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TI公司的LMH6881是全差分可編高速放大器,工作頻率高達1GHz, 100Ω輸入,低阻抗輸出,增益數字控制,從6dB到26dB,小信號帶寬2400MHz,100MHz時的OIP3為44dBm, HD3為-100dBc,噪音為9.7dB,主要用在示波器前端,頻譜分析儀增益區塊,差分ADC驅動器,IF/RF和基帶增益驅塊和醫療圖像.本文介紹了LMH6881主要特性,典型應用電路和 評估板LMH6881EVAL主要特性,電路圖和PCB布局圖. The LMH6881 is a fully differential amplifier optimized for signal path applications up to 1000 MHz. The LMH6881 has a 100Ω input and a low impedance output. The gain is digitally controlled over a 20 dB range from 26dB to 6dB. The LMH6881 is designed to replace fixed gain differential amplifiers with a single, flexible gain device. It has been designed to provide good noise figure and OIP3 over the entire gain range. This design feature is highlighted by the Dynamic Range Figure (DRF). The DRF is defined as the input third order intercept point (IIP3) minus the noise figure (NF). Traditional variable gain amplifiers generally have the best OIP3 and NF performance at maximum gain only. The LMH6881 is a high-speed, high-performance, programmable differential amplifier. With a bandwidth of 2.4GHz and high linearity of 44dBm OIP3, the LMH6881 is suitable for a wide variety of signal conditioning applications. The LMH6881 programmable differential amplifier family combines the best of both fully differential amplifiers and variable gain amplifiers. It offers superior noise and distortion performance over the entire gain range without external resistors, enabling the use of just one device and one design for multiple applications requiring different gain settings. The LMH6881 is an easy-to-use amplifier that can replace both fully differential, fixed gain amplifiers as well as variable gain amplifiers. The LMH6881 requires no external gain-setting components and supports gain settings from 6dB to 26dB with small, accurate 0.25-dB gain steps. With an input impedance of 100 Ohms the LMH6881 is easy to drive from a variety of sources such as mixers or filters. The LMH6881 also supports 50 Ohm single ended signal sources and supports both DC- and AC-coupled applications. Parallel gain control allows the LMH6881 to be soldered down in a fixed gain so that no control circuit is required. If dynamic gain control is desired, the LMH6881 can be changed with SPI compatible serial commands or with the parallel pins. The LMH6881 is fabricated in TI’s CBiCMOS8 proprietary complementary silicon germanium process and is available in a space saving, thermally enhanced 24-pin Lead Quad QFN package. The same amplifier is offered in a dual package as the LMH6882. LMH6881主要特性: Small Signal Bandwidth: 2400MHz OIP3 @ 100 MHz: 44dBm HD3 @ 100 MHz: −100dBc Noise Figure: 9.7dB Voltage Gain Range: 26dB to 6dB Voltage Gain Step Size 0.25dB Input Impedance 100Ω Parallel and Serial Gain Control Power Down Capability LMH6881應用: ● Oscilloscope Front End ● Spectrum Analyzer Gain Block ● Differential ADC Driver ● Differential Cable Driver ● IF / RF and Baseband Gain Blocks ● Medical Imaging 
圖1.LMH6881典型應用電路 LMH6881評估板LMH6881EVAL The LMH6881EVAL evaluation board is designed to aid in the characterization of Texas Instrument’s High Speed LMH6881 Differential Amplifier. In order to use the full bandwidth of the LMH6881 the LMH6881EVAL board has been built with a high performance dielectric on the top layer (ROGERS RO4350B). This high performance dielectric provides well matched impedance and low loss to frequencies beyond 1GHz. All signal paths are routed on the top layer. The remainder of the board is conventional FR4. Use the evaluation board as a guide for high frequency layout and as a tool to aid in device testing and characterization. 評估板LMH6881EVAL主要特性: Fully assembled Flexible configuration High performance Roger’s dielectric Parallel or serial control USB to SPI adapter
圖2.評估板LMH6881EVAL外形圖
圖3.評估板LMH6881EVAL和USB2ANY的連接圖
圖4.評估板LMH6881EVAL電路圖
圖5.評估板LMH6881EVAL金屬布局圖(頂層)
圖6.評估板LMH6881EVAL差分信號輸入配置圖
圖7.評估板LMH6881EVAL單端輸入信號配置圖 詳情請見: 
lmh6881[1].pdf
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