Operator`s manual

19 Signametrics
The Current functions are protected with a 2.5 A, 250 V fuse internal to the DMM. The 2.4mA and 24mA
ranges utilize a 10 shunt, while the 240mA and 2.4A ranges use a 0.1 shunt. In addition to the shunt
resistors, there is some additional parasitic resistance in the current measurement path associated with the
fuse and the internal wiring. The maximum burden voltage is about 250mV.
Warning! Applying voltages greater than 35 V to the I+, I- terminals can cause personal injury and/or
damage to your DMM! Think before applying any inputs to these terminals!
4.2.1 Improving DC Current Measurements
When making sensitive DC current measurements disconnect all terminals not associated with the
measurement. User the Relative function while in the desired DC current range to zero out any residual
error. Using the S-Cal (DMMCalibrate ()) prior to activating Relative will improve accuracy further.
Although the DMM is designed to withstand up-to 2.4A indefinitely, be aware that excessive heat may be
generated when measuring higher AC or DC currents. If allowed to rise this heat may adversely effect
subsequent measurements. In consideration with this effect, it is recommended that whenever practical,
higher current measurements be limited to short time intervals.
4.3 Resistance Measurements
The key for stable and accurate Resistance measurements, with low test voltage, is in the number of
current sources used. This DMM uses five. The V, 2 + provides the positive terminal and the V, 2-
negative terminal of the current source. The DMM measures resistance by 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 DMM
measure Resistance using six ranges; 240 to 24 M. Use the V,2+, V,2- terminals for this function.
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 SMU2064,
which has Extended Resistance Measurement function as well as active guarding.
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 4-3.