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PDF LT1256CN Data sheet ( Hoja de datos )
| Número de pieza | LT1256CN | |
| Descripción | 40MHz Video Fader and DC Gain Controlled Amplifier | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1251/LT1256
40MHz Video Fader and
DC Gain Controlled Amplifier
FEATURES
s Accurate Linear Gain Control: ±1% Typ, ±3% Max
s Constant Gain with Temperature
s Wide Bandwidth: 40MHz
s High Slew Rate: 300V/µs
s Fast Control Path: 10MHz
s Low Control Feedthrough: 2.5mV
s High Output Current: 40mA
s Low Output Noise
45nV/√Hz at AV = 1
270nV/√Hz at AV = 100
s Low Distortion: 0.01%
s Wide Supply Range: ±2.5V to ±15V
s Low Supply Current: 13mA
s Low Differential Gain and Phase: 0.02%, 0.02°
U
APPLICATIONS
s Composite Video Gain Control
s RGB, YUV Video Gain Control
s Video Faders, Keyers
s Gamma Correction Amplifiers
s Audio Gain Control, Faders
s Multipliers, Modulators
s Electronically Tunable Filters
, LTC and LT are registered trademarks of Linear Technology Corporation.
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.
TYPICAL APPLICATION
Two-Input Video Fader
1
IN1
2
0V TO 2.5V
CONTROL
IC
3
4
5
NULL
V–
6
7
LT1251/LT1256
++
12
––
CONTROL
++
C
IC IFS
FS
––
5k 5k
14
IN2
13
12 2.5VDC
INPUT
11 IFS
10
9 V+
8
RF2 RF1
1.5k 1.5k
VOUT
1251/56 TA01
LT1256
Gain Accuracy vs Control Voltage
5
4
VS = ±5V
VFS = 2.5V
3
2
1
0
–1
–2
–3
( )( )–4
GAIN ACCURACY (%) =
AVMEAS
–
VC
2.5
100
–5
0 0.5 1.0 1.5 2.0 2.5
CONTROL VOLTAGE (V)
1251/56 TA02
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LT1251/LT1256
LT1251
Gain vs Control Voltage
1.0
0.8
IN2
0.6
VFS = 2.5V
0.4
IN1
0.2
0
0 0.5 1.0 1.5 2.0 2.5
CONTROL VOLTAGE (V)
1251/56 G01
LT1251/LT1256
Control Path Bandwidth
10
8
VOLTAGE DRIVE VC
VS = ±5V
6
4
2
0
PIN 4 NOT IN SOCKET
–2
–4
–6
–8
–10
10k
100k 1M 10M
FREQUENCY (Hz)
100M
1251/56 G04
THD Plus Noise vs Frequency
10
VS
=
CC
± 5V,
VIN
=
1VRMS
AV = 1, RF = 1.5k, VFS = 2.5V
1
VC
=
CC
10%
0.1
0.01
VC
=
CC
50%
0.001
10
VC
=
CC
100%
100 1k 10k
FREQUENCY (Hz)
100k
1251/56 G08
LT1256
Gain vs Control Voltage
1.0
0.8
IN2
0.6
VFS = 2.5V
0.4
IN1
0.2
0
0 0.5 1.0 1.5 2.0 2.5
CONTROL VOLTAGE (V)
1251/56 G02
LT1251/LT1256
Control Path Bandwidth
10
8
VOLTAGE DRIVE RC
VC = GND
6 VS = ±5V
4
2
0
–2
–4
–6
–8
–10
10k
100k
1M
10M
FREQUENCY (Hz)
100M
1251/56 G05
2nd and 3rd Harmonic Distortion
vs Frequency
–20
VS
=
CC
± 5V
AV = 1
–30 RF = 1.5k
RL = 1k
VO = 2VP-P
–40 VC = VFS = 2.5V
– 50
2ND
– 60
3RD
–70
1
10
FREQUENCY (MHz)
100
1251/56 G09
Spot Input Noise Voltage and
Current vs Frequency
100
–in
10
en
+in
1
10 100
1k 10k
FREQUENCY (Hz)
1251/56 G06
Undistorted Output Voltage
vs Frequency
8
AV = 10
7
6
AV = 1
5
4
3 VS = ±5V
RL = 1k
2 RF = 1.5k
VC = VFS = 2.5V
1
100k
1M
10M
FREQUENCY (Hz)
100M
1251/56 G07
3rd Order Intercept vs Frequency
50
VS
=
CC
±15V
45 AV = 1
RF = 1.5k
40 RL = 100Ω
VC = VFS = 2.5V
35
30
25
20
15
10
0
5 10 15 20 25 30
FREQUENCY (MHz)
1251/56 G10
5
5 Page 
LT1251/LT1256
APPLICATIONS INFORMATION
100
VFS = 2.5V
millivolts of the negative supply can drive the NULL pin.
The AM modulator application shows an LT1077 driving
the NULL pin to eliminate the output DC offset voltage.
RF2 = 4.3k
50
RF2 = 1.5k
0
0 0.5 1.0 1.5 2.0 2.5
CONTROL VOLTAGE (V)
1251/56 F01
Figure 1. Linear Gain Control from 0 to 101
Capacitive Loads
Increasing the value of the feedback resistor reduces the
bandwidth and open-loop gain of the LT1251/LT1256;
therefore, the pole introduced by a capacitive load can be
overcome. If there is little or no resistive load in parallel
with the load capacitance, the output stage will resonate,
peak and possibly oscillate. With a resistive load of 150Ω,
any capacitive load can be accommodated by increasing
the feedback resistor. If the capacitive load cannot be
paralleled with a DC load of 150Ω, a network of 200pF in
series with 100Ω should be placed from the output to
ground. Then the feedback resistor should be selected for
best response.
The Null Pin
Pin 6 can be used to adjust the gain of an internal current
mirror to change the output offset. The open circuit
voltage at Pin 6 is set by the full scale current IFS flowing
through 200Ω to the negative supply. Therefore, the NULL
pin sits 100mV above the negative supply with VFS equal
to 2.5V. Any op amp whose output swings within a few
Crosstalk
The amount of signal from the off input that appears at the
output is a function of frequency and the circuit topology.
The nature of a current feedback input stage is to force the
voltage at the inverting input to be equal to the voltage at
the noninverting input. This is independent of feedback
and forced by a buffer amplifier between the inputs. When
the LT1251/LT1256 are operating noninverting, the off
input signal is present at the inverting input. Since one end
of the feedback resistor is connected to this input, the off
signal is only a feedback resistor away from the output.
The amount of unwanted signal at the output is deter-
mined by the size of the feedback resistor and the output
impedance of the LT1251/LT1256. The output impedance
rises with increasing frequency resulting in more crosstalk
at higher frequencies. Additionally, the current that flows
in the inverting input is diverted to the supplies within the
chip and some of this signal will also show up at the
output. With a 1.5k feedback resistor, the crosstalk is
down about 86dB at low frequencies and rises to – 78dB
at 1MHz and on to – 60dB at 6MHz. The curves show the
details.
Distortion
When only one input is contributing to the output (VC = 0%
or 100%) the LT1251/LT1256 have very low distortion. As
the control reduces the output, the distortion will increase.
The amount of increase is a function of the current that
flows in the inverting input. Larger input signals generate
more distortion. Using a larger feedback resistor will
reduce the distortion at the expense of higher output
noise.
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1256CN.PDF ] | |
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