Interference By Magnetic Fields; Resistance Measurement; Two-Wire Resistance Measurement; Four-Wire Resistance Measurement - Hameg HM8112-3 Benutzerhandbuch

R e s i s t a n c e M e a s u r e m e n t
4.5

Interference by magnetic fields

If the measuring cables are in the vicinity of ac magnetic fields,
a series mode interference signal will be induced. Such a source
of interference may be a cable carrying high mains frequency
currents or a transformer. Twisted pairs of measuring cables
will minimize the pick-up of magnetic interference in the vicinity
of a magnetic field. Measuring cables should not float around
freely nor should they be moved during a measurement, be-
cause this may also cause erroneous measurements. A greater
distance to the interfering field or shielding are further means
to minimize interference.

5 Resistance Measurement

The HM8112-3 measures resistances by injecting currents,
2 and 4 wire circuits are possible. A current from a precision
current generator is sent through the resistor R, the voltage
drop is measured.
5.1

Two-wire resistance measurement

A current from a current generator flows through the DUT
and the measuring cables' RL. The voltage drop across R is
measured. But there is also a small voltage drop across the
measuring cables. This is why it is necessary, especially when
measuring small resistances ( < 1 kΩ) to carefully compensate
for the measuring cables' resistances and thermoelectric volta-
ges by using the offset correction feature. This is performed by
connecting both measuring cables to one side od the DUT, i.e.
shorting them, then the button ZERO
This eliminates the sources of error like cable resistance, con-
tact resistance, and thermoelectric voltages at the junctions of
dissimilar metals.
If no offset correction was performed, a value for R will be dis-
played which consists of the sum of all resistances within the
measurement circuit, the result will hence be too high by the
amount of cable and other resistances.
R
L
R
R
L
In practice, usually cables of 1 m length are used which have
a resistance of 10 .. 20 mΩ. If the resistor to be measured is
100 Ω, this will cause an error of 0.04 %. With small resistances,
especially in the 100 Ω range, the cable resistance thus beco-
mes remarkable. In these ranges 4-wire measurements are
recommended.
46
Subject to change without notice
should be pushed.
4
DMM
U I
V
m
m
5.2

Four-wire resistance measurement

In order to prevent the measuring problems caused by the cable
resistances, the 4-wire circuit is used for all small resistors. In
a 4-wire measurement circuit also a current from a precision
current source flows through the resistor R. The voltage drop
across R is taken off directly by two more cables and measured,
and this voltage drop is strictly proportional to the resistance
value only.
R
L
R
L1
R
R
L1
R
L
The „outer" connections SOURCE of the 4-wire resistance
terminals are the ones which force the measuring current via
the cables with their resistances R
measured. The „inner" measuring cables with their resistances
R are connected to the V-SENSE- INPUT of the measuring
L1
instrument which has a high input resistance, hence the voltage
drop across R is neglegible.
L1
In both the 2-wire and 4-wire circuits shielded cables
should be used for the measurement of large resi-
stances (> 100 kΩ), the screen should be connected
to ground in order to prevent interference from
other voltage sources (like mains frequency hum).
The cables should also have a high insulation resi-
stance (e.g. Teflon insulation), otherwise leakage
current problems could arise, caused by the par-
allel connection of the DUT, R, and the insulation
resistance.
It is also advantageous to select a longer integra-
tion time > 1s in order to suppress interference by
the longer integration of the measuring signal.
5.3

Power dissipation of the resistors

A source of error, often overlooked when measuring resistive
sensors (e.g. temperatur sensors), is the power dissipation in
the resistors to be measured and their ensuing self-heating.
Especially with sensors with a high temperature coefficient the
measuring result can be adversely affected. The influence of
this source of error can be reduced by proper range selection.
The following table lists the power dissipation at full scale in
the various ranges.
Range Measuring current
100 Ω 1 mA
1 kΩ 1 mA
10 kΩ 100 µA
100 kΩ 10 µA
1 MΩ 1 µA
10 MΩ 100 mA
DMM
I
V
U
m
m
through the resistor to be
L
Power dissipation
at full scale reading
100 µW
1 mW
100 µW
10 µW
1 µW
100 mW