4-wire Ohm makes very repeatable low ohms measurements, from 10 m
to 200 k
. It is not
recommended to use
4W
when making measurements above 200 k
.
Figure 4-1. The
I,4
+
and I
,4
-
sense leads should be closest to the body of the resistor when making 4-
Wire resistance measurements.
4.3.3 Effects of Thermo-Voltaic Offset
Resistance measurements are sensitive to Thermo-Voltaic (Thermal EMF) errors. These error voltages
can be caused by poor test leads, relay contacts and other elements in the measurement path. They affect
all measurement methods, including 2-Wire and 4-Wire. To quantify this error, consider a system in
which signals are routed to the DMM via a relay multiplexing system. Many vendors of switching
products do not provide Thermal EMF specification, and it is not uncommon to find relays that have more
than 50
V. With several relay contacts in the path, the error can be significant. This error can be
measured using the DMM’s 240mV DC range. To do this, close a single relay that is not connected to any
load, wait for a short time (about 2 minutes), than measure the voltage across the shorted relay contacts.
Make sure to short the DMM leads and set ‘relative’ to clear the DMM offset prior to the measurement.
To calculate worst-case error, count all relay contacts, which are in series with the measurement (
V,
+,
V,
-
terminals in 2-Wire, and
I+, I-
terminals in 4-Wire mode). Multiply this count by the Thermal EMF
voltage. Use Ohms law to convert this voltage to resistance error as in the following table.
Range Ohms
Current
DMM
Resolution
Error due to
10
V EMF
Error due to
100
V EMF
Error due to
1mV EMF
240
1 mA
0.1m
10
m
100
m
1
2.4 k
1 mA
1 m
10
m
100
m
1
24 k
100 uA
10 m
100
m
1
10
240 k
10 uA
0.1
1
10
100
2.4 M
1 uA
1
10
100
10
24 M
100 nA
10
100
1
k
100
Figure 4-2. Resistance measurement errors contributed by Thermo-Voltaic offset.
4.4 Diode Characterization
The Diode measurement function is used for characterizing semiconductor part types. This function is
designed to display a semiconductor device’s forward or reverse voltage. The DMM forces a current and
measures voltage drop. The available source currents for diode I/V characterization include five DC
current values, 100
A, 1
A, 10
A, 100
A and 1 mA. The maximum diode voltage that can be
measured with this function is 2.4V. For higher voltages see the leakage test function.
Applications include I/V characteristics of Diodes, LEDs, Low voltage Zener diodes, Band Gap devices,
as well as IC
testing and polarity checking.
Signametrics
20
