GB4551 Video Buffer with Strobed DC Restore DATA SHEET FEATURES DESCRIPTION • accurate clamping to within ±1mV The GB4551 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 +2V above the clamp level in order to prevent damage to crosspoints or other circuits connected to the output. • ultra low differential gain (0.03% typ.) and differential phase (0.03o typ.) • wideband unity gain, can typically be flattened to ±0.1dB at 30MHz • can be configured as a gain stage with reduced bandwidth. • drives 100pF loads • convenient 8 pin SIP packaging The clamping action occurs when the STROBE (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. The GB4551 will operate from ± 9V to ± 12V power supplies and will directly interface with Gennum’s video crosspoint switches. The 8 pin SIP package is ideally suited for space restricted board layouts. PIN CONNECTIONS GB4551 APPLICATIONS • Input buffering and DC restoration to crosspoint switches 1 2 3 4 5 6 7 8 • Accurate back porch video signal clamping • Inter-circuit buffering and DC restoration. Pin No. GB4551 Function 1 +IN non-inverting input 2 VCC 3 4 5 6 7 8 STB GND CX VEE - IN OUT positive power supply strobe ground external capacitor negative power supply inverting input output (+2V OUTPUT CLAMP) CX +IN 10k 500 + 30k 12pF + 10k –IN - ORDERING INFORMATION STROBE Part Number Package Type GB4551-CSA 8 Pin SIP 0 to 70o C OUT 7mA NOM. Temperature Range VEE GND VCC All resistors in ohms, all capacitors in microfarads unless otherwise stated. SIMPLIFIED CIRCUIT DIAGRAM Document No. 520 - 52 - 2 GENNUM CORPORATION P.O. Box 489, Stn A, Burlington, Ontario, Canada L7R 3Y3 Japan Branch: B-201 Miyamae Village, 2-10-42 Miyamae, Suginami-ku Tokyo 168, Japan tel. (905) 632-2996 fax: (905) 632-2055 tel. (03) 3247-8838 fax: (03) 3247-8839 ABSOLUTE MAXIMUM RATINGS PARAMETER VALUE/UNITS ±13.5 V Supply Voltage 0° C ≤ TA ≤ 70° C Operating Temperature Range Storage Temperature Range -65° C ≤ TS ≤ 150° C Lead Temperature (Soldering, 10 sec) 260° C ±5 V Differential Video Input Voltage Strobe Input Voltage -V ≤ VSTROBE ≤ +V s s CAUTION CLASS 1 ESD SENSITIVITY V S = ± 10V, TA= 0°C to 70°C, R L = 10kΩ, CL = 10pF, unless otherwise shown ELECTRICAL CHARACTERISTICS POWER SUPPLIES PARAMETER SYMBOL Supply Voltage VS ±9 ± 10 ± 12 V + Supply Current I+ - 9 13.8 mA – Supply Current I- - 9 13.8 mA Frequency Response Full Power Bandwidth SIGNAL PATH FPBW Note: 1. Operating Range MIN TYP MAX UNITS at ± 0.1dB (NOTE 1, see Fig. 5) 25 - - MHz at -3dB VIN = 1V p-p 30 - - MHz Phase Delay at 3.58MHz (see Fig. 4) - -3.0 - deg Phase Variation at 3.58MHz - ± 0.1 ± 0.2 deg Differential Gain ∂g at 3.58MHz (VIN = 40 IRE) - 0.03 0.05 % Differential Phase ∂p at 3.58MHz (VIN = 40 IRE) - 0.03 0.05 degrees Max. Input Voltage VIN(MAX) AV =+1 Clipping occurs at: 1.8 2.0 2.2 V Input Resistance RIN 25 30 - kΩ Input Capacitance CIN AV =+1 - 2.0 - pF Output Resistance ROUT AV =+1, ƒ = 0 to1MHz - 8.6 - Ω AV =+1, ƒ = 100MHz - 53 - Ω VIN =+1V p-p - ±1 ±2 mV Clamp Accuracy STROBE CONDITIONS Strobe Pulse Width t P STROBE 500 1000 - ns Strobe Level VINHI 2.0 - - V VINLO - - 0.8 V The frequency peaking can be flattened by placing a small value series resistor at the GB4551 output. The value of the resistor will depend on the load capacitance and the amount of flattening required. Figure 5 shows the frequency response for three combinations of RS and CL. 520 - 52 - 2 2 A 500 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 ± 100 mV (± 14 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 20,000:1 to ensure closed loop stability. DETAILED DESCRIPTION The GB4551 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 75 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. The OTA receives its inputs from a differential low pass filter. This filter has a corner frequency of 600 