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ADI技术专家的分析： 自激最常见的产生原因是输入信号线与输出信号线布线过近引起的。除此之外，还有可能是以下原因： 1， 主要自激原因可能来自电源及PCB布线。首先请您测试一下，如果将输入端接地，看输出端是否有输出自激信号。建议您检查一下您的直流供电电源，是否有足够的去耦处理，直流供电电源如果有交流分量的话，会引起放大器自激震荡。 2， 两级放大的话级间也可能产生自激源，建议在两级间增加滤波网络。 3， 还有就是您的AD8367的用途，如果用作VGA，您可以减少Vgain的控制电压，即减少放大倍数，先测试一下，看是否还会自激。如果用作AGC，那么您要将MODE管脚输入置低，形成负反馈电路，并且检查一下是否第一级AD8367处在深负反馈产生自激。 ADI国外专家的意见（英文好的可以看看，第三点值得参考：高增益高带宽的这类器件可能不稳定，我的问题就是用的电阻做的宽带匹配，自激现象消失，但电阻匹配损耗大） This is not the first timecascaded AD8367 have been used to extend the dynamic range. There are afew things one must keep in mind while cascading the VGAs to extend the dynamicrange: 1) The input linearity of thecascade will be very low at high gain settings 2) The sensitivity will be poorat low gain settings 3) High max gain and highbandwidth devices like the AD8367 may be potentially unstable, and it may benecessary to apply some resistive padding between the stages to ensure a stablesolution. Starting with the third point,it will likely be necessary to add ~10dB of padding between the two VGA's (or all three VGAs if you need it, right now, I’ll just speak to two cascadedVGAs). The AD8367 needs to be sourced and loaded in a 200-Ohm impedanceenvironment to ensure stability. However, the output of the AD8367 presents a50-Ohm impedance. If you directly cascade the devices the 2nd stage VGA will bepresented by the 50-Ohm impedance of the first stages output, and may cause the2nd stage to oscillate. As a result a 200-Ohm to 200-Ohm resistive pad willlikely be needed between the two stages. A 10dB pad should be enough to isolatethe 50-Ohm source impedance of the first stage VGA from the input of the secondstage. If the cascade is properly stabilized, you should be able to achieve~90dB of gain adjustment range. You will probably have toeither LC matching or baluns/transformer on the input of the first VGA and theoutput of the second VGA to get the 50 Ohms to 200 Ohm match. This willhelp with the MDS and the Noise Figure of the system. 也建议您在级间增加pi衰网络。
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