Appendix 1 Four-Terminal (Voltage-Drop) Method
A1
Appendix
The Four-Terminal method is essential for measuring very small resistance values.
With two-terminal measurements (Fig. 1), the resistance of the test leads is included in the measured resis-
tance, resulting in measurement errors.
The four-terminal method (Fig. 2) consists of current source terminals (H
CUR
, L
CUR
) to provide constant cur-
rent, and voltage detection terminals (H
POT
, L
POT
) to detect voltage drop.
Because of the high input impedance of the voltmeter, measurement requires practically no current flow
through the leads connecting the voltage detection terminals to the test object, practically eliminating the
effects of lead and contact resistance on the measurement.
Appendix
Appendix 1 Four-Terminal (Voltage-Drop)
Method
Two-Terminal Measurement Method Four-Terminal Measurement Method
Measurement current I flows through test object
resistance R
0
as well as lead resistances r
1
and
r
2
.
The voltage to be measured is obtained by E = I
(r
1
+ R
0
+ r
2
), which includes lead resistances r
1
and r
2
.
Current I flows from r
2
through DUT resistance
R
0
and through r
1
. The high input impedance of
the voltmeter allows only negligible current flow
through r
3
and r
4
.
So the voltage drop across r
3
and r
4
is practically
nil, and voltage E across the measurement
terminals and voltage E
0
across test object
resistance R
0
are essentially equal, allowing test
object resistance to be measured without being
affected by r
1
to r
4
.
Figure 1.
r
1
r
2
E
I
Resistance R
0
Voltmeter
Constant current source
Figure 2.
r
1
r
2
E
I
Resistance R
0
Voltmeter
Constant current source
r
3
r
4
E
0
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