LT1251/LT1256
40MHz Video Fader and
DC Gain Controlled Amplifier
FEATURES
s
s
s
s
s
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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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s
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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
s
s
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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
LT1251/LT1256
ABSOLUTE
MAXIMUM
RATINGS
Total Supply Voltage (V
+
to V
–
) .............................. 36V
Input Current ......................................................
±15mA
Input Voltage on Pins 3,4,5,10,11,12 ............... V
–
to V
+
Output Short-Circuit Duration (Note 1) ........ Continuous
Specified Temperature Range (Note 2) ....... 0°C to 70°C
Operating Temperature Range ............... – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Junction Temperature (Note 3)............................ 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
PACKAGE/ORDER INFORMATION
TOP VIEW
IN1
FB1
V
C
I
C
R
C
NULL
V
–
1
2
3
4
5
6
7
N PACKAGE
14-LEAD PDIP
+
–
+
–
+
14
13
12
11
10
9
8
IN2
FB2
V
FS
I
FS
R
FS
V
+
V
OUT
ORDER PART
NUMBER
LT1251CN
LT1251CS
LT1256CN
LT1256CS
(Note 2)
1
CONTROL
2
–
+
C
FS
–
S PACKAGE
14-LEAD PLASTIC SO
T
JMAX
= 150°C,
θ
JA
= 70°C/ W (N)
T
JMAX
= 150°C,
θ
JA
= 100°C/ W (S)
Consult factory for Industrial and Military grade parts.
SIG AL A PLIFIER AC CHARACTERISTICS
0°C
≤
T
A
≤
70°C, V
S
=
±5V,
V
IN
= 1V
RMS
, f = 1kHz, A
VMAX
= 1, R
F1
= R
F2
= 1.5k, V
FS
= 2.5V, I
C
= I
FS
= NULL = Open, Pins 5,10 = GND,
unless otherwise noted.
SYMBOL
2%IN1
10%IN1
20%IN1
30%IN1
40%IN1
50%IN1
60%IN1
70%IN1
80%IN1
90%IN1
98%IN1
2%IN2
10%IN2
20%IN2
30%IN2
40%IN2
50%IN2
60%IN2
70%IN2
80%IN2
90%IN2
98%IN2
PARAMETER
2% Input 1 Gain
10% Input 1 Gain
20% Input 1 Gain
30% Input 1 Gain
40% Input 1 Gain
50% Input 1 Gain
60% Input 1 Gain
70% Input 1 Gain
80% Input 1 Gain
90% Input 1 Gain
98% Input 1 Gain
2% Input 2 Gain
10% Input 2 Gain
20% Input 2 Gain
30% Input 2 Gain
40% Input 2 Gain
50% Input 2 Gain
60% Input 2 Gain
70% Input 2 Gain
80% Input 2 Gain
90% Input 2 Gain
98% Input 2 Gain
Gain Drift with Temperature
(Worst Case at 30% Gain)
CONDITIONS
V
C
(Pin 3) = 0.05V
V
C
(Pin 3) = 0.25V
V
C
(Pin 3) = 0.50V
V
C
(Pin 3) = 0.75V
V
C
(Pin 3) = 1.00V
V
C
(Pin 3) = 1.25V
V
C
(Pin 3) = 1.50V
V
C
(Pin 3) = 1.75V
V
C
(Pin 3) = 2.00V
V
C
(Pin 3) = 2.25V
V
C
(Pin 3) = 2.45V
V
C
(Pin 3) = 2.45V
V
C
(Pin 3) = 2.25V
V
C
(Pin 3) = 2.00V
V
C
(Pin 3) = 1.75V
V
C
(Pin 3) = 1.50V
V
C
(Pin 3) = 1.25V
V
C
(Pin 3) = 1.00V
V
C
(Pin 3) = 0.75V
V
C
(Pin 3) = 0.50V
V
C
(Pin 3) = 0.25V
V
C
(Pin 3) = 0.05V
V
C
(Pin 3) = 0.75V
V
C
(Pin 3) = 0.75V
LT1251
LT1256
q
q
q
q
q
q
q
q
q
q
q
LT1251
LT1256
LT1251
LT1256
q
q
q
q
q
q
q
q
q
q
q
q
q
LT1251
LT1256
N Package
S Package
q
q
MIN
0
0.1
7
17
27
37
47
57
67
77
87
99.9
95.0
0
0.1
7
17
27
37
47
57
67
77
87
99.9
95.0
TYP
MAX
0.1
5.0
13
23
33
43
53
63
73
83
93
100.0
99.9
0.1
5.0
13
23
33
43
53
63
73
83
93
100.0
99.9
50
400
UNITS
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
%
ppm/°C
ppm/°C
2
U
W
U
U
W W
W
W
U
LT1251/LT1256
SIG AL A PLIFIER AC CHARACTERISTICS
0°C
≤
T
A
≤
70°C, V
S
=
±5V,
V
IN
= 1V
RMS
, f = 1kHz, A
VMAX
= 1, R
F1
= R
F2
= 1.5k, V
FS
= 2.5V, I
C
= I
FS
= NULL = Open, Pins 5,10 = GND,
unless otherwise noted.
