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LT1228CS8 Datasheet(PDF) 15 Page - Linear Technology

Part # LT1228CS8
Description  100MHz Current Feedback Amplifier with DC Gain Control
PDF  20 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT1228CS8 Datasheet(HTML) 15 Page - Linear Technology

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15
LT1228
Video DC Restore (Clamp) Circuit
LT1228 • TA13
–
+
–
+
gm
CFA
VOUT
8
3
2
5
V–
3k
RF
RG
4
10k
3k
5V
LOGIC
INPUT
RESTORE
0.01
µF
VIDEO
INPUT
2N3906
1
6
V+
7
200
Ω
1000pF
NOT NECESSARY IF THE SOURCE RESISTANCE IS LESS THAN 50
Ω
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 ISET is zero. With ISET 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 RF and RG. When the logic
signal is low, the PNP turns on and ISET goes to about 1mA.
Then the transconductance amplifier charges the capaci-
tor 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 RG to the
video input instead of to ground.
S
APPLICATI
TYPICAL
Video Fader
LT1228 • TA12
–
+
–
+
1k
100
Ω
gm
LT1223
CFA
VOUT
3
2
5
1
VIN1
1k
–
+
gm
3
1k
VIN2
10k
5.1k
10k
5.1k
10k
100
Ω
2
1k
5
1
–5V
VS = ±5V
The video fader uses the transconductance amplifiers
from two LT1228s in the feedback loop of another current
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.



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