Operator`s manual

37 Signametrics
forcing a current, and measuring a voltage, which the DMM converts and displays as a 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
In the 2-Wire resistance measurement the DMM sources current and measure resuting voltage. The
SMU2060 measure Resistance using six ranges; 240 to 24 M. The SMU2064 adds two ranges; 24 
and 240 M. It also has a specialized extended resistance measurement of. Connect the resistor to be
measured to the top two terminals; V,2+, V,2-. Disconnect the I,2+ and I,2- terminals in order to
reduce error due to leakage and noise, as well as better safety.
If the resistor to be measured is less than 24 k, you may null out any lead resistance errors by first
shorting the ends of the V,2+ and V,2- test leads together and performing a Relative operation
(DMMSetRelative under program control). Making measurements above 200 k, you should consider
shielded or twisted leads to minimize noise pickup. Further improvement can be achieved using guarding
(section 4.3.5).
It is a good idea to be aware of the test voltages, particularly when measuring a circuite that includes
semiconductors. To reduce this voltage, select a higher resistance range (lower current). For instance,
measuring 10k resistor using the 24k range (100uA), results in 1V test voltage, which will turn on
semiconductor junctions, resulting in lower resistance reading. To avoid this error, select the 240k range
(10uA), which will result in 100mV and will read the 10k a lot more accurately (see section 2.3 for
resistance ranges vs. current). For characterizing semiconductor part types, use the Diode measurement
function.
For applications requiring voltage and current controlled resistance measurements, use the Extended
Resistance Measurement function as well as active guarding is available with the SMU2064.
4.3.2 4-Wire Ohm Measurements
4-wire Ohms measurements are advantageous for making measurements below 200 k, eliminating lead
resistance errors. The V,2+ and V,2- terminals apply a current source stimulus to the resistance, and
the I,4+ and I,4- 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 4-1 for the proper connection. The sense
leads should be closest to the body of the resistor. Observe the limits on the lead resistance of the test
current source lines spelled out in section 2.3.2.
4-wire Ohm makes very repeatable low ohms measurements, from 100  (10  for SMU2064) to
240 k. It is not recommended to use 4W when making measurements above 100 k, although 4-wire
ohms measurements are facilitated up to 240 k. 4-wire measurements are disabled above 240 k since
the extra set of leads can actually degrade the accuracy, due to additional leakage and noise paths.