Keithley 2700, User Manual, Page: 115 / 500

3-24 Basic DMM Operation Model 2700 Multimeter/Switch System User’s Manual
Offset-compensated ohms
The presence of thermal EMFs (V
EMF
) can adversely affect low-resistance measurement
accuracy. To overcome these unwanted offset voltages, you can use offset-compensated
ohms (OCOMP). Offset-compensated ohms measurements can be performed on the 100
Ω
,
1k
Ω
, and 10k
Ω
ranges for the
Ω
4 function. It cannot be done on the
Ω
2 function.
NOTE The various instrument operations, including OCOMP, are performed on the
input signal in a sequential manner. See
“Signal processing sequence,”
page D-2 , for details. It includes a flowchart showing where in the processing
sequence that the OCOMP operation is performed.
For a normal resistance measurement, the Model 2700 sources a current (I) and measures
the voltage (V). The resistance (R) is then calculated (R=V/I) and the reading is displayed.
For offset-compensated ohms, two measurements are performed: one normal resistance
measurement, and one using the lowest current source setting.
The offset-compensated ohms reading is then calculated as follows:
Offset-compensated ohms reading =
Δ
V/
Δ
I
where:
Δ
V = V2 - V1
Δ
I = I2 - I1
V1 is the voltage measurement with the current source at its normal level.
V2 is the voltage measurement using the lowest current source setting.
The above 2-point measurement process and reading calculation eliminates the resistance
contributed by the presence of V
EMF
.
Enabling/disabling offset-compensated ohms
Offset-compensated ohms is enabled by pressing SHIFT and then OCOMP. When
enabled, the OCOMP annunciator is on. Offset-compensated ohms is disabled by again
pressing SHIFT and then OCOMP.