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 GB4551 while the other input is connected to the ground reference. Optimal frequency response for the GB4551 occurs with load capacitances in the range of 80 pF. For smaller loads, an external capacitor can be added to extend the bandwidth of the device. As shown in Figure 7, a small resistor, ROUT, should be included in series with the GB4551 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 5. In the application shown in figure 1, the input to pin 3 of the GB4551, STROBE, is provided by the back porch output of the GS4881 sync separator. The GS4881 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 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 STROBE low, the OTA sources or sinks a current depending on whether the output of the GB4551 is below or above ground, respectively. The output current from the OTA is integrated by the hold capacitor connected to pin 5, CX. The resulting voltage is buffered and fed to the non-inverting input of the op-amp through a 30 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. The method of adjusting the DC level of the video signal used in the GB4551 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 ± 1 mV is achieved by the GB4551. TEST CIRCUITS -10V +10V 0.1 0.1 4 2 6 10 COMPOSITE VIDEO INPUT GB4551 1 8 NON-POLAR COLOR BURST FILTER 5 3 CLAMPED VIDEO OUTPUT 7 470p 0.1 +10V 2 GS4881 SYNC. SEP. 4 8 0.1 5 6 10V 0V All resistors in ohms, all capacitors in microfarads unless otherwise stated. 680k 0.1 BACK PORCH PULSE Fig.1 Clamping Operation 3 520 - 52 - 2 +10V -10V 0.1 +5V 0.1 0.1 4 NETWORK ANALYSER 2 10 6 RS GB4551 1 4 8 1 CLC110 8 5 Sweep: 1-100MHz Level: 0dBM 5 3 7 75 CL VIDEO OUTPUT TO NETWORK ANALYSER I/P 10k 470p 0.1 +5V +5V All resistors in ohms, all capacitors in microfarads unless otherwise stated. Fig. 2 Differential Gain and Phase, Frequency Response TYPICAL PERFORMANCE CURVES (Unless otherwise shown VS = ± 10V) 0.0 CL = 100pF 5 -0.5 4 -1.0 3 -1.5 2 -2.0 PHASE(o) GAIN (dB) 6 1 0 47pF 27pF -2 VIN = 100mVp-p -4.0 10pF -3 CL = 100pF -3.0 -3.5 -1 V IN = 100mVp-p CL = 10pF -2.5 RS = 0Ω -4.5 -5.0 -4 1 10 100 1 3 Fig. 3 Gain vs Frequency 10 Fig. 4 Phase vs Frequency 0.3 0.050 0.2 0.045 R S / CL V IN = 40IRE 33Ω/68pF 0.1 0.040 dg (%), dp (°) 0.0 -0.1 GAIN (dB) 5 FREQUENCY (MHz) FREQUENCY (MHz) -0.2 18Ω/270pF -0 .3 -0.4 0.035 0.030 0.025 dg 0.020 0.015 dp 27Ω/120pF -0.5 0.010 -0.6 0.005 -0.7 0.0000 1 10 1 1 50 5 10 10 FREQUENCY (MHz) FREQUENCY (MHz) Fig. 6 Differential Gain & Phase vs Frequency Fig. 5 Flattened Frequency Response 520 - 52 - 2 3 4 BORDER -10V +10V COMPOSITE VIDEO INPUT GT4123 INPUT FROM (GB4551) 0.1 0.1 7 5 3 COMPOSITE 4 VIDEO INPUT 2 OUT GB4551 CONTROL STROBE ROUT 8 GT4123 5 3 7 GS4881 SYNC. SEP. 4 0.1 VID B OUT VID A GB4551s 1 VIDEO OUTPUT 1 3 CONTROL INSERT 8 0.1 GT4123 5V 0V 680k 5 TO OTHER 5 6 7 STROBE 470p +5V 2 GB4551 VID A NON-POLAR COLOR BURST FILTER 0.1 1 6 10 1 VID B BACK PORCH PULSE BACKGROUND VIDEO IN (FROM GB4551) 7 5 3 KEY INPUT VID B VID A 1 VIDEO OUTPUT 2 CONTROL All resistors in ohms, all capacitors in microfarads unless otherwise stated. Fig. 7 GB4551 Driving Several GT4123 Video Mixers APPLICATION CIRCUIT The above circuit represents a possible use of the GB4551 as an input back-porch clamp for video mixers in a simple Production Switcher. For simplicity, the STROBE input is derived from the GS4881 sync separator IC, driven from the composite input signal. The value of ROUT is chosen to roll-off the frequency response of the GB4551. The value is usually between 15 and 47 ohms depending on the total load capacitance seen by the device. The GB4551 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 GT4123. 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 1992 Gennum Corporation. All rights reserved. Printed in Canada. 5 520 - 52 - 2