LT1256CN - 40MHz Video Fader Amplifier | Analog Devices
料号： LT1256CN ✓ 量产中LT1256CN 概述
A video fader is a controlled-gain summing amplifier used to blend two signal sources smoothly and repeatably, typically in broadcast video equipment, audio consoles, keyers, and AGC loops. The LT1256 belongs to the current-feedback amplifier family, itself a subset of operational amplifiers, inside the broader category of analog signal-conditioning integrated circuits. Its linear control characteristic means a control voltage produces a proportional change in signal contribution, which makes the part well suited to automated mix and fade systems rather than purely manual potentiometer fading.
Key features include two independently weighted inputs, DC gain control in addition to the fading function, 40 MHz bandwidth with 300 V/us slew rate for full-amplitude video signals, and a wide 5 V to 30 V supply range that allows the part to run from legacy analog rails or modern low-voltage systems. Because the control loop is linear rather than logarithmic, the part can also be used as a programmable attenuator, an analog multiplier element in filter and modulation blocks, and as the forward path of a closed-loop gain-control system.
The current-feedback topology is the key architectural point: in a current-feedback amplifier the closed-loop bandwidth is largely set by the external feedback resistor rather than by the closed-loop gain, so large-signal performance stays consistent as the control voltage changes gain. The result is that signal amplitude does not visibly change color or slew rate while fading, and distortion stays low at large video amplitudes.
Typical applications include broadcast video switchers and mixers, video keyers and titlers, audio mixing consoles, automatic gain control front ends for cameras and RF receivers, lab-grade attenuator and filter test setups, and general instrumentation where a variable, precisely weighted gain element is needed.
When laying out an LT1256 design, treat the feedback resistor and the compensation capacitor as part of the gain-setting network: keep them close to the device, use a 0.1 uF ceramic bypass on each supply rail, and verify output swing headroom at the highest control voltage, because the output typically approaches the supply rails on a single-supply configuration.
This page consolidates manufacturer specifications, package and lifecycle detail, verified drop-in alternatives, comparison parameters, and practical layout guidance in one place, so engineers can qualify the LT1256CN for a new mixer or fade block without cross-checking several documents.
直接替换替代料： LT1256CN ——相同封装，引脚兼容。需要 PCB 改动的不同封装料号已排除。
LT1256CN#PBF
✅ 直接替换📋 参考替代料（不在目录中）
LT1251CN#PBF
✅ 直接替换 ⚠️ 规格未核验📋 参考替代料（不在目录中）
LT1256CS#PBF
✅ 直接替换📋 参考替代料（不在目录中）
LT1256CN 最大额定值与电气特性
| Device Type | Video Fader and DC Gain Controlled Amplifier |
| Amplifier Topology | Current Feedback |
| Small-Signal Bandwidth (-3 dB) | 40 MHz |
| Slew Rate | 300 V/us |
| Supply Voltage Range | 5 V to 30 V |
| Number of Channels | 1 |
| Number of Inputs | 2 |
| 输出数量 | 1 |
| Gain Control Characteristic | Linear control circuit |
| 封装 | 14-pin PDIP (N-14) |
| 安装类型 | Through-hole |
| 生命周期状态 | PRODUCTION |
| Lead-Free Ordering Code | LT1256CN#PBF |
| Manufacturer Documentation | LT1256 Data Sheet, 24 pages (per distributor datasheet listing) |
LT1256CN 14-pin pdip (n-14) 引脚配置指南
LT1256CN（14-pin pdip (n-14) 封装）的引脚配置。 该模拟元件具有输入、输出和电源引脚。请参阅制造商数据手册以了解失调调零、补偿和使能引脚配置。适用于信号调理和放大器电路。
暂无详细引脚数据： LT1256CN.
请参阅 数据手册 以查看完整引脚配置。
典型应用
LT1256CN 适用于 6 应用s: Broadcast Video Mixer and Switcher, Video Keyer and Titler Overlay, Audio Mixing Console and Program Bus, Closed-Loop Automatic Gain Control Front End, Programmable Attenuator and Filter Test Equipment, RF and IF Receiver Signal Conditioning.
Broadcast Video Mixer and Switcher
The LT1256CN fits video mixers because it sums two weighted inputs under a linear control voltage while preserving 40 MHz bandwidth and 300 V/us slew rate, so composite and component video signals keep their edges intact during a fade. Placing the device as the final stage of a mixing bus lets a control MCU ramp the fade rather than relying on a mechanical fader pot. The 5 V to 30 V supply range covers both legacy studio rails and modern low-voltage boards, while the single output drives the next stage through a conventional 75 ohm video network. The trade-off is that the control characteristic is linear rather than logarithmic, so the control ramp must be shaped in software if a logarithmic fader law is desired.
推荐
Video Keyer and Titler Overlay
In a keyer or titler, the overlay source must be weighted against the background video with precise, repeatable control, which is exactly what the LT1256CN two-input, linear-control architecture provides. The 40 MHz bandwidth and 300 V/us slew rate handle full-amplitude video without visible color shift or ringing, and the 5 V to 30 V supply span allows the amplifier to share an existing studio rail without extra regulation. Because the same device also provides a DC gain control path, it can double as the clip/gain element ahead of the key comparator. Design care is needed at the output to preserve headroom when the control voltage drives the contribution to its maximum.
推荐
Audio Mixing Console and Program Bus
Although marketed as a video fader, the LT1256CN is a general dual-input controlled-gain summing amplifier, so it serves audio program buses where a voltage-controlled crossfade is needed rather than a passive potentiometer. Its linear control characteristic makes gain a computable function of the control voltage, which simplifies console automation and calibration routines. The wide supply range supports unbalanced professional audio rails, and the high slew rate keeps fast transients from clipping through the mixer stage. The limitation is that a single pair of inputs gives only two-way weighting, so multi-channel consoles use it per bus, and the audio path should be checked for distortion at the highest control settings.
