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GB4571 Video Buffer with Precision Strobed DC Restore
DATA SHEET
FEATURES * accurate clamping to within 1 mV * less than 1 mV clamping distortion * variable clamping range (VEE + 3.0 V to VCC - 4.5 V) * ultra low differential gain and differential phase * fast recovery from sudden changes in DC offsets * less than 0.15 phase delay variation at 3.58 MHz * 0.1 dB flatness to 80 MHz * 4.5 V to 13.2 V supply voltage range
DESCRIPTION The GB4571 is a high performance video buffer with DC restore used primarily for accurate back porch clamping. Key features include operation from 4.5 V to 13.2 V supply voltages, fast recovery from sudden changes in DC offset, 0.1 dB flatness to 80 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 GS4881 or GS4882) is applied to the GB4571's STROBE input. While STROBE is low, the GB4571 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 1 mV is achieved by the GB4571. APPLICATIONS
PIN CONNECTIONS GB4571
C HOLD V IN V CLAMP STROBE 1 2 3 4 8 7 6 5 VCC VOUT VEE GND
* * * *
Clamping of NTSC, PAL, SECAM & RGB Video Signals Multimedia Graphics Overlay Production Switchers Linear Keyers
SIMPLIFIED CIRCUIT DIAGRAM AVAILABLE PACKAGING
VCC 8
8 pin PDIP 8 pin SOIC
VIN COUPLING CAP
2
7
VOUT
ORDERING INFORMATION
6 VEE
Part Number
Package
Temperature Range
GB4571-CDA GB4571-CKA GB4571-CTA
8 pin PDIP 8 pin SOIC 8 pin SOIC Tape
0 to 70C 0 to 70C
NON-LNEAR V TO I CONVERTER
HIGH Z BUFFER
+ 3 VCLAMP
4
STROBE
0 to 70C
5 GND 1 CHOLD
Document No. 521 - 17 - 01
GENNUM CORPORATION P.O. Box 489, Stn A, Burlington, Ontario, Canada L7R 3Y3 tel. (905) 632-2996 fax: (905) 632-2055
Japan Branch: A-302, Miyamae Village, 2-10-42, Miyamae, Suginami-ku, Tokyo 168, Japan tel. (03) 3247-8838 fax. (03) 3247-8839
ABSOLUTE MAXIMUM RATINGS
PARAMETER Supply Voltage Operating Temperature Range Storage Temperature Range Lead Temperature (soldering, 10 seconds) Differential Video Input Voltage Strobe Input Voltage VALUE/UNITS 13.5 V 0C TA 70C -65C T 150C S 260C 5 V -VS V STROBE +VS
V S = 5 V, TA = 0 to 70C, RL = 10k, CL = 10 pF, unless otherwise shown.
CAUTION
ELECTROSTATIC SENSITIVE DEVICES
DO NOT OPEN PACKAGES OR HANDLE EXCEPT AT A STATIC-FREE WORKSTATION
ELECTRICAL CHARACTERISTICS
PARAMETER Supply Voltage POWER SUPPLIES + Supply Current - Supply Current Small Signal B.W. (flattened response) Phase Delay Differential Gain Differential Phase Input Voltage Input Resistance Input Capacitance Output Resistance
SYMBOL
CONDITIONS
MIN 4.5 -
TYP 5.0 10 10 -
MAX 13.2 15 15 -
UNITS V mA mA MHz
VS I-
I+ 0.1 dB after insertion loss, VIN=100 mV p-p oD g p VIN RIN CIN ROUT I.L. PSRR = 0 to 1 MHz = 100 MHz at 3.58 MHz and 4.43 MHz at 3.58 MHz and 4.43 MHz at 3.58 MHz and 4.43 MHz
B.W.
