in Figure, the signal is provided by the back porch pulse of the GS sync separator. The GS generates back porch pulses which remain at horizontal rate throughout the vertical interval. Constant rate pulses eliminate the possibility of introducing a DC offset due to a change in duty cycle VIDEO k COLOR BURST FILTER 0. GB D DC RESTORE 3.0nF R OUT OAD CLAMPED VIDEO OUTPUT 0k GS SYNC SEPARATOR BACK PORCH PULSE All resistors in ohms, all capacitors in microfarads unless otherwise stated. Fig. GB Typical Application Circuit 3 of - - 0

4 +V -V +V NETWORK ANALYSER Sweep: -00MHz Level: 0dBM *0 3 GB 0.0 CLC0 0k 0. VIDEO OUTPUT TO NETWORK ANALYSER I/P +V +V All resistors in ohms, all capacitors in microfarads unless otherwise stated. * This input capacitor must be shorted when performing Differential Gain and Differential Phase tests. Fig. Frequency Response Test Circuit GB TYPICAL PERFORMANCE CURVES (V S = ± V, unless otherwise specified) GAIN (db) = ø = pf = pf = 0pF = 0pF = 0pF PHASE (deg) = ø = 0pF = 0pF = 0pF = 00pF = pf Fig. 3 Gain vs Frequency - 0 Fig. Phase vs Frequency GAIN (db) = 30Ω = pf = Ω = pf PHASE (deg) = 30Ω = pf Fig. Flattened Frequency Response - 0 Fig. Phase vs Frequency of

5 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 September 99 Gennum Corporation. All rights reserved. Printed in Canada. of - - 0