推荐
Closed-Loop Automatic Gain Control Front End
The LT1256CN works well as the forward element of an AGC loop because its control input accepts a DC feedback voltage while the 5 V to 30 V supply range lets the same device sit at the sensor or receiver signal level without a separate regulator. A rectifier and error amplifier measure output amplitude and drive the control pin until the output sits at the chosen reference. Because the bandwidth is set mainly by the external feedback resistor, the loop gain and phase margin remain predictable as the control voltage moves across its range. The main design caution is loop stability: keep the loop crossover well below 40 MHz and keep the feedback compensation components close to the device.
推荐
Programmable Attenuator and Filter Test Equipment
Laboratory attenuator and filter response test setups need a repeatable, voltage-controlled gain element, and the LT1256CN provides that with a linear control law plus the DC gain function that lets a fixture set both attenuation and reference level from a single amplifier. Its 40 MHz bandwidth covers video-band and wideband filter characterization, and the through-hole 14-pin PDIP package is convenient for prototyping and instrument retrofit where surface-mount rework is undesirable. The trade-off is that a through-hole package consumes more board area than the SOIC option, and the control pin should be kept away from fast-switching nodes to avoid control-signal pickup. Repeatable calibration relies on the linear control characteristic.
推荐
RF and IF Receiver Signal Conditioning
Receiver chains frequently need a weighted two-input summing point before filtering, for example to combine a main path with a test injection or to fade between antennas. The LT1256CN handles this as a current-feedback amplifier with 40 MHz bandwidth and 300 V/us slew rate, which is adequate for IF-band conditioning while keeping the amplitude stable as the control voltage changes. Using the DC gain control path, the same device can trim gain in the presence of a detected signal level, supporting soft AGC behavior ahead of a demodulator. The design limit is the 5 V to 30 V supply envelope and input common-mode range, which must be respected when the IF level sits near the rail.
推荐
推荐产品汇总
工程参考数据： LT1256CN ——对比、设计指导和合规信息。
选型指南
与替代料对比
| 参数 | 本产品 | LT1256CN#PBF | LT1251CN#PBF | LT1256CS#PBF |
|---|---|---|---|---|
| Brand | Analog Devices | Analog Devices | Analog Devices | Analog Devices |
| Package | 14-pin PDIP (N-14) | 14-pin PDIP (N-14) - same | 14-pin PDIP (N-14) - same | 14-pin PDIP (N-14) order option (SOIC version also offered) |
| Small-Signal Bandwidth (-3 dB) | 40 MHz | 40 MHz | 40 MHz | 40 MHz |
| Supply Voltage Range | 5 V to 30 V | 5 V to 30 V | 5 V to 30 V | 5 V to 30 V |
| DC Gain Control Function | Yes | Yes | No - fading function only | Yes |
| Number of Inputs / Outputs | 2 inputs / 1 output | 2 inputs / 1 output | 2 inputs / 1 output | 2 inputs / 1 output |
| Amplifier Topology | Current Feedback | Current Feedback | Current Feedback | Current Feedback |
| Lead-Free Finish Option | Yes (LT1256CN#PBF code available) | Yes - lead-free as standard | Yes - lead-free as standard | Yes - lead-free as standard |
关键差异点
- Combines a video fader and a DC gain control function in a single amplifier （对比 LT1251CN#PBF)
- Drop-in lead-free path for existing PDIP layouts （对比 LT1256CS#PBF)
- Wide single-supply operating range for studio and low-voltage rails alike （对比 LT1256CS#PBF)
设计说明
Place the feedback resistor and any compensation capacitor directly between the output pin and the inverting input, keeping traces short and free of other routing. In a current-feedback amplifier the feedback impedance sets the bandwidth, so a long or inductive feedback trace can shift the gain-bandwidth product and cause peaking or oscillation. Keep the 0.1 uF ceramic bypass capacitor at the supply pins and a bulk capacitor close to the device, and route the control input away from the high-impedance input node to minimize control-signal pickup.
Estimated: with a 5 V single supply, a 30 V supply, and a 1 V output at a 40 MHz small-signal level, the internal dissipation is far below any thermal limit, but the real constraint is output headroom rather than heat. On a 5 V rail the output can only swing between the rails, so full-amplitude video waveforms clip unless the signal is biased mid-supply. Confirm the maximum output swing and the control-pin input range in the manufacturer data sheet before relying on the 5 V operating point for full-scale composite video.
Do not treat the control input as a logic-level command without checking its voltage range and input impedance in the data sheet; overdriving it can clamp the internal control current and distort the fade. Avoid substituting the SOIC package option onto a PDIP land pattern, and confirm the lead-free versus tin-lead finish against your assembly's solder and RoHS requirements. Finally, remember that the control characteristic is linear, so an unmodified ramp gives a linear amplitude fade rather than the logarithmic taper users expect from a studio fader.
Because closed-loop bandwidth in a current-feedback amplifier is set by the feedback resistor rather than by the closed-loop gain, changing the control voltage does not change the signal color the way a voltage-feedback amplifier can. Use the data sheet's recommended feedback resistor value and keep input and output routing away from clocked and switching nodes to preserve the 40 MHz, 300 V/us performance. In video paths, terminate the output into the standard 75 ohm network and check for ringing on the edge of a 40 MHz-equivalent transition before locking the layout.
合规信息
A lead-free ordering code (LT1256CN#PBF) is offered alongside the standard LT1256CN, and the #PBF suffix denotes the lead-free finish. RoHS, REACH, halogen-free and conflict-minerals statements for the specific ordering codes are not published on the accessible product material, so they are reported as unknown rather than assumed.
