Operation
4-6
Four-terminal ohms measurements
For general purpose testing, the Keithley Model 196
can be used to make four-terminal resistance measure-
ments of each thick Þlm. As shown in Figure 4-5,
OHMS HI and OHMS SENSE HI are connected to one
matrix row, and OHMS LO and OHMS SENSE LO are
connected to another matrix row. With this conÞgura-
tion, the resistance of each resistor element and/or
combined elements can be measured by closing the ap-
propriate crosspoints. In Figure 4-5, crosspoints 1!1
(Row 1, Column 1) and 2!3 (Row 2, Column 3) are
closed to measure the combined resistance of R1 and
R2.
Figure 4-4
Thick film resistor network testing
1
2
3
4
H
1 345 6789102
R
1
R
2
R
3
R
4
R
1
R
2
R
4
R
3
Model 230
Model 196
Ohms Sense
Volts Ohms
Volts/Ohms HI
Output
Common
Measure V or
4-terminal Ω
Source V
Ohms Sense HI
Volts/Ohms LO
Ohms Sense LO
TF-1 TF-2
Sense Output
Sense Common
LHLHLHLHL HLHLHLHLHL
Rows
H
L
H
L
H
L
H
L
Cols
7012
The effects of thermal EMFs generated by relay con-
tacts and connections can be cancelled by using the off-
set compensated ohms feature of the Model 196. (The
Model 7012 has been designed to keep relay EMF at a
minimal level.) To compensate for thermal EMFs, close
two crosspoints (such as 1!1!1 and 1!2!1), this will short
the input of the Model 196, enable zero to cancel inter-
nal offset, and then enable offset compensated ohms.
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