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屏蔽与防护

为了降低静电干扰，利用具有地电位的导体包围样本和线缆（屏蔽）。

为了防止漏电流影响测量，利用具有同样电势的导体对至静电计[1]输入端的连接进行包围（防护）。测量电流时，需要将防护装置连接至电压源的LO端。

在高真空系统（真空室）中，它通常接地，起到屏蔽作用。样本支架应当连接至防护电势，以防止漏电流。只要利用接地导体对测量室（如加热装置）内的信号线缆进行屏蔽，同时利用栅格对泵浦（如涡轮分子泵浦）进行覆盖，就可以进行低噪声测量[2](1fAp-p)。对于需要超高真空的应用，可以使用特氟纶（Teflon）®电缆，因为这种电缆几乎不会漏气。如果没有三轴馈通，可以在连接处使用绝缘的同轴馈通(如BNC) ，并增加屏蔽筒，对线缆的外部屏蔽层和真空室进行连接。

Shielding and Guarding

To reduce electrostatic interference, surround the sample and the cables with a conductor at ground potential (shield).

To prevent leakage currents from affecting the measurements, surround the connection to the electrometer input by a conductor at the same potential (guard). For measuring currents, this is done by connecting the guard to the LO terminal of the voltage source.

In a high vacuum system, the vacuum chamber, which is commonly grounded, acts as the shield. The sample holder should be connected to guard potential to prevent leakage currents. Low noise measurements (1fAp-p) can be made as long as the signal cabling inside the chamber (e.g., heating elements) is shielded with a grounded conductor and the pumps (e.g., turbomolecular pumps) are covered with a grid. For applications that require ultra-high vacuum, use Teflon® wires because they outgas very little. If a triax feedthrough isn’t available, use an insulated coaxial feedthrough (e.g., BNC) in conjunction with an additional shielding cylinder that connects the outer shield of the cable with the vacuum chamber.

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[1]静电计http://www.keithley.com.cn/products/dcac/sensitive/highresistance/?mn=6514

[2]低噪声测量http://www.keithley.com.cn/products/dcac/sensitive/acdc/?mn=2010

[3]6517A型静电计/高阻表http://www.keithley.com.cn/products/localizedproducts/highresistance/6517a