LT1251/LT1256
40MHz Video Fader and
DC Gain Controlled Amplifier
FEATURES
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DESCRIPTION
The LT
®
1251/LT1256 are 2-input, 1-output, 40MHz cur-
rent feedback amplifiers with a linear control circuit that
sets the amount each input contributes to the output.
These parts make excellent electronically controlled vari-
able gain amplifiers, filters, mixers and faders. The only
external components required are the power supply by-
pass capacitors and the feedback resistors. Both parts
operate on supplies from
±2.5V
(or single 5V) to
±15V
(or single 30V).
Absolute gain accuracy is trimmed at wafer sort to mini-
mize part-to-part variations. The circuit is completely
temperature compensated.
The LT1251 includes circuitry that eliminates the need for
accurate control signals around zero and full scale. For
control signals of less than 2% or greater than 98%, the
LT1251 sets one input completely off and the other
completely on. This is ideal for fader applications because
it eliminates off-channel feedthrough due to offset or gain
errors in the control signals.
The LT1256 does not have this on/off feature and operates
linearly over the complete control range. The LT1256 is
recommended for applications requiring more than 20dB
of linear control range.
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Accurate Linear Gain Control:
±
1% Typ,
±
3% Max
Constant Gain with Temperature
Wide Bandwidth: 40MHz
High Slew Rate: 300V/µs
Fast Control Path: 10MHz
Low Control Feedthrough: 2.5mV
High Output Current: 40mA
Low Output Noise
45nV/√Hz at A
V
= 1
270nV/√Hz at A
V
= 100
Low Distortion: 0.01%
Wide Supply Range:
±2.5V
to
±15V
Low Supply Current: 13mA
Low Differential Gain and Phase: 0.02%, 0.02°
APPLICATIONS
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Composite Video Gain Control
RGB, YUV Video Gain Control
Video Faders, Keyers
Gamma Correction Amplifiers
Audio Gain Control, Faders
Multipliers, Modulators
Electronically Tunable Filters
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
Two-Input Video Fader
5
IN1
1
+
LT1256
Gain Accuracy vs Control Voltage
4
V
S
=
±5V
V
FS
= 2.5V
LT1251/LT1256
+
–
+
–
14
2
–
+
IN2
GAIN ACCURACY (%)
2
0V TO 2.5V
CONTROL
I
C
3
4
5
NULL
V
–
6
7
1
CONTROL
C
5k
I
C
I
FS
3
2
1
0
–1
–2
–3
13
12 2.5VDC
INPUT
11
I
FS
10
9
8
V
OUT
1251/56 TA01
FS
5k
–
R
F2
1.5k
R
F1
1.5k
V
+
–4 GAIN ACCURACY (%) = A
VMEAS
–
–5
0
0.5
U
U
U
(
V
C
100
2.5
)
(
)
2.5
1.5
2.0
1.0
CONTROL VOLTAGE (V)
1251/56 TA02
1
LINER [ LINEAR TECHNOLOGY ]
