本帖最后由 newton64 于 2016-9-18 15:16 编辑
在PXI的规范2.2中提到（硬件模块的规范应该和CPCI相近）：
RECOMMENDATION: Analog component grounds SHOULD be referenced to the module front panel
ground.
RECOMMENDATION: The front panel ground and the PXI (digital) backplane ground SHOULD be
separated so that the high speed clock edges do not produce spurious signals in the analog circuits.
OBSERVATION: The ground of high speed digital circuits should be returned back to the PXI ground and
not the analog ground
OBSERVATION: Analog circuits should be grounded to the front panel of the module.
根据以上摘出的几条来看，规范为了免除数字信号对模拟的影响建议将两个地分割，数字地走背板地，模拟地走前面板地。
那么我就有些疑惑了：
1.前面板地就是通过前面板连接pcb侧边条进而连接导轨的“外壳地”吧？由于有大量机械连接（前面板到pcb，pcb到导轨），其接触电阻应该不小。另外作为外壳地，出于屏蔽作用应该会接收很多交流的辐射。以上这些不会影响模拟地的稳定吗？
2.规范中提到了建议模拟地和数字地间接一电阻，但怎么感觉下面两条有点矛盾呢？
OBSERVATION: Having the front panel ground and the digital ground entirely separated may make the
module unduly sensitive to static handling problems. This can be alleviated by adding a resistor between the
two grounds whose value is significantly higher than the impedance of the PXI ground to the chassis power
supply ground, a value of greater than 10 ohms and less than 100 k ohms being adequate is most cases. The
presence of a 10 ohm connection is unlikely to introduce significant current since it is much higher than the
resistance from the PXI backplane ground to the power supply.
OBSERVATION: Modules should be designed to maximize the impedance above 100 kHz between the
PXI backplane ground and the module front panel.
其中黑体字部分有些冲突？
3.规范中同时建议了下图的处理方法：
个人认为这种π型网络加上数字模拟电路布局的分割已经足以使模拟信号很干净了，应该都使用背板的地作为回流也可以，为什么规范要建议走前面板地呢？
求高手指点，谢谢！
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