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3.5.5 Four-Lead Sensor Measurement 35
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3.5.5 Four-Lead Sensor
Measurement All sensors, including both two-lead and four-lead can be measured with a four-lead
technique. The purpose of a four-lead measurement is to eliminate the effect of lead
resistance on the measurement. If it is not taken out, lead resistance is a direct error
when measuring a sensor.
In a four-lead measurement, current leads and voltage leads are run separately up to
the sensor. With separate leads there is little current in the voltage leads, so their
resistance does not enter into the measurement. Resistance in the current leads will
not change the measurement as long as the voltage compliance of the current source
is not reached. When two-lead sensors are used in four-lead measurements, the short
leads on the sensor have an insignificant resistance.
3.5.6 Two-Lead Sensor
Measurement There are times when crowding in a cryogenic system forces users to read sensors in a
two-lead configuration because there are not enough feedthroughs or room for lead
wires. If this is the case, plus voltage to plus current and minus voltage to minus cur-
rent leads are attached at the back of the instrument or at the vacuum feedthrough.
The error in a resistive measurement is the resistance of the lead wire run with cur-
rent and voltage together. If the leads contribute 2
)
or 3
)
to a 10 k
)
reading, the
error can probably be tolerated. When measuring voltage for diode sensors, you can
calculate the error in voltage as the lead resistance times the current, typically 10 µA.
For example: a 10
)
lead resistance times 10 µA results in a 0.1 mV error in voltage.
Given the sensitivity of a silicon diode at 4.2 K, the error in temperature would be only
3 mK. At 77 K the sensitivity of a silicon diode is lower so the error would be close to
50 mK. Again, this may not be a problem for every user. Connectors are also a big
source of error when making two-lead measurements. Connector contact resistance
is unpredictable and changes with time and temperature. Minimize interconnections
when making two-lead measurements. Refer to FIGURE 3-6 for an image of a two-
lead sensor measurement.
FIGURE 3-5 4-lead measurement
+
+
–
–
I
V
V
I
+
+
–
–
I
V
V
I
Diode
(option only) Resistive sensor