SYMBOL
PARAMETER
Gain Supply Rejection
External Resistor Gain
50% Input 1
Slew Rate
Control Feedthrough
Full Power Bandwidth
Small-Signal Bandwidth
Differential Gain (Notes 4,5)
Differential Phase (Notes 4,5)
THD
Total Harmonic Distortion
CONDITIONS
V
C
= 1.25V, V
S
=
±5V
to
±15V
Pins 5,10 = Open, External 5k Resistors
from Pins 4,11 to Ground, V
C
= 1.25V
V
IN
=
±2.5V,
V
O
at
±2V,
R
L
= 150Ω
V
C
= 1.25VDC + 2.5V
P-P
at 1kHz
V
O
= 1V
RMS
V
S
=
±5V
V
S
=
±15V
Control = 0% or 100%
Control = 25% or 75%
Control = 0% or 100%
Control = 25% or 75%
Gain = 100%
Gain = 50%
Gain = 10%
10% to 90%, V
O
= 100mV
V
O
= 100mV
V
O
= 100mV
0.1%,
∆V
O
= 2V
MIN
q
q
q
SR
BW
t
r
, t
f
OS
t
PD
t
S
Rise Time, Fall Time
Overshoot
Propagation Delay
Settling Time
SIG AL A PLIFIER DC CHARACTERISTICS
SYMBOL
V
OS
PARAMETER
Input Offset Voltage
Input Offset Voltage Drift
Noninverting Input Bias Current
Inverting Input Bias Current
Inverting Input Bias Current Null Change
Input Noise Voltage Density
Noninverting Input Noise Current Density
Inverting Input Noise Current Density
Input Resistance
Input Capacitance
Input Voltage Range
Common Mode Rejection Ratio
Inverting Input Current Common Mode Rejection
PSRR
Power Supply Rejection Ratio
Noninverting Input Current Power Supply Rejection
Inverting Input Current Power Supply Rejection
0°C
≤
T
A
≤
70°C, V
S
=
±5V,
V
CM
= 0V, V
FS
= 2.5V, I
C
= I
FS
= NULL = Open, Pins 5,10 = GND, unless otherwise noted.
CONDITIONS
Either Input
Difference Between Inputs
Either Input
Either Input
Difference Between Inputs
Null (Pin 6) Open to V
–
f = 1kHz
f = 1kHz
f = 1kHz
Either Noninverting Input
Either Noninverting Input
V
S
=
±5V
V
S
= 5V
V
CM
= – 3V to 3V
V
S
= 5V, V
CM
= 2V to 3V, V
O
= 2.5V
V
CM
= – 3V to 3V
V
S
= 5V, V
CM
= 2V to 3V, V
O
= 2.5V
V
S
=
±5V
to
±15V
V
S
=
±5V
to
±15V
V
S
=
±5V
to
±15V
MIN
q
q
q
q
q
q
I
IN+
I
IN–
e
n
+i
n
–i
n
R
IN
C
IN
CMRR
W
W
U
U
TYP
0.03
45
150
300
2.5
20
30
40
0.02
0.90
0.02
0.55
0.002
0.015
0.4
11
3
10
65
MAX
0.10
55
UNITS
%/V
%
V/µs
mV
P-P
MHz
MHz
MHz
%
%
DEG
DEG
%
%
%
ns
%
ns
ns
–3
– 2.5
– 30
–1
– 280
q
q
q
q
q
q
q
q
q
q
q
5
±3
2
55
50
TYP
2
1
10
0.5
10
0.5
– 170
2.7
1.5
29
17
1.5
±3.2
61
57
0.07
0.17
76
30
30
MAX
5
3
2.5
30
1
– 60
3
0.25
0.70
100
200
70
UNITS
mV
mV
µV/°C
µA
µA
µA
µA
nV/√Hz
pA/√Hz
pA/√Hz
MΩ
pF
V
V
dB
dB
µA/
V
µA/
V
dB
nA/V
nA/V
3
LT1251/LT1256
SIG AL A PLIFIER DC CHARACTERISTICS
SYMBOL
A
VOL
R
OL
V
OUT
PARAMETER
Large-Signal Voltage Gain
Transresistance,
∆V
OUT
/∆I
IN–
Maximum Output Voltage Swing
0°C
≤
T
A
≤
70°C, V
S
=
±5V,
V
CM
= 0V, V
FS
= 2.5V, I
C
= I
FS
= NULL = Open, Pins 5,10 = GND, unless otherwise noted.
