Datasheet
LT1228
15
1228fd
TYPICAL APPLICATIONS
Video Fader Video DC Restore (Clamp) Circuit
LT1228 • TA12
–
+
–
+
1k
100Ω
g
m
LT1223
CFA
V
OUT
3
2
5
1
V
IN1
1k
–
+
g
m
3
1k
V
IN2
10k
5.1k10k
5.1k10k
100Ω
2
1k
5
1
–5V
V
S
= ±5V
The video fader uses the transconductance amplifiers
from two LT1228s in the feedback loop of another cur-
rent feedback amplifier, the LT1223. The amount of signal
from each input at the output is set by the ratio of the
set currents of the two LT1228s, not by their absolute
value. The bandwidth of the current feedback amplifier
is inversely proportional to the set
current in this
configuration. Therefore, the set currents remain high
over most of the pot’s range, keeping the bandwidth over
15MHz even when the signal is attenuated 20dB. The pot
is set up to completely turn off one LT1228 at each end
of the rotation.
LT1228 • TA13
–
+
–
+
g
m
CFA
V
OUT
8
3
2
5
V
–
3k
R
F
R
G
4
10k
3k
5V
LOGIC
INPUT
RESTORE
0.01µF
VIDEO
INPUT
2N3906
1
6
V
+
7
200Ω
1000pF
NOT NECESSARY IF THE SOURCE RESISTANCE IS LESS THAN 50Ω
The video restore (clamp) circuit restores the black level
of the composite video to zero volts at the beginning of
every line. This is necessary because AC coupled video
changes DC level as a function of the average brightness
of the picture. DC restoration also rejects low frequency
noise such as hum.
The circuit has two inputs: composite video and a logic
signal. The logic signal is
high except during the back
porch time right after the horizontal sync pulse. While
the logic is high, the PNP is off and I
SET
is zero. With I
SET
equal to zero the feedback to Pin 2 has no affect. The
video input drives the noninverting input of the current
feedback amplifier whose gain is set by R
F
and R
G
. When
the logic signal is low, the
PNP turns on and I
SET
goes to
about 1mA. Then the transconductance amplifier charges
the capacitor to force the output to match the voltage at
Pin 3, in this case zero volts.
This circuit can be modified so that the video is DC coupled
by operating the amplifier in an inverting configuration.
Just ground the video input shown and connect R
G
to the
video input instead of
to ground.