Datasheet

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LT1251/LT1256

40MHz Video Fader and

DC Gain Controlled Amplifier

TYPICAL APPLICATION

Two-Input Video Fader

–

IN1 IN2

2.5VDC

INPUT



1.5k

OUT



1.5k

–

NULL

1251/56 TA01

0V TO 2.5V

CONTROL

–

1

2

3

4

5

6

14

13

12

11

10

9

CONTROL

LT1251/LT1256

CFS

5k5k

LT1256

Gain Accuracy vs Control Voltage

FEATURES

DESCRIPTION

■

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

= 1

270nV/√Hz at A

= 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°

, LTC and LT are registered trademarks of Linear Technology Corporation.

APPLICATIONS

■

Composite Video Gain Control

■

RGB, YUV Video Gain Control

■

Video Faders, Keyers

■

Gamma Correction Amplifiers

■

Audio Gain Control, Faders

■

Multipliers, Modulators

■

Electronically Tunable Filters

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.

CONTROL VOLTAGE (V)

GAIN ACCURACY (%)

5

4

3

2

1

0

–1

–2

–3

–4

–5

2.0

1251/56 TA02

0.5

1.0

1.5

2.5

= ±5V

= 2.5V

100

GAIN ACCURACY (%) = A

VMEAS

–

()



2.5

()

Summary of content (24 pages)