CONDITIONS
V
O
= – 3V to 3V, R
L
= 150Ω
V
O
= – 2.75V to 2.75V, R
L
= 150Ω
V
O
= – 3V to 3V, R
L
= 150Ω
V
O
= – 2.75V to 2.75V, R
L
= 150Ω
No Load
R
L
= 150Ω
V
S
=
±15V,
No Load
V
S
= 5V, V
CM
= 2.5V, (Note 6)
V
S
=
±5V
V
S
= 5V, V
CM
= V
O
= 2.5V
V
C
= V
FS
= 2.5V
V
C
= V
FS
= 1.25V
V
C
= V
FS
= 0V
V
C
= V
FS
= 2.5V, V
S
=
±15V
V
C
= V
FS
= 0V, V
S
=
±15V
MIN
83
83
0.75
0.75
±4.0
±3.0
±2.75
±14.0
1.2
±30
±20
TYP
93
1.8
±4.2
±3.5
±14.2
3.8
±40
±30
13.5
7.5
1.3
14.5
1.4
MAX
UNITS
dB
dB
MΩ
MΩ
V
V
V
V
V
mA
mA
mA
mA
mA
mA
mA
I
O
I
S
Maximum Output Current
Supply Current
CO TROL A D FULL SCALE A PLIFIER CHARACTERISTICS
0°C
≤
T
A
≤
70°C, V
S
=
±5V,
V
FS
= 2.5V, I
C
= I
FS
= NULL = Open, Pins 5,10 = GND, unless otherwise noted.
SYMBOL
PARAMETER
Control Amplifier Input Offset Voltage
Full-Scale Amplifier Input Offset Voltage
Control Amplifier Input Resistance
Full-Scale Amplifier Input Resistance
Control Amplifier Input Bias Current
Full-Scale Amplifier Input Bias Current
Internal Control Resistor
Internal Full-Scale Resistor
Resistor Temperature Coefficient
Control Path Bandwidth
Control Path Rise and Fall Time
Control Path Transition Time
Control Path Propagation Delay
CONDITIONS
Pin 4 to Pin 3
Pin 11 to Pin 12
MIN
q
q
q
q
q
q
R
C
R
FS
The
q
denotes specifications which apply over the specified operating
temperature range.
Note 1:
A heat sink may be required depending on the power supply
voltage.
Note 2:
Commercial grade parts are designed to operate over the
temperature range of – 40°C to 85°C but are neither tested nor guaranteed
beyond 0°C to 70°C. Industrial grade parts specified and tested over
– 40°C to 85°C are available on special request. Consult factory.
