Offset-Compensated Ohms - Keithley 2701 User Manual

3-24 Basic DMM Operation

Offset-compensated ohms

The presence of thermal EMFs (V
accuracy. To overcome these unwanted offset voltages, you can use offset-compensated
ohms (OCOMP). Offset-compensated ohms measurements can be performed on the 100Ω ,
1kΩ , and 10kΩ ranges for the Ω4 function. It cannot be done on the Ω2 function.
NOTE
For a normal resistance measurement, the Model 2701 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 = ΔV/ΔI
where: ΔV = V2 - V1
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
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.
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.
ΔI = I2 - I1
) can adversely affect low-resistance measurement
EMF
.
EMF
Model 2701 User's Manual