PAGE 1
LT1251/LT1256 40MHz Video Fader and DC Gain Controlled Amplifier U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 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 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.
PAGE 2
LT1251/LT1256 W U U W W W 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)........
PAGE 3
LT1251/LT1256 W U SIG AL A PLIFIER AC CHARACTERISTICS 0°C ≤ TA ≤ 70°C, VS = ±5V, VIN = 1VRMS, f = 1kHz, AVMAX = 1, RF1 = RF2 = 1.5k, VFS = 2.5V, IC = IFS = NULL = Open, Pins 5,10 = GND, unless otherwise noted.
PAGE 4
LT1251/LT1256 W U SIG AL A PLIFIER DC CHARACTERISTICS 0°C ≤ TA ≤ 70°C, VS = ±5V, VCM = 0V, VFS = 2.5V, IC = IFS = NULL = Open, Pins 5,10 = GND, unless otherwise noted. SYMBOL AVOL PARAMETER Large-Signal Voltage Gain ROL Transresistance, ∆VOUT /∆IIN– CONDITIONS VO = – 3V to 3V, RL = 150Ω VO = – 2.75V to 2.75V, RL = 150Ω VO = – 3V to 3V, RL = 150Ω VO = – 2.75V to 2.
PAGE 5
LT1251/LT1256 U W TYPICAL PERFORMANCE CHARACTERISTICS LT1256 Gain vs Control Voltage LT1251 Gain vs Control Voltage 100 0.8 0.8 IN2 0.6 IN2 GAIN (V/V) VFS = 2.5V 0.4 0.6 VFS = 2.5V 0.4 IN1 IN1 0.2 0.2 0 0 0.5 0 1.5 2.0 1.0 CONTROL VOLTAGE (V) 0.5 0 2.5 1.5 2.0 1.0 CONTROL VOLTAGE (V) 10 4 0 PIN 4 NOT IN SOCKET –4 8 0 –2 –4 –6 –8 –8 1M 10M FREQUENCY (Hz) 100k 1M 10M FREQUENCY (Hz) 1251/56 G04 VS = ±5V, VIN = 1VRMS AV = 1, RF = 1.5k, VFS = 2.5V 3 VS = ±5V RL = 1k RF = 1.
PAGE 6
LT1251/LT1256 U W TYPICAL PERFORMANCE CHARACTERISTICS Bandwidth vs Feedback Resistance, AV = 1, RL = 100Ω PEAKING ≤ 0.5dB PEAKING ≤ 5.0dB –3dB BANDWIDTH (MHz) 60 50 VS = ±15V 40 VS = 5V 30 VS = ±5V 20 45 PHASE 4 50 VS = ±15V 40 VS = 5V 30 –45 2 –90 1 –135 GAIN 0 –225 –2 VS = ±5V 1.6 1.0 1.2 1.4 0.8 FEEDBACK RESISTANCE (kΩ) 10 1.8 0.6 1.6 1.0 1.2 1.4 0.
PAGE 7
LT1251/LT1256 U W TYPICAL PERFORMANCE CHARACTERISTICS Supply Current vs Full-Scale Voltage Supply Current vs Full-Scale Current 14 TA = 125°C 4 10 8 TA = – 55°C 6 4 2 0 0.5 1.0 2.0 1.5 FULL-SCALE VOLTAGE, VFS (V) 0 V – +1 300 400 100 200 FULL-SCALE CURRENT, IFS (µA) V– –50 500 TA = –55°C TA = 125°C 100 0 –100 –200 –300 150 TA = –55°C TA = 25°C TA = 125°C 50 0 –50 –100 –150 250 100 150 200 300 50 NULL VOLTAGE, REFERENCED TO V – (mV) 1.7 0 1.1 TA = –55°C 0.
PAGE 8
LT1251/LT1256 U W TYPICAL PERFORMANCE CHARACTERISTICS Slew Rate vs Full-Scale Reference Voltage Power Supply Rejection Ratio vs Frequency Slew Rate vs Temperature 80 350 AV = 1 VS = ±15V 200 SLEW RATE (V/µs) 250 VS = ±5V 150 100 50 POWER SUPPLY REJECTION RATIO (dB) 350 300 SLEW RATE (V/µs) VS = ±5V AV = 1 NO LOAD 300 250 200 0 VS = ±5V AV = 1 RF = 1.5k VC = VFS = 2.5V POSITIVE 60 NEGATIVE 50 40 30 20 10 0 0 0.5 1.0 2.0 1.5 FULL-SCALE REFERENCE VOLTAGE (V) 2.
PAGE 9
LT1251/LT1256 W W SI PLIFIED SCHE ATIC VCC Q8 + Q10 Q9 Q19 Q11 + + + I2 I1 I4 I5 Q16 Q5 Q1 Q6 Q2 Q7 R1 250Ω VC Q3 Q20 Q17 Q13 Q12 R2 250Ω R3 250Ω VFS IC Q4 Q14 I3 R4 250Ω IFS Q15 RC 5k + Q18 RFS 5k + I6 RC RFS VEE VCC Q30 Q29 Q31 Q32 R5 200Ω R6 200Ω Q52 Q53 D1 D2 Q38 Q36 Q37 Q21 R7 200Ω Q54 Q39 Q56 Q22 Q41 Q45 Q40 Q55 Q44 FB1 IN1 IN2 OUT FB2 I7 Q42 Q46 Q43 Q57 Q47 Q58 Q25 Q26 Q48 Q49 Q50 Q51 D3 D4 Q59 Q23 Q24 Q27 Q28 Q33 Q34 Q35