Note 3:
T
J
is calculated from the ambient temperature T
A
and the power
dissipation P
D
according to the following formulas:
LT1251CN/LT1256CN:
T
J
= T
A
+ (P
D
• 70°C/W)
LT1251CS/LT1256CS:
T
J
= T
A
+ (P
D
• 100°C/W)
4
W
U
W
U
q
q
q
q
q
q
q
q
q
q
q
q
q
17.0
9.5
1.8
18.5
2.0
U
T
A
= 25°C
T
A
= 25°C
Small Signal, V
C
= 100mV, (Note 7)
Small Signal, V
C
= 100mV, (Note 7)
0% to 100%
Small Signal,
∆V
C
= 100mV
V
C
from 0% or 100%
25
25
– 750
– 750
3.75
4
TYP
5
5
100
100
– 300
– 300
5
5
0.2
10
35
150
50
90
MAX
15
15
6.25
6
UNITS
mV
mV
MΩ
MΩ
nA
nA
kΩ
kΩ
%/°C
MHz
ns
ns
ns
ns
Note 4:
Differential gain and phase are measured using a Tektronix
TSG120YC/NTSC signal generator and a Tektronix 1780R Video
Measurement Set. The resolution of this equipment is 0.1% and 0.1°. Five
identical amplifier stages were cascaded giving an effective resolution of
0.02% and 0.02°.
Note 5:
Differential gain and phase are best when the control is set at 0%
or 100%. See the Typical Performance Characteristics curves.
Note 6:
Tested with R
L
= 150Ω to 2.5V to simulate an AC coupled load.
Note 7:
Small-signal control path response is measured driving R
C
(Pin 5)
to eliminate peaking caused by stray capacitance on Pin 4.
LT1251/LT1256
TYPICAL PERFORMANCE CHARACTERISTICS
LT1251
Gain vs Control Voltage
1.0
1.0
SPOT NOISE (nV/√Hz OR pA/√Hz)
0.8
IN2
GAIN (V/V)
0.6
GAIN (V/V)
V
FS
= 2.5V
0.4
IN1
0.2
0
0
0.5
1.5
2.0
1.0
CONTROL VOLTAGE (V)
2.5
1251/56 G01
LT1251/LT1256
Control Path Bandwidth
10
8
6
VOLTAGE GAIN (dB)
VOLTAGE GAIN (dB)
10
VOLTAGE DRIVE V
C
V
S
= ±5V
OUTPUT VOLTAGE (V
P-P
)
4
2
0
–2
–4
–6
–8
–10
10k
PIN 4 NOT IN SOCKET
100k
1M
10M
FREQUENCY (Hz)
THD Plus Noise vs Frequency
10
V
S
= ±5V, V
IN
= 1V
RMS
A
V
= 1, R
F
= 1.5k, V
FS
= 2.5V
CC
–30
1
THD + NOISE (%)
3RD ORDER INTERCEPT (dBm)
CC
DISTORTION (dBc)
V
C
= 10%
0.1
V
C
= 50%
CC
0.01
V
C
= 100%
CC
0.001
10
100
1k
10k
FREQUENCY (Hz)
100k
1251/56 G08
U W
LT1256
Gain vs Control Voltage
100
Spot Input Noise Voltage and
Current vs Frequency
0.8
IN2
0.6
V
FS
= 2.5V
–i
n
10
0.4
IN1
0.2
e
n
+i
n
1
10
100
1k
FREQUENCY (Hz)
10k
1251/56 G06
0
0
0.5
1.5
2.0
1.0
CONTROL VOLTAGE (V)
2.5
1251/56 G02
LT1251/LT1256
Control Path Bandwidth
8
VOLTAGE DRIVE R
C
V
C
= GND
V
S
= ±5V
7
6
5
4
3
2
8
6
4
2
0
–2
–4
–6
–8
Undistorted Output Voltage
vs Frequency
A
V
= 10
A
V
= 1
V
S
= ±5V
R
L
= 1k
R
F
= 1.5k
V
C
= V
FS
= 2.5V
1M
10M
FREQUENCY (Hz)
100M
1251/56 G07
100M
1251/56 G04
–10
10k
100k
1M
10M
FREQUENCY (Hz)
100M
1251/56 G05
1
100k
2nd and 3rd Harmonic Distortion
vs Frequency
–20
V
S
= ±5V
A
V
= 1
R
F
= 1.5k
R
L
= 1k
V
O
= 2V
P-P
V
C
= V
FS
= 2.5V
CC
3rd Order Intercept vs Frequency
50
45
40
35
30
25
20
15
V
S
= ±15V
A
V
= 1
R
F
= 1.5k
R
L
= 100Ω
V
C
= V
FS
= 2.5V
CC
–40
–50
2ND
3RD
–60
–70
1
10
FREQUENCY (MHz)
100
1251/56 G09
10
0
5
10
15
20
FREQUENCY (MHz)
25
30
1251/56 G10
5
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