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.
Clamping of NTSC, PAL, SECAM & RGB Video Signals
Multimedia Graphics Overlay
Production Switchers
Linear Keyers
Part Number
Package
Temperature Range
GB4571-CDA 8 pin PDIP
0
to 70
C
GB4571-CKA 8 pin SOIC
0
to 70
C
GB4571-CTA 8 pin SOIC Tape
0
to 70
C
DATA SHEET
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
GB4571 Video Buffer with
Precision Strobed DC Restore
FEATURES
DESCRIPTION
AVAILABLE PACKAGING
8 pin PDIP
8 pin SOIC
PIN CONNECTIONS
1
2
3
4
8
7
6
5
V
CC
V
OUT
V
EE
GND
C
HOLD
V
IN
V
CLAMP
STROBE
GB4571
ORDERING INFORMATION
APPLICATIONS
accurate clamping to within
1 mV
less than 1 mV clamping distortion
variable clamping range (V
EE
+ 3.0 V to V
CC
- 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
Document No. 521 - 17 - 01
SIMPLIFIED CIRCUIT DIAGRAM
C
HOLD
GND
VCC
V
EE
V
IN
COUPLING
CAP
V
OUT
STROBE
V
CLAMP
-
+
7
3
1
5
2
HIGH Z
BUFFER
NON-LNEAR
V TO
I
CONVERTER
4
6
8
2
521 - 17 - 00
Supply Voltage
V
S
4.5
5.0
13.2
V
+ Supply Current
I+
-
10
15
mA
- Supply Current
I-
-
10
15
mA
Small Signal B.W.
B.W.
0.1 dB after insertion loss,
80
-
-
MHz
Phase Delay
D
at 3.58 MHz and 4.43 MHz
-2.85
-3.0
-3.15
deg
Differential Gain
g
at 3.58 MHz and 4.43 MHz
-
0.01
0.03
%
Differential Phase
p
at 3.58 MHz and 4.43 MHz
-
0.01
0.05
deg
Input Voltage
V
IN
V
EE
+3
-
V
CC
-3
V
Input Resistance
R
IN
25
-
-
k
Input Capacitance
C
IN
-
2.0
-
pF
Output Resistance
R
OUT
= 0 to 1 MHz
-
8.6
10
= 100 MHz
-
20
40
Insertion Loss
I.L.
= 100 kHz
-
0.03
0.05
dB
Power Supply Rejection Ratio
PSRR
= 1 kHz
50
-
-
dB
= 10 MHz
20
-
-
dB
PP Signal / RMS Noise
S/N
V
SIG
=1 Vp-p
75
-
-
dB
Strobe Pulse Width
t
PWL
Active low
2.0
2.5
-
s
Accuracy
V
IN
= 0.5V to 2Vp-p,
-
1
2
mV
STROBE pulses at H rate
Recovery Time
Scan rate 15.7kHz & 2
s strobe pulses
0.5V offset
-
1
3
ms
0.1V offset
-
190
600
s
Clamping Distortion
Distortion amplitude
-
-
1
mV
Distortion duration (amp. within 0.1 mV)
-
-
3
s
Clamp Voltage Range
V
CLAMP
V
IN
= 1 Vp-p
V
EE
+3
- V
CC
-4.5
V
Input Logic High
V
IH
1.8
-
-
V
Input Logic Low
V
IL
-
-
0.8
V
Strobe Input Current
I
IL
,
I
IH
-
-
20
A
ABSOLUTE MAXIMUM RATINGS
PARAMETER
VALUE/UNITS
Supply Voltage
13.5 V
Operating Temperature Range
0
C
T
A
70
C
Storage Temperature Range
-65
C
T
S
150
C
Lead Temperature (soldering, 10 seconds)
260
C
Differential Video Input Voltage
5 V
Strobe Input Voltage
-V
S
V
STROBE
+V
S
PARAMETER
SYMBOL
CONDITIONS MIN TYP MAX UNITS
POWER
SUPPLIES
ELECTRICAL CHARACTERISTICS
V
S
=
5 V, T
A
= 0
to 70
C, R
L
= 10k
, C
L
= 10 pF, unless otherwise shown.
CAUTION
ELECTROSTATIC
SENSITIVE DEVICES
DO NOT OPEN PACKAGES OR HANDLE
EXCEPT AT A STATIC-FREE WORKSTATION
(flattened response)
V
IN
=100 mV p-p
SIGNAL
PATH
STROBE
V
CLAMP
NOTE:
1.
VA or B
= +1 Vp-p output taken from OUTPUT
2 .
VC
= +1 Vp-p output taken from VA
or
VB
3
521 - 17 - 00
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
V
CLAMP
and V
IN
than it does for proportionally smaller
differences between V
CLAMP
and V
IN
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 C
IN
:C
HOLD
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 C
IN
and C
HOLD
.
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.
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, R
OUT
, should
be included in series with the GB4571 output to obtain optimal
response flatness. For a nominal load of 18 pF, R
OUT
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 V
CLAMP
respectively.
The output current from the OTA is integrated by the hold
capacitor connected to pin 1, C
HOLD
. C
HOLD
develops a
corrective voltage during the period when the clamp is enabled
and holds the corrective voltage while the clamp is disabled.
All resistors in ohms, all capacitors in microfarads unless otherwise stated.
Fig. 1 GB4571 Typical Application Circuit
680k
1.0nF
GB4571
STROBED
DC RESTORE
1 4
2 7
VIDEO
COLOR
BURST
FILTER
0.1
6
4
8
5
2
CLAMPED
VIDEO
OUTPUT
BACK
PORCH
PULSE
0.1
0.1
0.1
0.1
GS4881
SYNC
SEPARATOR
C
LOAD
C
HOLD
V
CLAMP
R
OUT
V
CC
V
CC
V
EE
75k
22
8 6
3
5
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