PAGE 10
LT1251/LT1256 U W U U APPLICATIONS INFORMATION Supply Voltage The LT1251/LT1256 are high speed amplifiers. To prevent problems, use a ground plane with point-to-point wiring and small bypass capacitors (0.01µF to 0.1µF) at each supply pin. For good settling characteristics, especially driving heavy loads, a 4.7µF tantalum within an inch or two of each supply pin is recommended. The LT1251/LT1256 can be operated on single or split supplies. The minimum total supply is 4V (Pins 7 to 9).
PAGE 11
LT1251/LT1256 U W U U APPLICATIONS INFORMATION 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. 100 GAIN (V/V) VFS = 2.5V Crosstalk RF2 = 4.3k 50 RF2 = 1.5k 0 0 0.5 1.5 2.0 1.0 CONTROL VOLTAGE (V) 2.5 1251/56 F01 Figure 1.
PAGE 12
LT1251/LT1256 U U W U APPLICATIONS INFORMATION Signal Path Description RF1 RG1 I1 R1 2 – 1 1 V1 I1 K + IO Σ V2 14 2 13 I2 RG2 I2 VO C ROL + 8 +1 1–K – R2 RF2 1251/56 BD Figure 2. Signal Path Block Diagram V2 Figure 2 is the basic block diagram of the LT1251/LT1256 signal path with external resistors RG1, RF1, RG2 and RF2. Both input stages are operating as noninverting amplifiers with two input signals V1 and V2.
PAGE 13
LT1251/LT1256 U W U U APPLICATIONS INFORMATION In low gain applications, R1 and R2 are small compared to the feedback resistors and therefore we can simplify the equation to: ( ) + (RG1)(RF1) (RG2)(RF2) VO = KV1 1 − K V2 RG1 + RF1 RG2 + RF2 ( ) 1− K 1 + sROLC K + + ROL RF1 RF2 Note that the denominator causes a gain error due to the open-loop gain (typically 0.1% for frequencies below 20kHz) and for mismatches in RF1 and RF2. A 1% mismatch in the feedback resistors results in a 0.
PAGE 14
LT1251/LT1256 U U W U APPLICATIONS INFORMATION gain) is ±3% as detailed in the electrical tables. By using a 2.5V full-scale voltage and the internal resistors, no additional errors need be accounted for. Control Circuit Description V+ VC 3 IC + C IFS + RC – 11 4 5 VFS FS – IC 12 RC 5k CONTROL V TO I RFS 5k 10 IFS RFS FULL SCALE V TO I 1251/56 F03 Figure 3.
PAGE 15
LT1251/LT1256 U TYPICAL APPLICATIONS AM Modulator with DC Output Nulling Circuit 0.1µF LT1256 1 1MHz CARRIER 14 + 50Ω 2 – + 1 2 – 13 CONTROL 220k 2.5VDC INPUT 3 0.1µF 4 AUDIO MODULATION + – FS 5k 5 220k + IC IFS C – 5k 12 11 NULL 6 9 7 8 V– RF1 1.5k RF2 1.5k 10 V+ VOUT 220k V+ 0.1µF – LT1077 + 1251/56 TA03 V– Single Supply Noninverting AC Amplifier with Digital Gain Control Single Supply Inverting AC Amplifier C1 10µF + V1 V + RG1 1.5k RF1 1.
PAGE 16
LT1251/LT1256 U TYPICAL APPLICATIONS Controlled Gain, Voltage-to-Current Converter (Current Source) RF 1k RF 1k RG 100Ω ×4 1 2 V IN 14 13 LT1256 + 1 – 8 RO 1k IOUT + – + 2 LT1363 VC RC RFS VFS 3 5 10 12 CONTROL VOLTAGE RF 1k – 2.5VDC INPUT RF 1k ( ) V RF VC IOUT = IN RO RG VFS 1251/56 TA09 OUTPUT RESISTANCE DEPENDS ON MATCHING OF RESISTORS Variable Lowpass, Highpass and Allpass Filter R2 R1 – V IN INVERTED HIGHPASS LT1252 R3 + BASIC VARIABLE INTEGRATOR R R C ALLPASS 1.
PAGE 17
