Operator`s manual

27 Signametrics
4.3 Resistance and Leakage Measurements
Resistance is measured using eight (six in the SM2040) precision current sources, with the DMM displaying the
resistance value. Most measurements can be made in the 2-wire mode. The 4-wire ohms is used to make low value
resistance measurements. All resistance measurement modes are susceptible to Thermo-Voltaic (Thermal EMF)
errors. See section 4.3.5 for details.
4.3.1 2-Wire Ohm Measurements
The DMM measure using 330Ω to 33 MΩ ranges. The SM2042 and SM2044 add 33 Ω and 330 MΩ ranges. Use
the V,Ω+, V,Ω- terminals, being certain to always disconnect the I+, I- terminals.
Most resistance measurements can be made using the simple 2-wire Ohms method. Simply connect V,Ω+ to one
end of the resistor, and the V,Ω- to the other end. If the resistor to be measured is less than 30 kΩ, you should null
out any lead resistance errors by first touching the V,Ω+ and V,Ω- test leads together and then performing a
Relative function. If making measurements above 300 kΩ, you should use shielded or twisted leads to minimize
noise pickup. This is especially true for measurements above 1 MΩ.
You may also want to control the Ohms current used in making resistance measurements. (See the Specifications
section, "Resistance, 2-wire and 4-wire", for a table of resistance range vs. current level.) All of the Ohms ranges of
the SM2040 have enough current and voltage compliance to turn on diode junctions. For characterizing
semiconductor part types, use the Diode measurement function. To avoid turning on a semiconductor junction, you
may need to select a higher range (lower current). When checking semiconductor junctions, the DMM displays a
resistance value linearly related to the voltage across the junction.
For applications requiring resistance measurements higher than 330 MΩ, the Extended Resistance Measurement
method as well as active guarding is available with the SM2044.
4.3.2 4-Wire Ohm Measurements
4-wire Ohms measurements are advantageous for making measurements below 330 kΩ, eliminating lead resistance
errors. The Voltage (V,Ω) Input terminals serve as a current source to stimulus the resistance, and the I, 4WΩ Input
terminals are the sense inputs. The Source + and Sense + leads are connected to one side of the resistor, and the
Source - and Sense - leads are connected to the other side. Both Sense leads should be closest to the body of the
resistor. See Figure 4-3.
4-wire Ohm makes very repeatable low ohms measurements, from 100 µΩ (10 µΩ for SM2042 & SM2044) to
330 kΩ. It is not recommended to use 4WΩ when making measurements above 100 kΩ, although 4-wire ohms
measurements are facilitated up to 330 kΩ. 4-wire measurements are disabled above 330 kΩ since the extra set of
leads can actually degrade the accuracy, due to additional leakage and noise paths.