80
-2.85 VEE+3 25 50 20 75 2.0 -
-3.0 0.01 0.01 2.0 8.6 20 0.03 2.5 1
-3.15 0.03 0.05 VCC-3 10 40 0.05 2
deg % deg V k pF dB dB dB dB s mV
Insertion Loss SIGNAL PATH Power Supply Rejection Ratio
= 100 kHz = 1 kHz = 10 MHz
PP Signal / RMS Noise Strobe Pulse Width Accuracy
S/N t PWL
VSIG =1 Vp-p Active low VIN = 0.5V to 2Vp-p, STROBE pulses at H rate Scan rate 15.7kHz & 2s strobe pulses 0.5V offset 0.1V offset
Recovery Time
VEE+3 1.8 -
1 190 -
3 600 1 3 VCC-4.5 0.8 20
ms s mV s V V V A
Clamping Distortion
Distortion amplitude Distortion duration (amp. within 0.1 mV)
VCLAMP
Clamp Voltage Range Input Logic High
VCLAMP VIH VIL I IL, IIH
VIN = 1 Vp-p
STROBE
Input Logic Low Strobe Input Current
NOTE:
1. V A or B = +1 Vp-p output taken from OUTPUT 2 . VC = +1 Vp-p output taken from V A or V B
521 - 17 - 00
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DETAILED DESCRIPTION The GB4571 is intended for video applications requiring precision DC restoration. The GB4571's signal path consists of a simple Darlington emitter follower for maximum bandwidth performance. With this configuration, the GB4571's small signal frequency response remains with 0.1 dB of 0 dB out to 80 MHz. Optimal frequency response for the GB4571 occurs with load capacitance in the range of 18 pF. For smaller loads, an external capacitor can be added to maintain the bandwidth of the device. As shown in Figure 1, a small resistor, ROUT, should be included in series with the GB4571 output to obtain optimal response flatness. For a nominal load of 18 pF, ROUT should be chosen to be approximately 30 . 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 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 GB4571, while the other input is connected to the clamp voltage reference. When enabled by taking STROBE low, the OTA sources or sinks current depending on whether the output of the GB4571 is below or above VCLAMP respectively. The output current from the OTA is integrated by the hold capacitor connected to pin 1, CHOLD. C HOLD 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 GB4571 input through a non-linear V to I converter. This feedback current transfers charge to the GB4571'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 GB4571's input for larger differences between VCLAMP and V IN than it does for proportionally smaller differences between VCLAMP and VIN 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 400 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 CIN :CHOLD should be maintained at approximately 20,000:1 to ensure closed loop stability. Additional reductions in recovery time from changes in DC offset may be obtained by reducing the values of CIN and CHOLD. In the application shown in Figure 1, the STROBE signal is provided by the back porch pulse of the GS4881 sync separator. The GS4881 generates back porch pulses which remain at horizontal rate throughout the vertical interval. Constant rate STROBE pulses eliminate the possibility of introducing a DC offset due to a change in duty cycle.
VCC 0.1
VEE 0.1
8 VIDEO COLOR BURST FILTER 22 VCLAMP 0.1
6 ROUT CLAMPED VIDEO CLOAD OUTPUT
GB4571 2 STROBED 7 DC RESTORE
3 CHOLD 1.0nF VCC 1 4 5
75k
8 GS4881 SYNC SEPARATOR 4 5 6
2
0.1
BACK PORCH PULSE 680k 0.1
All resistors in ohms, all capacitors in microfarads unless otherwise stated.
Fig. 1 GB4571 Typical Application Circuit
3
521 - 17 - 00
+5V 0.1
-5V
0.1 0.1
+5V
NETWORK ANALYSER Sweep: 1-100MHz Level: 0dBM 75
3 *10 2
8
6
5 5 7
GB4571
1 4
RS
4
1
CLC110
8
5 CL 0.01 +5V 10k
VIDEO OUTPUT TO NETWORK ANALYSER I/P
0.1
+5V
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. 2 Frequency Response Test Circuit
GB4571 TYPICAL PERFORMANCE CURVES (VS = 5 V, unless otherwise specified)
6.0
RS = o
4.0
CL = 68pF CL = 82pF CL = 120pF PHASE (deg)
1 0 -1 -2 -3 -4 -5 -6 -7
RS = o
GAIN (dB)
CL = 180pF
2.0
CL = 270pF CL = 180pF CL = 120pF C L = 100pF CL = 82pF
CL = 270pF
0.0
-2.0 1 10 100
-8
2
10
FREQUENCY (MHz)
FREQUENCY (MHz)
Fig. 3 Gain vs Frequency
Fig. 4 Phase vs Frequency
0.1
-0.5 -1 -1.5
0.05
0
PHASE (deg)
GAIN (dB)
RS = 30 CL = 15pF RS = 28 CL = 18pF
-2 -2.5 -3.0 -3.5 -4
RS = 30 CL = 15pF
-0.05
-0.1
-0.15
-0.2
1
10
100
2
10
FREQUENCY (MHz) FREQUENCY (MHz)
Fig. 6 Phase vs Frequency Fig. 5 Flattened Frequency Response
521 - 17 - 00
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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. (c) Copyright September 1994 Gennum Corporation. All rights reserved. Printed in Canada.
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521 - 17 - 00

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