Keithley 2657A, Reference Manual, Page: 699 / 805

Section 9: Frequently asked questions (FAQs) Model 2657A High Power System SourceMeter® Instrument Reference Manual
9-6 2657A-901-01 Rev. B/December 2012
How do I make low-current measurements?
Low-current connections
Low-current measurements (<1 mA) are subject to errors caused by leakage currents and leakage
resistances in the signal path. Model 2657A 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.
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.
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. To protect the operator
from hazardous live voltages, you must make a separate protective earth (safety ground) connection
to the safety shield.
A safety shield must be used whenever hazardous voltages (>30 V RMS, 42 V peak) will be present
in the test circuit. To prevent electrical shock that could cause injury or death, never use the Model
2657A 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.
WARNING