Keithley 7011-S Instruction Manual page 56

Low-level resistance measurements
Many times, it is necessary to make resistance mea-
surements with either lower voltage sensitivity or
higher currents than are available with ordinary
DMMs. Examples of cases where low-level resistance
measurements may be necessary include the testing of
PC board traces, contacts, bus bars, and low resistance
shunts.
Figure 44 shows a typical test con6guation for a
switching system capable of testing a number of low
resistance devices. The Model 220 Current Source forc-
es current through the device under test, while the
Model 182 Sensitive Digital Voltmeter measures there-
suliing voltage across the device.
will have a detrimental effect on measurement accwa-
cy unless steps are taken to avoid them (the Model 7011
has been designed to keep relay EMF at a minimal lev-
el.). Thermal FMF effects can be virtually eliminated by
taking two voltage measurements, E, and E,, the first
with the current, I, flowing in one direction, and the
second with a current, I, of the same magnitude flow-
ing in the opposite direction. The resistance can then be
calculated as follows:
Note that simply reversing the current source polarity
will result in a 2X accuracy specification change. To
avoid this problem, matrix switching could be added
to the test system to reverse the current. See paragraph
4.4.3.
Since low voltage levels are being measured, thermal
F.MF offsets generated by relay and connector contacts
L---J
Dud 1%?B MUX
r---1
r-.--
1
I a:G I
I I
L-l
L:-:J L-1-J
Figure 4-6
Low resistance testing
4-8