LT1251/LT1256 U TYPICAL APPLICATIONS Logarithmic Gain Control (Noninverting) 6k 15 – 1 V IN 600Ω 200Ω VFS = 2.5V LT1251/LT1256 2 1 + 14 8 V + 13 2 – VOUT GAIN (dB) 2k + 9 7 V– 0 VC RC RFS VFS 3 5 10 12 CONTROL VOLTAGE 1.5k <1dB ERROR 2.5VDC INPUT AV = 24dB –15 1251/56 TA07a 0 ( ) VC – 0.5 VFS 2.5 1.25 CONTROL VOLTAGE (V) 1251/56 TA07b Logarithmic Gain Control (Inverting) 6k 15 VFS = 2.5V LT1251/LT1256 2 – 1 1 V IN + 14 6k 8 V + 13 – 2 VOUT GAIN (dB) 1.
PAGE 18
LT1251/LT1256 U TYPICAL APPLICATIONS 3.58MHz Phase Shifter R1 470Ω R2 1k C1 0.001µF – VIN R5 430Ω 1/2 LT1253 R3 470Ω R'2 1k C'1 0.001µF R7 150Ω + R9 1.5k C5 50pF 1/2 LT1253 R8 910Ω R'3 470Ω 1 R'9 1.5k + 2 8 14 + 13 – 2 VC 13 R'4 1k VC RC RFS VFS 3 5 10 12 R11 150Ω R'6 430Ω C '5 50pF C '2 100pF R'8 910Ω LT1256 + 1 – 8 + – 2 VC RC RFS VFS 3 5 10 12 R'10 1.5k 2.5V C3, 100pF 2.5V C'3, 100pF VC R'11 150Ω R12, 10k R'12, 10k C'4 0.002µF C4 0.
PAGE 19
LT1251/LT1256 U TYPICAL APPLICATIONS State Variable Filter with Adjustable Frequency and Q 1k 1k HPOUT 1k – VIN 500pF 1k BPOUT LT1252 + 1.5k LT1256 2 – 1 + 14 – 500Ω 500pF 1k 8 + 13 1.5k 1 1.5k 2 2 1 VC RC RFS VFS 3 5 10 12 14 VFS Vω 1k 13 500pF LT1256 – + 1 8 LPOUT + – 2 VC RC RFS VFS 3 5 10 12 1.5k 1.5k LT1256 – 1 8 + + 2 – VFS RFS RC 12 10 5 VC 3 VFS VQ 1.5k VFS Vω 2 1 500pF 1k 1251/56 TA15a 14 13 VFS = 2.5V 1.
PAGE 20
LT1251/LT1256 W W ACRO ODEL For PSpiceTM * * Linear Technology LT1251/LT1256 VIDEO FADER MACROMODEL * Written: 3-11-1994 BY WILLIAM H. GROSS.
PAGE 21
LT1251/LT1256 W W ACRO ODEL *full scale amp IBFS 12 0 –300NA RIFS 12 0 100MEG C12 12 0 1PF R12 12 12A 1600 CBWFS 12A 0 10PF EFS 12B 0 12A 0 1.0 VOSFS 12B 11 –5MV C11 11 0 1PF RFS 11 10 5K C10 10 0 1PF * *generating K *** the next two lines are for the LT1251 EK K 0 TABLE {I(VOSC)/I(VOSFS)}= (–100,0) (0.04,0) (0.1,0.11) + (0.9,0.907) (0.95,1.0) (100,1.0) *** the next two lines are for the LT1256 *EK K 0 TABLE {I(VOSC)/I(VOSFS)}= (–100,0) (0,0) (0.2,0.21) *+ (0.9,0.9) (1.0,1.0) (100,1.
PAGE 22
LT1251/LT1256 W W ACRO ODEL LT1251/LT1256 Macro Model for PSpice PIN # IN NODE # IN K GENERATOR FIRST INPUT STAGE R1 27Ω VOS1 2A NOISE GENERATOR K 1 EN 2 C1 1.5pF IB1 500nA RI1 17M C2 1pF 2B E1 R2 27Ω VOS2 13A 14 RNOISE2 200k IB2 450nA 6 C13 1pF 13B E2 RN2 400Ω VNULL 6A 13 RI2 17M RNOISE1 200k NULL CIRCUIT SECOND INPUT STAGE C14 1.5pF RDUMMY 1M EK GNULL RN1 200Ω 6B C6 1pF 7 CONTROL AMP SUPPLY CURRENTS R3 1.
PAGE 23
LT1251/LT1256 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. N Package 14-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.130 ± 0.005 (3.302 ± 0.127) 0.300 – 0.325 (7.620 – 8.255) 0.770* (19.558) MAX 0.045 – 0.065 (1.143 – 1.651) 0.015 (0.380) MIN 0.005 (0.125) MIN 0.100 ± 0.010 (2.540 ± 0.254) *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm) ( +0.635 8.255 –0.
PAGE 24
LT1251/LT1256 U TYPICAL APPLICATIONS 4-Quadrant Multiplier as a Double-Sideband Suppressed-Carrier Modulator Modulation Gain vs Control Voltage 1.0 VS = ±5V VFS = 2.5V 0.8 LT1256 1 14 2 MODULATION – 0.6 + 1 2 – 0.4 13 GAIN (V/V) + RG1 1.5k CONTROL 1MHz CARRIER 3 0.1µF 4 10k* + – C 5k 5 + IC IFS FS 5k – 12 11 RF2 1.5k 10 RF1 1.5k 0.2 0 –0.2 –0.4 –0.6 6 9 7 8 10k V– –0.8 V+ –1.0 VOUT 2.5VDC INPUT 0 0.5 1.5 2.0 1.0 CONTROL VOLTAGE, PIN 3 (V) 2.