Keithley 2651A Reference Manual page 54

Section 2: General operation
Hazardous voltages may be present on the output and guard terminals. To prevent electrical
shock that could cause injury or death, NEVER make or break connections to the Model 2651A
while the output is on. Power off the equipment from the front panel or disconnect the main
power cord from the rear of the Model 2651A before handling cables connected to the outputs.
Putting the equipment into standby does not guarantee the outputs are not powered if a
hardware or software fault occurs.
To take an ohms measurement:
1. Press the SRC key to select the desired source function, and then set the output source (current
or voltage, dependant on which function is selected) to the desired value based on the expected
resistance. See
source-measure procedure
in this section.
2. Press the LIMIT key to edit the voltage or current limit. When programming a voltage limit, set the
voltage limit above the maximum expected voltage across the resistor under test. When
programming a current limit, set the current limit at or above the maximum expected current
through the resistor under test. See
Front panel source-measure procedure
24) earlier in this section.
3. Press the MEAS key to display voltage or current, then make sure that AUTO measurement
range is on.
4. Press the MEAS key to display ohms.
5. Turn on the output, then note the reading on the display. If necessary, press the TRIG key to
display continuous readings. Turn off the output when finished.
6. Press the OUTPUT ON/OFF control to turn the output on.
7. View the displayed reading (press the TRIG key if necessary). When finished, press the OUTPUT
ON/OFF control again to turn the output off.
Ohms sensing
Ohms measurements can be made using either 2-wire or 4-wire sensing. See
(on page 2-42) for information on connections and sensing methods.
The 2-wire sensing method has the advantage of requiring only two test leads. However, as shown in
2-wire resistance sensing, test lead resistance can seriously affect the accuracy of 2-wire resistance
measurements, particularly with lower resistance values. The 4-wire sensing method shown in
minimizes or eliminates the effects of lead resistance by measuring the voltage across the resistor
under test with a second set of test leads. Because of the high input impedance of the voltmeter, the
current through the sense leads is negligible, and the measured voltage is essentially the same as the
voltage across the resistor under test.
2-32
Model 2651A High Power System SourceMeter® Instrument Reference Manual
Step 1 (see "Step 1: Select and set source level" on page 2-24) of
(see "Front-panel source-measure procedure" on page 2-24) earlier
Step 2
WARNING
(see "Step 2: Set compliance limit" on page 2-25) of
(see "Front-panel source-measure procedure" on page 2-
Front-panel
DUT test connections
2651A-901-01 Rev. A / March 2011