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Keysight Technologies
Achieve Accurate Resistance Measurements
with the 34980A Multifunction Switch
Measure Unit
Application Note
Abstract
When you make multiple resistance measurements, accuracy can be improved with compensation
and control. This is particularly true when resistance values being measured become very small or
when small changes in resistance need to be detected. This Application Note offers several options to
help you make accurate low resistance measurements. It provides a basic overview of how to ensure
accurate 2-wire, 3-wire and 4-wire measurements with the Keysight 34980A Multifunction Switch/
Measure unit with built-in digital multimeter (DMM) and a plug-in multiplexer module.
03 | Keysight | Achieve Accurate Resistance Measurements with the 34980A Multifunction Switch Measure Unit - Application Note
Multiplexer relay
The Challenge of Two-Wire Resistance S+
RR RL
Measurements
V+
Two-wire resistance measurements are the simplest
to make. After all, what test engineer has not picked Isource R unknown
up a handheld ohm meter to make this measurement?
This basic concept can be extended to making multiple RR RL
V-
measurements by using a 2-wire multiplexer such as the
Keysight 34980A Multifunction Switch/Measure unit with
built-in DMM and a 2-wire multiplexer module like the S-
34921A or the 34922A.
Figure 2. Two-wire technique where RR is the relay contact
The drawing at Figure 2 shows an unknown resistance resistance and RL is the lead resistance. R is the resistance to be
measured.
connected to an ohm meter through a multiplexer and
a length of wire. In addition to the unknown resistance 2 Wire Error
(Runknown), the resistance of the lead wires (RL) and the
contact resistance of any closed relays (RR) equal the 60
total resistance measured. In applications where the 50
Resistance error
unknown resistance is relatively high or where accuracy 40
requirements are low, a two-wire measurement may be 30
acceptable. 20
10
The plot at Figure 3, shows how the error in the measure-
0
ment changes as the value of Runknown becomes smaller.
1 10 100 1000 10000
The additional resistance of the lead length and relay
causes a resistance error that becomes substantial as R unknown
Runknown is similar to RL + RR.
Figure 3. Two-wire error.
A reference resistor (Rreference) can also be used to Multiplexer relays
compensate for relay resistance and the lead length. The RR RL
S+
reference resistor may be of a similar value to Runknown and
should be located such that lead lengths are comparable. I loop RR RL
The technique involves measuring the reference resistor, V+
calculating an error based on deviation from the known V loop
value and using this error to compensate the measurement Isource V R unknown
of Runknown. Corrections are typically made in software after
making sequential measurement of Runknown and Rreference. RR RL
V-
RR RL
The technique is best applied in situations where a
2-wire system is in place and there is a desire to improve
S-
accuracy without making substantial changes. The refer-
ence resistor could be a low value or even a short in some
Figure 4. 4-wire technique where one loop provides current to
situations. The key is to have a trusted reference that Runknown, creating a voltage drop, measured in the low current V
is relatively insensitive to environmental factors and on loop. The current in the V loop is small due to the high impedence
the same order of the Runknown value to provide a trusted of the voltmeter, therefore RR and RL can be neglected and
measurement. This techinique should be validated before Runknown is accurately measured.
relying on measurement results.
04 | Keysight | Achieve Accurate Resistance Measurements with the 34980A Multifunction Switch Measure Unit - Application Note
Benefits of a 4-Wire Technique 0