View GB4551_300966.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

GB4551 Video Buffer with Strobed DC Restore
DATA SHEET
FEATURES * accurate clamping to within 1mV * 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
DESCRIPTION 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. 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. 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 CONNECTIONS
Pin No. 1 2 3 4 5 6 7 8
GB4551 +IN VCC STB GND CX VEE - IN OUT
Function non-inverting input positive power supply strobe ground external capacitor negative power supply inverting input output
CX
(+2V OUTPUT CLAMP)
+IN
10k 500
30k
+ 12pF
+ 10k -IN
-
ORDERING INFORMATION
STROBE 7mA NOM. VEE VCC
All resistors in ohms, all capacitors in microfarads unless otherwise stated.
OUT
Part Number GB4551-CSA
Package Type 8 Pin SIP
Temperature Range 0 to 70o C
GND
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 Supply Voltage Operating Temperature Range Storage Temperature Range Lead Temperature (Soldering, 10 sec) Differential Video Input Voltage Strobe Input Voltage VALUE/UNITS 13.5 V 0 C TA 70 C -65 C TS 150 C 260 C 5 V -V VSTROBE +V s s
CAUTION CLASS 1 ESD SENSITIVITY
ELECTRICAL CHARACTERISTICS
PARAMETER POWER SUPPLIES Supply Voltage + Supply Current - Supply Current Frequency Response Full Power Bandwidth Phase Delay Phase Variation Differential Gain Differential Phase SIGNAL PATH Max. Input Voltage Input Resistance Input Capacitance Output Resistance g p
V S = 10V, TA= 0C to 70C, R L = 10k, CL = 10pF, unless otherwise shown
SYMBOL VS I+ I-
CONDITIONS Operating Range
MIN 9 -
TYP 10 9 9 -3.0 0.1 0.03 0.03 2.0 30 2.0 8.6 53 1 1000 -
MAX 12 13.8 13.8 0.2 0.05 0.05 2.2 2 0.8
UNITS V mA mA MHz MHz deg deg % degrees V k pF mV ns V V
at 0.1dB (NOTE 1, see Fig. 5) FPBW at -3dB VIN = 1V p-p at 3.58MHz (see Fig. 4) at 3.58MHz at 3.58MHz (VIN = 40 IRE) at 3.58MHz (VIN = 40 IRE) AV =+1 Clipping occurs at:
25 30 1.8 25
VIN(MAX) RIN CIN ROUT
AV =+1 AV =+1, = 0 to1MHz AV =+1, = 100MHz
500 2.0 -
Clamp Accuracy STROBE Strobe Pulse Width Strobe Level t P STROBE VINHI VINLO
Note: 1.
VIN =+1V p-p
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
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 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 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. 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 k 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 500 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. 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. 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 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 0.1
-10V
0.1
4 COMPOSITE VIDEO INPUT COLOR BURST FILTER 0.1 470p
+10V
2
6
10 1 NON-POLAR 5 3 7
GB4551
8
CLAMPED VIDEO OUTPUT
2 GS4881 SYNC. SEP. 4 6
8 0.1 5
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 0.1
-10V
0.1 0.1
+5V
NETWORK ANALYSER Sweep: 1-100MHz Level: 0dBM 75
4 10 1
2
6
GB4551
5 3
RS 8 7 CL
4
1
CLC110
8
5 10k
VIDEO OUTPUT TO NETWORK ANALYSER I/P
470p +5V
All resistors in ohms, all capacitors in microfarads unless otherwise stated.
0.1 +5V
Fig. 2 Differential Gain and Phase, Frequency Response
TYPICAL PERFORMANCE CURVES (Unless otherwise shown VS = 10V)
6 5 4 3 0.0
CL = 100pF
-0.5 -1.0 -1.5
PHASE(o)
GAIN (dB)
2 1 0
-2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0
CL = 10pF CL = 100pF
47pF
-1
VIN = 100mVp-p
-2 -3 -4 1 10
27pF 10pF
VIN = 100mVp-p RS = 0
100
1
3
5
10
FREQUENCY (MHz)
FREQUENCY (MHz)
Fig. 3 Gain vs Frequency
Fig. 4 Phase vs Frequency
0.3 0.2
0.050
R S / CL
0.1 0.0 -0.1
0.045
33/68pF
0.040 0.035 0.030 0.025
VIN = 40IRE
GAIN (dB)
-0.2 -0 .3 -0.4 -0.5 -0.6 -0.7 1 10 50
dg (%), dp ()
18/270pF
dg
0.020 0.015
27/120pF
dp
0.010 0.005 0.0000 1
1 10
3
5
10
FREQUENCY (MHz)
FREQUENCY (MHz)
Fig. 5 Flattened Frequency Response
Fig. 6 Differential Gain & Phase vs Frequency
520 - 52 - 2
4
BORDER +10V COMPOSITE VIDEO INPUT 0.1
GT4123 7 5 3 VID B OUT VID A CONTROL STROBE
-10V
0.1
INPUT FROM (GB4551)
1
GB4551
COMPOSITE VIDEO INPUT COLOR BURST FILTER 0.1 10
4 1 NON-POLAR
2
6
GB4551
5 3
ROUT 8 GT4123 7 STROBE TO OTHER GB4551s INSERT 3 CONTROL GT4123 5 VID A 7 VID B OUT 1 VIDEO OUTPUT 1
470p +5V
2 GS4881 SYNC. SEP. 4 6
8 0.1 5
5V 0V
BACKGROUND VIDEO IN (FROM GB4551)
7 5 3
VID B VID A CONTROL
1 VIDEO OUTPUT 2
680k
0.1
BACK PORCH PULSE
KEY INPUT
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. (c) Copyright June 1992 Gennum Corporation. All rights reserved. Printed in Canada.
5
520 - 52 - 2

▲Up To Search▲

Price & Availability of GB4551

	To Download GB4551 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .