Keithley 7021 Instruction Manual page 56

Although the four-wire connection scheme minimizes prob-
lems caused by voltage drops, there is one other potentially
troublesome area associated with low resistance measure-
ments: thermal EMFs caused by the relay contacts. In order
to compensate for thermal EMFs, the offset-compensated
ohms feature of the Model 2000 DMM should be used.
Table 5-1
Paired channels in four-wire resistance example
Device Channel
under test pair
1 1 and 3
2 2 and 4
3 6 and 5
4 7 and 8
5 11 and 9
6 12 and 10
7 16 and 13
8 17 and 14
9 21 and 15
10 22 and 18
11 26 and 19
12 27 and 20
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Connection
designations
Bank A, In 1 and
Bank B, In 3
Bank A, In 2 and
Bank B, In 4
Bank A, In 6 and
Bank B, In 5
Bank A, In 7 and
Bank B, In 8
Bank A, In 11 and
Bank B, In 9
Bank A, In 12 and
Bank B, In 10
Bank A, In 16 and
Bank B, In 13
Bank A, In 17 and
Bank B, In 14
Bank A, In 21 and
Bank B, In 15
Bank A, In 22 and
Bank B, In 18
Bank A, In 26 and
Bank B, In 19
Bank A, In 27and
Bank B, In 20
Low-level resistance measurements
Many times, it is necessary to make resistance measure-
ments with either lower voltage sensitivity or higher cur-
rents than are available with ordinary DMMs. Examples of
cases where low-level resistance measurements may be nec-
essary include the testing of PC board traces, contacts, bus
bars, and low resistance shunts.
Figure 5-7 shows a typical test configuration for a switching
system capable of testing a number of low resistance
devices. The Model 220 Current Source forces current
through the device under test, while the Model 182 Sensi-
tive Digital Voltmeter measures the resulting voltage across
the device.
Since low voltage levels are being measured, thermal EMF
offsets generated by relay and connector contacts will have a
detrimental effect on measurement accuracy unless steps are
taken to avoid them. (The Model 7021 has been designed to
keep relay EMF at a minimal level.) Thermal EMF effects
can be virtually eliminated by taking two voltage measure-
ments, E and E , the first with the current, I, flowing in one
1 2
direction, and the second with a current, I, of the same mag-
nitude, flowing in the opposite direction. The resistance can
then be calculated as follows:
R =
Note that reversing the current source polarity will result in
a 2 × accuracy specification change. To avoid this problem,
matrix switching could be added to the test system to
reverse the current. See Section 4.
Operation
E – E
2 1
--------------- -
2I
5-9