LT1251/LT1256
40MHz Video Fader and
DC Gain Controlled Amplifier
U
FEATURES
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).
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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
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45nV/√Hz at AV = 1
270nV/√Hz at AV = 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°
Absolute gain accuracy is trimmed at wafer sort to mini-
mize part-to-part variations. The circuit is completely
temperature compensated.
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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.
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APPLICATIONS
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Composite Video Gain Control
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RGB, YUV Video Gain Control
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Video Faders, Keyers
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Gamma Correction Amplifiers
TheLT1256doesnothavethison/offfeatureandoperates
linearly over the complete control range. The LT1256 is
recommended for applications requiring more than 20dB
of linear control range.
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Audio Gain Control, Faders
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Multipliers, Modulators
Electronically Tunable Filters
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATION
LT1256
Two-Input Video Fader
Gain Accuracy vs Control Voltage
5
V
V
= ±5V
FS
S
4
3
= 2.5V
LT1251/LT1256
CONTROL
1
2
3
4
5
6
7
14
13
12
11
10
9
IN1
IN2
+
–
+
–
1
2
2
1
2.5VDC
INPUT
0V TO 2.5V
CONTROL
+
–
+
–
I
FS
I
C
FS
5k
C
0
I
FS
I
R
R
F1
–1
–2
–3
C
F2
1.5k
1.5k
5k
+
V
2.5
NULL
V
C
100
(
–4 GAIN ACCURACY (%) =
A
–
)
VMEAS
(
)
8
–
V
–5
0
0.5
1.0
1.5
2.0
2.5
V
1251/56 TA01
OUT
CONTROL VOLTAGE (V)
1251/56 TA02
1