High-Speed, Closed-Loop Buffer# Technical Datasheet: LMH6559MFX Differential Amplifier
Manufacturer : National Semiconductor Corporation (NSC)
## 1. Application Scenarios
### 1.1 Typical Use Cases
The LMH6559MFX is a high-speed, fully differential amplifier designed to convert single-ended signals to differential outputs, or to buffer and drive differential signals. Its primary use cases include:
* Single-Ended to Differential Conversion: A core function, ideal for interfacing single-ended signal sources (like sensors, DACs, or RF mixers) with differential-input ADCs. This improves noise immunity and common-mode rejection in the signal chain.
* Differential Line Driving: Driving signals over twisted-pair cables (e.g., in professional video, communications backplanes, or test equipment) where maintaining signal integrity and rejecting ground noise is critical.
* ADC Input Buffering: Placed directly before high-speed, high-resolution ADCs (such as pipeline or SAR ADCs) to provide a low-impedance drive, isolate the ADC's sampling glitches from the source, and set the optimal common-mode voltage.
* Active Filtering: Used as the gain element in differential active filter topologies (e.g., Sallen-Key) for anti-aliasing or signal conditioning in intermediate frequency (IF) stages.
### 1.2 Industry Applications
* Communications Infrastructure: Driving high-speed ADCs in wireless base station receivers, microwave backhaul equipment, and cable modem termination systems (CMTS). Its wide bandwidth supports common IF frequencies.
* Test & Measurement: Serving as a front-end buffer for high-bandwidth oscilloscopes, arbitrary waveform generators, and spectrum analyzers, ensuring clean signal delivery to acquisition systems.
* Medical Imaging: In ultrasound systems, it can buffer and drive signals from transducer arrays to the analog-to-digital conversion stage.
* Professional Video & Broadcast: Differential driving of high-speed video signals (HD-SDI, 3G-SDI) across backplanes or between equipment racks.
### 1.3 Practical Advantages and Limitations
Advantages:
* High Speed: Features a -3dB bandwidth of 900 MHz (G=+2) and a slew rate of 4100 V/µs, suitable for fast transient signals.
* Fully Differential Architecture: Inherently rejects even-order harmonics and common-mode noise, improving system dynamic range.
* Output Common-Mode Control: An internal common-mode feedback loop sets a stable output common-mode voltage, which can also be adjusted externally via the `VOCM` pin to match ADC requirements.
* Low Distortion: Excellent harmonic distortion performance (e.g., -80 dBc HD2/HD3 at 20 MHz) is critical for high-fidelity signal processing.
* Disable Function: The `EN/DIS` pin allows the device to be placed in a low-power shutdown state, useful for power-sensitive or multiplexed applications.
Limitations:
* Fixed Gain: Internally configured for a fixed gain of +2 V/V (6 dB). Different gains require external resistors, which can complicate design and affect bandwidth.
* Power Supply Sensitivity: Performance is optimized for ±5V supplies. Operation outside recommended ranges degrades bandwidth, distortion, and output swing.
* Thermal Considerations: In a small SOT-23-6 package (MFX), power dissipation is limited. Continuous operation at maximum output swing into low-impedance loads may require thermal analysis.
* Input Common-Mode Range: Not rail-to-rail. The input voltage must remain within the specified range relative to the supply rails to maintain linear operation.
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
* Pitfall 1: Improper
