PCB设计不当，会对电子设备的可靠性产生不良影响。以下关于PCB地线设计方法描述错误的是()。
- A.只由数字电路组成的PCB的地线系统时，可将接地线做成闭环路
- B.电路板上既有高速逻辑电路，又有线性电路时，应尽量加大线性电路的接地面积
- C.接地线应尽可能加粗，使它能通过3倍于PCB的允许电流
- D.在低频电路中，通常信号的工作频率小于1 MHz，它的布线和元件间的电感影响较小，而接地电路形成的环流对干扰影响较大，因而应采用就近多点接地
正确答案及解析
正确答案
解析
设计只由数字电路组成的PCB的地线系统时，将接地线做成闭环路可以明显提高抗噪声能力。在低频电路中，通常信号的工作频率小于1 MHz，它的布线和元件间的电感影响较小，而接地电路形成的环流对干扰影响较大，因而应采用一点接地。当信号工作频率大于10MHz时，地线阻抗变得很大，此时应尽量降低地线阻抗，应采用就近多点接地。当工作频率在1～10 MHz时，如果采用一点接地，则其地线长度不应超过波长的 1/20，否则应采用多点接地法。电路板上既有高速逻辑电路，又有线性电路时，应使它们尽量分开，而两者的地线不要相混，分别与电源端地线相连。要尽量加大线性电路的接地面积。若接地线很细，接地电位则随电流的变化而变化，致使电子设备的定时信号电子不稳，抗噪声性能变坏。因此应将接地线尽量加粗，使它能通过三倍于PCB的允许电流。如有可能，接地线的宽度应大于3 mm。
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