Operation
4-7
Four-wire resistance tests
More precise measurements over a wider range of system
and DUT conditions can be obtained by using the four-wire
measurement scheme shown in Figure 4-7. Here, separate
sense leads from the Model 2000 DMM are routed through
the multiplexer to the resistor under test. The extra set of
sense leads minimizes the effects of voltage drops across the
test leads. Note, however, that an extra two poles of switch-
ing are required for each resistor tested. For this reason, only
16 resistors per card can be tested using this configuration
because two channels must close at the same time.
The example shown in Figure 4-7 tests four devices using
banks A and B. The appropriate channel pairs to close for the
example are shown in Table 4-1.
Figure 4-7
Four-wire resistance testing
Table 4-1
Paired channels in four-wire resistance example
Note that banks A and B must be electrically isolated. Like-
wise, if you were to configure a dual 1
×
8 four-wire resistance
test by externally connecting the outputs of banks A and B
and banks C and D using customer-supplied jumpers, you
must electrically isolate banks A and B from banks C and D.
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.
Measurement considerations
Many measurements made with the Model 7035 are subject
to various effects that can seriously affect low-level measure-
ment accuracy. The following paragraphs discuss these
effects and ways to minimize them.
Path isolation
The path isolation is simply the equivalent impedance
between any two test paths in a measurement system. Ideally,
the path isolation should be infinite, but the actual resistance
and distributed capacitance of cables and connectors results
in less than infinite path isolation values for these devices.
HI
LO
POWER
Model 2000
DMM
1
Dual 1
×
4 MUX
DUTs
(4)
A. Test Configuration
4
5
8
1
4
Input HI
Input LO
Sense
Ω
4
Wire HI
Sense
Ω
4 Wire
LO
7035
R
B. Simplified Equivalent Circuit
Input HI
Sense
Ω
4 Wire HI
Input LO
Model 2000
DMM
7035
DUT
Ω
Sense
Ω
4 Wire LO
Device
under test
(DUT)
Channel
pair
Connection
designations
1
1 and 5
Bank A, IN 1 and
Bank B, IN 5
2
2 and 6
Bank A, IN 2 and
Bank B, IN 6
3
3 and 7
Bank A, IN 3 and
Bank B, IN 7
4
4 and 8
Bank A, IN 4 and
Bank B, IN 8
Содержание 7035
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