Tektronix Keithley 2651A Reference Manual page 820

Model 2651A High Power System SourceMeter® Instrument Reference Manual
How do I make low-current measurements?
Low-current measurements (<1 mA) are subject to errors caused by leakage currents and leakage
resistances in the signal path. Model 2651A instruments are equipped with triaxial connectors to
minimize these problems. To assure accurate low-level measurements, the integrity of the signal path
must be maintained to the device under test (DUT), including using both low-noise triaxial cables and
a suitable test fixture.
Low-current connections
The figure below shows typical connections for low-current measurements. The DUT in this example
could be a low-current semiconductor device, a high-megohm resistor, or any other passive or active
electronic device requiring low-current measurements. Note that the DUT is enclosed in both a guard
shield and a safety shield, which is necessary when hazardous voltages are used.
The inner shield (guard) of the HI triaxial cable is connected to the test fixture guard shield. The guard
shield prevents leakage currents from affecting the measurements. The outer cable shield (chassis
ground or protective earth (safety ground)) is connected to the safety shield.
A safety shield must be used whenever hazardous voltages (>30 V
present in the test circuit. To prevent electrical shock that could cause injury or death, never
use the Model 2651A in a test circuit that may contain hazardous voltages without a properly
installed and configured safety shield.
Connect the enclosure of all metal test fixtures to protective earth (safety ground).
Nonconductive test fixtures must be rated to double the maximum capability of the test
equipment in the system. Failure to attach the ground wires to a known protective earth may
result in electric shock.
Additional connections for redundant protective earth (safety ground) may be required.
2651A-901-01 Rev. C October 2021
Section 9: Frequently asked questions
, 42 V ) will be
RMS PEAK
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