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
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 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.
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
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
: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.
V
CC
0.1
V
EE
0.1
8
VIDEO
COLOR
BURST
FILTER
22
V
CLAMP
0.1
6
R
OUT
CLAMPED
VIDEO
C
LOAD
OUTPUT
GB4571
2
STROBED
7
DC RESTORE
3
C
HOLD
1.0nF
V
CC
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
GENNUM [ GENNUM CORPORATION ]
