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Grasp shielding, guarding in high impedance apps

Posted: 12 Dec 2013 ?? ?Print Version ?Bookmark and Share

Keywords:shielding? grounding? measurements? high impedance? electrostatic field?

Consider a a well-shielded and grounded single SMU test system; In the example shown in figure 5a, if the measurement LO terminal were to be grounded to earth at the DUT LO terminal, either directly or through a capacitance, ground currents would flow in the measurement leads, and remote sensing would have to be used to eliminate the error voltage generated by the E-field between the two safety grounds. In figure 5b, if the shield surrounding the DUT were connected to earth, current would not flow through the measurement LO connection. In this case, the capacitance from the DUT to its surrounding shield should be minimised. In figure 5c, the shield is connected to earth at the instrument through a current limiting resistor. In this case, the potential between safety earth grounds depicted by Vx does not force any current because only one safety earth ground has been introduced.

In all of these examples, the guard should be brought as close to the DUT as possible and dropped only after it is within the DUT shield.

Figure 5b: In this single SMU application, grounding LO at the DUT, either directly or through a capacitance, leads to no error currents in the measurement LO lead.

Figure 5c: With a single SMU, grounding the shield with a resistor, at the instrument leads to no error current in the measurement LO lead.

Next; a well-shielded and grounded multi-instrument test system; figure 5d introduces a second instrument into the test system. In this case, preventing all ground currents from entering into the measurement is difficult because there are two different safety ground connections. In this case, the resulting current can be reduced by connecting LO to safety earth with a high resistance at only a single point, by connecting both shields to the DUT shield, as shown in figure 5d. In this case, the bulk of the current will flow through both power transformers and through the shield system. Some current will flow through the measurement leads, so remote sensing will also be necessary.

Figure 5d: With two SMUs, the remote sense lines compensate for the current flowing in the measurement leads generated from the use of two different grounds.

Conclusions
Most measurement errors can be traced to currents coupled into the DUT or into the measurement leads from external electrostatic (high impedance) fields. Adding an electrostatic shield properly grounded to the instrument LO can totally eliminate these noise sources. In some instances, a guard must be used instead of or in addition to an electrostatic shield, for very low current applications. Differences in the safety ground caused by safety ground currents generated from line-operated equipment can also cause measurement errors if the current is allowed to flow through the measurement leads. Common mode current from the test system's instruments contribute to these errors.

The instrument power transformer supports this current so any connection to safety ground should be created as described. The safety shield used to maintain operator safety offers the added benefit of providing some low frequency RF shielding. If the instrument common is connected to safety ground with a relatively large resistor, the RF energy will not enter the instrument, and voltages due to EMI rectification can be minimised.

About the author
James Niemann is a Staff Engineer at Keithley Instruments, which is part of Tektronix. He is responsible for designing instrumentation used in low-level measurements. Niemann earned a Bachelor of Science degree in Electrical Engineering from the University of Akron. He has been awarded three patents for his work and has 24 years of experience in instrumentation design.

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